OpenFeign（一）

spring-cloud-openFeign源码深度解析

如何使用spring cloud feign

 1 @SpringBootApplication
 2 @EnableFeignClients
 3 public class WebApplication {
 4 
 5     public static void main(String[] args) {
 6         SpringApplication.run(WebApplication.class, args);
 7     }
 8 
 9     @FeignClient("name")
10     static interface NameService {
11         @RequestMapping("/")
12         public String getName();
13     }
14 }

从官网的例子中，可以看出是通过注解驱动的，所以从注解开始看起。

spring cloud feign是如何工作的

Feign涉及了两个注解，一个是@EnableFeignClients，用来开启 Feign，另一个是@FeignClient，用来标记要用 Feign 来拦截的请求接口。

 1 @Retention(RetentionPolicy.RUNTIME)
 2 @Target(ElementType.TYPE)
 3 @Documented
 4 @Import(FeignClientsRegistrar.class)
 5 public @interface EnableFeignClients {
 6 
 7     String[] value() default {};
 8 
 9     String[] basePackages() default {};
10 
11     Class<?>[] basePackageClasses() default {};
12 
13     Class<?>[] defaultConfiguration() default {};
14 
15     Class<?>[] clients() default {};
16 }

 1 @Target(ElementType.TYPE)
 2 @Retention(RetentionPolicy.RUNTIME)
 3 @Documented
 4 public @interface FeignClient {
 5 
 6     @AliasFor("name")
 7     String value() default "";
 8 
 9     @Deprecated
10     String serviceId() default "";
11 
12     @AliasFor("value")
13     String name() default "";
14     
15     String qualifier() default "";
16 
17     String url() default "";
18 
19     boolean decode404() default false;
20 
21     Class<?>[] configuration() default {};
22 
23     Class<?> fallback() default void.class;
24 
25     Class<?> fallbackFactory() default void.class;
26 
27     String path() default "";
28 
29     boolean primary() default true;
30 
31 }

@EnableFeignClients 是关于注解扫描的配置，比如扫描路径，配置等。@FeignClient 则是关于对该接口进行代理的时候，一些实现细节的配置，比如访问url是什么， fallback 方法，关于404的请求是抛错误还是正常返回。

注册client

先关注对EnableFeignClients 的处理，可以看出它使用了@Import(FeignClientsRegistrar.class)，看名字可知是一个注册器，通过扫描某个特性的类，将bean注册到IOC中。Spring 通过调用其 registerBeanDefinitions 方法来获取其提供的 bean definition。

1 public void registerBeanDefinitions(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {
2     //注册configuration
3     registerDefaultConfiguration(metadata, registry);
4     //注册注解
5     registerFeignClients(metadata, registry);
6 }

 1 private void registerDefaultConfiguration(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {
 2     //获取注解@EnableFeignClients 下设置的属性值
 3     Map<String, Object> defaultAttrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName(), true);
 4 
 5     if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
 6         String name;
 7         //判断传入的defaultConfiguration的是不是topClass，所谓topClass就是说此类不是别的类的内部类
 8         if (metadata.hasEnclosingClass()) {
 9             name = "default." + metadata.getEnclosingClassName();
10         }
11         else {
12             name = "default." + metadata.getClassName();
13         }
14         registerClientConfiguration(registry, name,defaultAttrs.get("defaultConfiguration"));
15     }
16 }

1 private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,Object configuration) {
2     //加载FeignClientSpecification bean
3     BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(FeignClientSpecification.class);
4     builder.addConstructorArgValue(name);
5     builder.addConstructorArgValue(configuration);
6     //注册
7     registry.registerBeanDefinition(name + "." + FeignClientSpecification.class.getSimpleName(),builder.getBeanDefinition());
8 }

这里会往 Registry 里面添加一个BeanDefinition，即 FeignClientSpecification，configuration是通过 EnableFeignClients 注解的 defaultConfiguration 参数传入。

 1 public void registerFeignClients(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {
 2     ClassPathScanningCandidateComponentProvider scanner = getScanner();
 3     scanner.setResourceLoader(this.resourceLoader);
 4 
 5     Set<String> basePackages;
 6 
 7     Map<String, Object> attrs = metadata
 8         .getAnnotationAttributes(EnableFeignClients.class.getName());
 9     // 扫描带有FeignClient注解的类
10     AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
11         FeignClient.class);
12     //获取@EnableFeignClients 中clients的值
13     final Class<?>[] clients = attrs == null ? null
14         : (Class<?>[]) attrs.get("clients");
15     if (clients == null || clients.length == 0) {
16         //如果没有设置，那么加入要扫描的注解和扫描的包
17         scanner.addIncludeFilter(annotationTypeFilter);
18         // 确定扫描的包路径列表
19         basePackages = getBasePackages(metadata);
20     }
21     else {
22         //如果设置了，最终扫出来的Bean必须是注解中设置的那些
23         final Set<String> clientClasses = new HashSet<>();
24         basePackages = new HashSet<>();
25         for (Class<?> clazz : clients) {
26             basePackages.add(ClassUtils.getPackageName(clazz));
27             clientClasses.add(clazz.getCanonicalName());
28         }
29         AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {
30             @Override
31             protected boolean match(ClassMetadata metadata) {
32                 String cleaned = metadata.getClassName().replaceAll("\$", ".");
33                 return clientClasses.contains(cleaned);
34             }
35         };
36         scanner.addIncludeFilter(
37             new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));
38     }
39     //循环扫描，并把根据注解信息，进行相关注册
40     for (String basePackage : basePackages) {
41         Set<BeanDefinition> candidateComponents = scanner
42             .findCandidateComponents(basePackage);
43         for (BeanDefinition candidateComponent : candidateComponents) {
44             if (candidateComponent instanceof AnnotatedBeanDefinition) {
45                 // verify annotated class is an interface
46                 AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
47                 AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
48                 //必须注解在interface上
49                 Assert.isTrue(annotationMetadata.isInterface(),
50                               "@FeignClient can only be specified on an interface");
51 
52                 Map<String, Object> attributes = annotationMetadata
53                     .getAnnotationAttributes(
54                     FeignClient.class.getCanonicalName());
55 
56                 String name = getClientName(attributes);
57                 registerClientConfiguration(registry, name,attributes.get("configuration"));
58 
59                 registerFeignClient(registry, annotationMetadata, attributes);
60             }
61         }
62     }
63 }
64 
65 private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
66     String className = annotationMetadata.getClassName();
67     BeanDefinitionBuilder definition = BeanDefinitionBuilder
68         .genericBeanDefinition(FeignClientFactoryBean.class);
69     validate(attributes);
70     //将属性设置到FeignClientFactoryBean 中
71     definition.addPropertyValue("url", getUrl(attributes));
72     definition.addPropertyValue("path", getPath(attributes));
73     String name = getName(attributes);
74     definition.addPropertyValue("name", name);
75     definition.addPropertyValue("type", className);
76     definition.addPropertyValue("decode404", attributes.get("decode404"));
77     definition.addPropertyValue("fallback", attributes.get("fallback"));
78     definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
79     //设置Autowire 类型
80     definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
81 
82     String alias = name + "FeignClient";
83     AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
84 
85     boolean primary = (Boolean)attributes.get("primary"); // has a default, won't be null
86 
87     beanDefinition.setPrimary(primary);
88 
89     String qualifier = getQualifier(attributes);
90     if (StringUtils.hasText(qualifier)) {
91         alias = qualifier;
92     }
93     //注册bean
94     BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
95                                                            new String[] { alias });
96     BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
97 }

另一个往 Registry 里面添加的 BeanDefinition则是FeignClientFactoryBean,负责注册FeignClient。

也就是说，Feign的注册一共分为一下几步：

扫描@EnableFeignClients注解，如果有defaultConfiguration属性配置，则将configuration注册到BeanDefinition中，如果不指定的话，spring 提供的默认配置是FeignClientsConfiguration。
扫描 basePackage 下面所有包含了 @FeignClient 注解的类
如果@EnableFeignClients中配置了clients属性，则扫描出来的bean只有在clients中配置的那些
循环扫描@FeignClient注解，如果配置了configuration，则将configuration按照 1 注册打BeanDefinition中，也就是说Feign既支持用作统一的默认的Config作为全局配置，也可以分别在@FeignClient中单独配置configuration 作为局部配置。
将@FeignClient中的其他配置设置到FeignClientFactoryBean中。
最后调用FeignClientFactoryBean#getObject来创建client实例。

加载配置项

接下来来看下 FeignClientFactoryBean，Spring Context 创建 Bean 实例时会调用它的 getObject 方法。

 1 public Object getObject() throws Exception {
 2     return getTarget();
 3 }
 4 
 5 /**
 6 * @param <T> the target type of the Feign client
 7 * @return a {@link Feign} client created with the specified data and the context information
 8 */
 9 <T> T getTarget() {
10     FeignContext context = applicationContext.getBean(FeignContext.class);
11     Feign.Builder builder = feign(context);//1
12 
13     if (!StringUtils.hasText(this.url)) {
14         //如果没有指定url,获取name值拼接默认url
15         String url;
16         if (!this.name.startsWith("http")) {
17             url = "http://" + this.name;
18         }
19         else {
20             url = this.name;
21         }
22         url += cleanPath();
23         return (T) loadBalance(builder, context, new HardCodedTarget<>(this.type,
24                                                                        this.name, url));
25     }
26     if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
27         this.url = "http://" + this.url;
28     }
29     String url = this.url + cleanPath();
30     Client client = getOptional(context, Client.class);
31     if (client != null) {
32         if (client instanceof LoadBalancerFeignClient) {
33             //使用ribbon提供的负载均衡
34             // not load balancing because we have a url,
35             // but ribbon is on the classpath, so unwrap
36             client = ((LoadBalancerFeignClient)client).getDelegate();
37         }
38         builder.client(client);
39     }
40     Targeter targeter = get(context, Targeter.class);
41     return (T) targeter.target(this, builder, context, new HardCodedTarget<>(
42         this.type, this.name, url));
43 }

如果未指定url，则根据client的name来拼接url，并开启负载均衡
如果指定了URL，没有指定client，那么就根据url来调用，相当于直连，没有负载均衡。如果没有指定client的话，可以使用负载均衡。现在的版本是默认开启负载均衡。

首先在1处可以看出通过feign(context)方法初始化了Feign.Builder,所以着重看一下这个方法：

 1 protected Feign.Builder feign(FeignContext context) {
 2     //获取FeignClientsConfiguration 中注册的bean ，设置到feign中
 3     FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);
 4     Logger logger = loggerFactory.create(this.type);
 5 
 6     // @formatter:off
 7     Feign.Builder builder = get(context, Feign.Builder.class)
 8         // required values
 9         .logger(logger)
10         .encoder(get(context, Encoder.class))
11         .decoder(get(context, Decoder.class))
12         .contract(get(context, Contract.class));
13     // @formatter:on
14 
15     configureFeign(context, builder);
16 
17     return builder;
18 }

这里被设置到builder的bean来自于FeignClientsConfiguration在启动时加载到了context中，这是spring的默认配置，即使在@EnableFeignClients和@FeignClient没有配置configuration也能保证可以使用。

 1 @Configuration
 2 public class FeignClientsConfiguration {
 3     ...
 4         @Bean
 5         @ConditionalOnMissingBean
 6         public Decoder feignDecoder() {
 7         return new OptionalDecoder(new ResponseEntityDecoder(new SpringDecoder(this.messageConverters)));
 8     }
 9 
10     @Bean
11     @ConditionalOnMissingBean
12     public Encoder feignEncoder() {
13         return new SpringEncoder(this.messageConverters);
14     }
15 
16     @Bean
17     @ConditionalOnMissingBean
18     public Contract feignContract(ConversionService feignConversionService) {
19         return new SpringMvcContract(this.parameterProcessors, feignConversionService);
20     }
21 
22     @Bean
23     @Scope("prototype")
24     @ConditionalOnMissingBean
25     public Feign.Builder feignBuilder(Retryer retryer) {
26         return Feign.builder().retryer(retryer);
27     }
28     ...
29 }

那么自定义的configuration在哪里加载呢，可以看到方法feign(context)中最后一行调用了configureFeign(context, builder)，来看一下这个方法。

  1 protected void configureFeign(FeignContext context, Feign.Builder builder) {
  2     //获取.properties的属性
  3     FeignClientProperties properties = applicationContext.getBean(FeignClientProperties.class);
  4     if (properties != null) {
  5         if (properties.isDefaultToProperties()) {
  6             //默认为true
  7             configureUsingConfiguration(context, builder);
  8             configureUsingProperties(properties.getConfig().get(properties.getDefaultConfig()), builder);
  9             configureUsingProperties(properties.getConfig().get(this.name), builder);
 10         } else {
 11             configureUsingProperties(properties.getConfig().get(properties.getDefaultConfig()), builder);
 12             configureUsingProperties(properties.getConfig().get(this.name), builder);
 13             configureUsingConfiguration(context, builder);
 14         }
 15     } else {
 16         configureUsingConfiguration(context, builder);
 17     }
 18 }
 19 //获取用户通过configuration @Bean的自定义配置
 20 protected void configureUsingConfiguration(FeignContext context, Feign.Builder builder) {
 21     Logger.Level level = getOptional(context, Logger.Level.class);
 22     if (level != null) {
 23         builder.logLevel(level);
 24     }
 25     Retryer retryer = getOptional(context, Retryer.class);
 26     if (retryer != null) {
 27         builder.retryer(retryer);
 28     }
 29     ErrorDecoder errorDecoder = getOptional(context, ErrorDecoder.class);
 30     if (errorDecoder != null) {
 31         builder.errorDecoder(errorDecoder);
 32     }
 33     //connectTimeoutMillis和readTimeoutMillis的默认值
 34     Request.Options options = getOptional(context, Request.Options.class);
 35     if (options != null) {
 36         builder.options(options);
 37     }
 38     Map<String, RequestInterceptor> requestInterceptors = context.getInstances(
 39         this.name, RequestInterceptor.class);
 40     if (requestInterceptors != null) {
 41         builder.requestInterceptors(requestInterceptors.values());
 42     }
 43 
 44     if (decode404) {
 45         builder.decode404();
 46     }
 47 }
 48 
 49 
 50 /***
 51      *
 52      * @param config 获取.properties中配置的bean
 53      * @param builder feign
 54      */
 55 protected void configureUsingProperties(FeignClientProperties.FeignClientConfiguration config, Feign.Builder builder) {
 56     if (config == null) {
 57         return;
 58     }
 59 
 60     if (config.getLoggerLevel() != null) {
 61         builder.logLevel(config.getLoggerLevel());
 62     }
 63     //设置connectTimeoutMillis和readTimeoutMillis的值，这里的属性值来自于.properties配置的
 64     if (config.getConnectTimeout() != null && config.getReadTimeout() != null) {
 65         builder.options(new Request.Options(config.getConnectTimeout(), config.getReadTimeout()));
 66     }
 67 
 68     if (config.getRetryer() != null) {
 69         Retryer retryer = getOrInstantiate(config.getRetryer());
 70         builder.retryer(retryer);
 71     }
 72 
 73     if (config.getErrorDecoder() != null) {
 74         ErrorDecoder errorDecoder = getOrInstantiate(config.getErrorDecoder());
 75         builder.errorDecoder(errorDecoder);
 76     }
 77 
 78     if (config.getRequestInterceptors() != null && !config.getRequestInterceptors().isEmpty()) {
 79         // this will add request interceptor to builder, not replace existing
 80         //这里只会往原有的interceptor中添加新的，而不会删掉原有的。
 81         for (Class<RequestInterceptor> bean : config.getRequestInterceptors()) {
 82             RequestInterceptor interceptor = getOrInstantiate(bean);
 83             builder.requestInterceptor(interceptor);
 84         }
 85     }
 86 
 87     if (config.getDecode404() != null) {
 88         if (config.getDecode404()) {
 89             builder.decode404();
 90         }
 91     }
 92 
 93     if (Objects.nonNull(config.getEncoder())) {
 94         builder.encoder(getOrInstantiate(config.getEncoder()));
 95     }
 96 
 97     if (Objects.nonNull(config.getDecoder())) {
 98         builder.decoder(getOrInstantiate(config.getDecoder()));
 99     }
100 
101     if (Objects.nonNull(config.getContract())) {
102         builder.contract(getOrInstantiate(config.getContract()));
103     }
104 }

把配置文件中的配置项在启动时初始化到FeignClientProperties中。

如果配置文件中没有配置，则将FeignClientsConfiguration中的bean作为默认值设置到builder。
如果配置文件中有配置，并且用默认加载顺序时，首先加载FeignClientsConfiguration中的bean，然后加载在注解中配置的configuration，最后加载配置文件中的。
如果不是默认加载顺序，则首先加载注解中配置的configuration，然后加载配置文件中的配置，最后加载FeignClientsConfiguration中的bean。注意，顺序在后面的配置会覆盖掉前面的

创建client实例

配置文件加载完之后，就是最关键的一步，创建实例.因为两种方式都是通过获取 Targeter 来生成动态代理类。这里拿出了负载均衡做例子。

 1 protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
 2                             HardCodedTarget<T> target) {
 3     Client client = getOptional(context, Client.class);
 4     if (client != null) {
 5         builder.client(client);
 6         Targeter targeter = get(context, Targeter.class);
 7         return targeter.target(this, builder, context, target);
 8     }
 9 
10     throw new IllegalStateException(
11         "No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-netflix-ribbon?");
12 }

在 FeignAutoConfiguration 里面，配置了Target，可以看出这里配置了两种相斥的bean。

 1 @Configuration
 2 @ConditionalOnClass(name = "feign.hystrix.HystrixFeign")
 3 protected static class HystrixFeignTargeterConfiguration {
 4     @Bean
 5     @ConditionalOnMissingBean
 6     public Targeter feignTargeter() {
 7         return new HystrixTargeter();
 8     }
 9 }
10 
11 @Configuration
12 @ConditionalOnMissingClass("feign.hystrix.HystrixFeign")
13 protected static class DefaultFeignTargeterConfiguration {
14     @Bean
15     @ConditionalOnMissingBean
16     public Targeter feignTargeter() {
17         return new DefaultTargeter();
18     }
19 }

如果 feign.hystrix.HystrixFeign`路径不存在，则直接用 FeignBuidler 中DefaultTargeter的 target 方法生成代理。

 1 class HystrixTargeter implements Targeter {
 2 
 3     @Override
 4     public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,Target.HardCodedTarget<T> target) {
 5         if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {
 6             return feign.target(target);
 7         }
 8         feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;
 9         SetterFactory setterFactory = getOptional(factory.getName(), context,
10                                                   SetterFactory.class);
11         if (setterFactory != null) {
12             builder.setterFactory(setterFactory);
13         }
14         Class<?> fallback = factory.getFallback();
15         if (fallback != void.class) {
16             return targetWithFallback(factory.getName(), context, target, builder, fallback);
17         }
18         Class<?> fallbackFactory = factory.getFallbackFactory();
19         if (fallbackFactory != void.class) {
20             return targetWithFallbackFactory(factory.getName(), context, target, builder, fallbackFactory);
21         }
22 
23         return feign.target(target);
24     }
25 }

1 class DefaultTargeter implements Targeter {
2 
3     @Override
4     public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,Target.HardCodedTarget<T> target) {
5         return feign.target(target);
6     }
7 }

到这里spring对于创建client实例工作基本完成。接下来主要步骤在feign中。

Feign是怎么工作的

构建接口动态代理

这里会直接调用Feign的target方法：

 1 public <T> T target(Target<T> target) {
 2     return build().newInstance(target);
 3 }
 4 
 5 private InvocationHandlerFactory invocationHandlerFactory =new InvocationHandlerFactory.Default();
 6 
 7 private QueryMapEncoder queryMapEncoder = new QueryMapEncoder.Default();
 8 
 9 public Feign build() {
10     SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
11         new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
12                                              logLevel, decode404, closeAfterDecode);
13     //handlersByName将所有参数进行封装，并提供解析接口方法的逻辑
14     ParseHandlersByName handlersByName =
15         new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,
16                                 errorDecoder, synchronousMethodHandlerFactory);
17     return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
18 }

ReflectiveFeign构造函数有三个参数：

ParseHandlersByName 将builder所有参数进行封装，并提供解析接口方法的逻辑
InvocationHandlerFactory 默认值是InvocationHandlerFactory.Default,通过java动态代理的InvocationHandler实现
QueryMapEncoder 接口参数注解@QueryMap时，参数的编码器，默认值QueryMapEncoder.Default

ReflectiveFeign 生成动态代理对象。

ReflectiveFeign#newInstance

 1 public <T> T newInstance(Target<T> target) {
 2     //为每个方法创建一个SynchronousMethodHandler对象，并放在 Map 里面。
 3     //targetToHandlersByName是构造器传入的ParseHandlersByName对象，根据target对象生成MethodHandler映射
 4     Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
 5     Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
 6     List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();
 7     //遍历接口所有方法，构建Method->MethodHandler的映射
 8     for (Method method : target.type().getMethods()) {
 9       if (method.getDeclaringClass() == Object.class) {
10         continue;
11       } else if(Util.isDefault(method)) {
12         //如果是 default 方法，说明已经有实现了，用 DefaultHandler接口default方法的Handler
13         DefaultMethodHandler handler = new DefaultMethodHandler(method);
14         defaultMethodHandlers.add(handler);
15         methodToHandler.put(method, handler);
16       } else {
17         //否则就用上面的 SynchronousMethodHandler
18         methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
19       }
20     }
21     // 创建动态代理，factory 是 InvocationHandlerFactory.Default，创建出来的是 ReflectiveFeign.FeignInvocationHanlder，也就是说后续对方法的调用都会进入到该对象的 inovke 方法。
22     InvocationHandler handler = factory.create(target, methodToHandler);
23     // 创建动态代理对象
24     T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(), new Class<?>[]{target.type()}, handler);
25     //将default方法直接绑定到动态代理上
26     for(DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
27       defaultMethodHandler.bindTo(proxy);
28     }
29     return proxy;
30   }

这段代码主要的逻辑是：

创建MethodHandler的映射，这里创建的是实现类SynchronousMethodHandler
通过InvocationHandlerFatory创建InvocationHandler
绑定接口的default方法，通过DefaultMethodHandler绑定

SynchronousMethodHandler和DefaultMethodHandler实现了InvocationHandlerFactory.MethodHandler接口，动态代理对象调用方法时，如果是default方法，会直接调用接口方法，因为这里将接口的default方法绑定到动态代理对象上了，其他方法根据方法签名找到SynchronousMethodHandler对象，调用其invoke方法。

创建MethodHandler方法处理器

ReflectiveFeign#apply

 1 public Map<String, MethodHandler> apply(Target key) {
 2     //通过contract解析接口方法，生成MethodMetadata列表，默认的contract解析Feign自定义的http注解
 3     List<MethodMetadata> metadata =contract.parseAndValidatateMetadata(key.type());
 4     Map<String, MethodHandler> result = new LinkedHashMap<String, MethodHandler>();
 5     for (MethodMetadata md : metadata) {
 6         //根据目标接口类和方法上的注解信息判断该用哪种 buildTemplate
 7         //BuildTemplateByResolvingArgs实现RequestTemplate.Factory，RequestTemplate的工厂
 8         BuildTemplateByResolvingArgs buildTemplate;
 9         if (!md.formParams().isEmpty() && md.template().bodyTemplate() == null) {
10             //如果有formParam，并且bodyTemplate不为空，请求体为x-www-form-urlencoded格式
11             //将会解析form参数，填充到bodyTemplate中
12             buildTemplate = new BuildFormEncodedTemplateFromArgs(md, encoder);
13         } else if (md.bodyIndex() != null) {
14             //如果包含请求体，将会用encoder编码请求体对象
15             buildTemplate = new BuildEncodedTemplateFromArgs(md, encoder);
16         } else {
17             //默认的RequestTemplate的工厂，没有请求体，不需要编码器
18             buildTemplate = new BuildTemplateByResolvingArgs(md);
19         }
20         //使用工厂SynchronousMethodHandler.Factory创建SynchronousMethodHandler
21         result.put(md.configKey(),
22                    factory.create(key, md, buildTemplate, options, decoder, errorDecoder));
23     }
24     return result;
25 }

这段代码的逻辑是：

通过Contract解析接口方法，生成MethodMetadata，默认的Contract解析Feign自定义的http注解
根据MethodMetadata方法元数据生成特定的RequestTemplate的工厂
使用SynchronousMethodHandler.Factory工厂创建SynchronousMethodHandler，这里有两个工厂不要搞混淆了，SynchronousMethodHandler工厂和RequestTemplate工厂，SynchronousMethodHandler的属性包含RequestTemplate工厂

SynchronousMethodHandlerFactory#create

1 public MethodHandler create(Target<?> target, MethodMetadata md,
2                             RequestTemplate.Factory buildTemplateFromArgs,
3                             Options options, Decoder decoder, ErrorDecoder errorDecoder) {
4     //buildTemplateFromArgs--RequestTemplate Facoory
5     return new SynchronousMethodHandler(target, client, retryer, requestInterceptors, logger,
6                                         logLevel, md, buildTemplateFromArgs, options, decoder,
7                                         errorDecoder, decode404);
8 }

Contract解析接口方法生成MethodMetadata

这里有两个Contract，一个是feign默认的Contract，另一个是Spring实现的Contract。

Feign.Contract-Contract.Default

feign默认的解析器是Contract.Default继承了Contract.BaseContract，解析生成MethodMetadata方法入口：

 1 @Override
 2 public List<MethodMetadata> parseAndValidatateMetadata(Class<?> targetType) {
 3 
 4     Map<String, MethodMetadata> result = new LinkedHashMap<String, MethodMetadata>();
 5     for (Method method : targetType.getMethods()) {
 6         //...
 7         MethodMetadata metadata = parseAndValidateMetadata(targetType, method);
 8         //...
 9         result.put(metadata.configKey(), metadata);
10     }
11     return new ArrayList<MethodMetadata>(result.values());
12 }
13 
14 protected MethodMetadata parseAndValidateMetadata(Class<?> targetType, Method method) {
15     MethodMetadata data = new MethodMetadata();
16     data.returnType(Types.resolve(targetType, targetType, method.getGenericReturnType()));
17     data.configKey(Feign.configKey(targetType, method));
18 
19     if(targetType.getInterfaces().length == 1) {
20         processAnnotationOnClass(data, targetType.getInterfaces()[0]);
21     }
22     //处理Class上的注解
23     processAnnotationOnClass(data, targetType);
24 
25     for (Annotation methodAnnotation : method.getAnnotations()) {
26         //处理方法注解
27         processAnnotationOnMethod(data, methodAnnotation, method);
28     }
29     //...
30     Class<?>[] parameterTypes = method.getParameterTypes();
31     Type[] genericParameterTypes = method.getGenericParameterTypes();
32     
33     Annotation[][] parameterAnnotations = method.getParameterAnnotations();
34     int count = parameterAnnotations.length;
35     for (int i = 0; i < count; i++) {
36         boolean isHttpAnnotation = false;
37         if (parameterAnnotations[i] != null) {
38             //方法参数注解
39             isHttpAnnotation = processAnnotationsOnParameter(data, parameterAnnotations[i], i);
40         }
41         if (parameterTypes[i] == URI.class) {
42             //参数类型是URI，后面构造http请求时，使用该URI
43             data.urlIndex(i);
44         } else if (!isHttpAnnotation) {
45             //如果没有被http注解，就是body参数
46             //...
47             data.bodyIndex(i);
48             data.bodyType(Types.resolve(targetType, targetType, genericParameterTypes[i]));
49         }
50     }
51 
52     if (data.headerMapIndex() != null) {
53         //@HeaderMap注解的参数必须是Map，key类型必须是String
54         checkMapString("HeaderMap", parameterTypes[data.headerMapIndex()], genericParameterTypes[data.headerMapIndex()]);
55     }
56 
57     if (data.queryMapIndex() != null) {
58         if (Map.class.isAssignableFrom(parameterTypes[data.queryMapIndex()])) {
59             //@QueryMap注解的参数如果是Map，key类型必须是String
60             checkMapKeys("QueryMap", genericParameterTypes[data.queryMapIndex()]);
61         }
62     }
63     return data;
64 }

1.处理Class上的注解

这里主要处理@Headers

 1 //处理@Headers注解
 2 protected void processAnnotationOnClass(MethodMetadata data, Class<?> targetType) {
 3     if (targetType.isAnnotationPresent(Headers.class)) {
 4         //被Headers注解
 5         String[] headersOnType = targetType.getAnnotation(Headers.class).value();
 6         //...
 7         //header解析成map，加到MethodMetadata中
 8         Map<String, Collection<String>> headers = toMap(headersOnType);
 9         headers.putAll(data.template().headers());
10         data.template().headers(null); // to clear
11         data.template().headers(headers);
12     }
13 }

2.处理方法上的注解

这里主要处理@RequestLine、@Body、@Headers

 1 //处理方法注解
 2 protected void processAnnotationOnMethod(MethodMetadata data, Annotation methodAnnotation,
 3                                          Method method) {
 4     Class<? extends Annotation> annotationType = methodAnnotation.annotationType();
 5     if (annotationType == RequestLine.class) {
 6         //@RequestLine注解
 7         String requestLine = RequestLine.class.cast(methodAnnotation).value();
 8         //...
 9         if (requestLine.indexOf(' ') == -1) {
10             //...
11             data.template().method(requestLine);
12             return;
13         }
14         //http请求方法
15         data.template().method(requestLine.substring(0, requestLine.indexOf(' ')));
16         if (requestLine.indexOf(' ') == requestLine.lastIndexOf(' ')) {
17             // no HTTP version is ok
18             data.template().append(requestLine.substring(requestLine.indexOf(' ') + 1));
19         } else {
20             // skip HTTP version
21             data.template().append(
22                 requestLine.substring(requestLine.indexOf(' ') + 1, requestLine.lastIndexOf(' ')));
23         }
24         //将'%2F'反转为'/'
25         data.template().decodeSlash(RequestLine.class.cast(methodAnnotation).decodeSlash());
26         //参数集合格式化方式，默认使用key=value0&key=value1
27         data.template().collectionFormat(RequestLine.class.cast(methodAnnotation).collectionFormat());
28 
29     } else if (annotationType == Body.class) {
30         //@Body注解
31         String body = Body.class.cast(methodAnnotation).value();
32         //...
33         if (body.indexOf('{') == -1) {
34             //body中不存在{，直接传入body
35             data.template().body(body);
36         } else {
37             //body中存在{，就是bodyTemplate方式
38             data.template().bodyTemplate(body);
39         }
40     } else if (annotationType == Headers.class) {
41         //@Header注解
42         String[] headersOnMethod = Headers.class.cast(methodAnnotation).value();
43         //...
44         data.template().headers(toMap(headersOnMethod));
45     }
46 }

3.处理参数上的注解

这里主要处理@Param、@QueryMap、@HeaderMap，只要有这三个注解，则isHttpAnnotation=true。

 1 //处理参数上的注解
 2 protected boolean processAnnotationsOnParameter(MethodMetadata data, Annotation[] annotations, int paramIndex) {
 3     boolean isHttpAnnotation = false;
 4     for (Annotation annotation : annotations) {
 5         Class<? extends Annotation> annotationType = annotation.annotationType();
 6         if (annotationType == Param.class) {
 7             //@Param注解
 8             Param paramAnnotation = (Param) annotation;
 9             String name = paramAnnotation.value();
10             //...
11             //增加到MethodMetadata中
12             nameParam(data, name, paramIndex);
13             //@Param注解的expander参数，定义参数的解释器，默认是ToStringExpander，调用参数的toString方法
14             Class<? extends Param.Expander> expander = paramAnnotation.expander();
15             if (expander != Param.ToStringExpander.class) {
16                 data.indexToExpanderClass().put(paramIndex, expander);
17             }
18             //参数是否已经urlEncoded，如果没有，会使用urlEncoded方式编码
19             data.indexToEncoded().put(paramIndex, paramAnnotation.encoded());
20             isHttpAnnotation = true;
21             String varName = '{' + name + '}';
22             if (!data.template().url().contains(varName) &&
23                 !searchMapValuesContainsSubstring(data.template().queries(), varName) &&
24                 !searchMapValuesContainsSubstring(data.template().headers(), varName)) {
25                 //如果参数不在path里面，不在query里面，不在header里面，就设置到formParam中
26                 data.formParams().add(name);
27             }
28         } else if (annotationType == QueryMap.class) {
29             //@QueryMap注解，注解参数对象时，将该参数转换为http请求参数格式发送
30             //...
31             data.queryMapIndex(paramIndex);
32             data.queryMapEncoded(QueryMap.class.cast(annotation).encoded());
33             isHttpAnnotation = true;
34         } else if (annotationType == HeaderMap.class) {
35             //@HeaderMap注解，注解一个Map类型的参数，放入http header中发送
36             //...
37             data.headerMapIndex(paramIndex);
38             isHttpAnnotation = true;
39         }
40     }
41     return isHttpAnnotation;
42 }

Spring.Contract-SpringMvcContract

feign默认的解析器是SpringMvcContract继承了Contract.BaseContract，解析生成MethodMetadata方法入口:

 1 @Override
 2 public MethodMetadata parseAndValidateMetadata(Class<?> targetType, Method method) {
 3     this.processedMethods.put(Feign.configKey(targetType, method), method);
 4     MethodMetadata md = super.parseAndValidateMetadata(targetType, method);
 5 
 6     RequestMapping classAnnotation = findMergedAnnotation(targetType,
 7                                                           RequestMapping.class);
 8     if (classAnnotation != null) {
 9         // produces - use from class annotation only if method has not specified this
10         // 如果Accept为空，则设置@RequestMapping的produces
11         if (!md.template().headers().containsKey(ACCEPT)) {
12             parseProduces(md, method, classAnnotation);
13         }
14 
15         // consumes -- use from class annotation only if method has not specified this
16         // 如果Content-Type为空，则设置@RequestMapping的consumes
17         if (!md.template().headers().containsKey(CONTENT_TYPE)) {
18             parseConsumes(md, method, classAnnotation);
19         }
20 
21         // headers -- class annotation is inherited to methods, always write these if
22         // present 设置heerders
23         parseHeaders(md, method, classAnnotation);
24     }
25     return md;
26 }

这里parseAndValidateMetadata调用了父类即BaseContract的方法，如上，也会分为3部来处理，这里只分析有区别的部分。解析完成之后，会根据在class上注解的@RequestMapping的属性值来设置到MethodMetadata中。

1.处理Class上的注解

这里主要处理@RequestMapping

 1 protected void processAnnotationOnClass(MethodMetadata data, Class<?> clz) {
 2     if (clz.getInterfaces().length == 0) {
 3         RequestMapping classAnnotation = findMergedAnnotation(clz,
 4                                                               RequestMapping.class);
 5         if (classAnnotation != null) {
 6             // Prepend path from class annotation if specified
 7             if (classAnnotation.value().length > 0) {
 8                 String pathValue = emptyToNull(classAnnotation.value()[0]);
 9                 pathValue = resolve(pathValue);
10                 if (!pathValue.startsWith("/")) {
11                     pathValue = "/" + pathValue;
12                 }
13                 //插入url
14                 data.template().insert(0, pathValue);
15             }
16         }
17     }
18 }

2.处理方法上的注解

这里主要处理@RequestMapping，和处理class时差不多

 1 protected void processAnnotationOnMethod(MethodMetadata data,
 2                                          Annotation methodAnnotation, Method method) {
 3     if (!RequestMapping.class.isInstance(methodAnnotation) && !methodAnnotation
 4         .annotationType().isAnnotationPresent(RequestMapping.class)) {
 5         return;
 6     }
 7 
 8     RequestMapping methodMapping = findMergedAnnotation(method, RequestMapping.class);
 9     // 设置HTTP Method
10     RequestMethod[] methods = methodMapping.method();
11     if (methods.length == 0) {
12         methods = new RequestMethod[] { RequestMethod.GET };
13     }
14     checkOne(method, methods, "method");
15     data.template().method(methods[0].name());
16 
17     // path
18     checkAtMostOne(method, methodMapping.value(), "value");
19     if (methodMapping.value().length > 0) {
20         String pathValue = emptyToNull(methodMapping.value()[0]);
21         if (pathValue != null) {
22             pathValue = resolve(pathValue);
23             // Append path from @RequestMapping if value is present on method
24             if (!pathValue.startsWith("/")
25                 && !data.template().toString().endsWith("/")) {
26                 pathValue = "/" + pathValue;
27             }
28             data.template().append(pathValue);
29         }
30     }
31 
32     // produces
33     parseProduces(data, method, methodMapping);
34 
35     // consumes
36     parseConsumes(data, method, methodMapping);
37 
38     // headers
39     parseHeaders(data, method, methodMapping);
40 
41     data.indexToExpander(new LinkedHashMap<Integer, Param.Expander>());
42 }

3.处理参数上的注解

这里主要处理@Param、@QueryMap、@HeaderMap

 1 protected boolean processAnnotationsOnParameter(MethodMetadata data,
 2                                                 Annotation[] annotations, int paramIndex) {
 3     boolean isHttpAnnotation = false;
 4 
 5     AnnotatedParameterProcessor.AnnotatedParameterContext context = new SimpleAnnotatedParameterContext(
 6         data, paramIndex);
 7     Method method = this.processedMethods.get(data.configKey());
 8     for (Annotation parameterAnnotation : annotations) {
 9         AnnotatedParameterProcessor processor = this.annotatedArgumentProcessors
10             .get(parameterAnnotation.annotationType());
11         if (processor != null) {
12             Annotation processParameterAnnotation;
13             // synthesize, handling @AliasFor, while falling back to parameter name on
14             // missing String #value():
15             processParameterAnnotation = synthesizeWithMethodParameterNameAsFallbackValue(
16                 parameterAnnotation, method, paramIndex);
17             //调用不同策略处理不同的注解
18             isHttpAnnotation |= processor.processArgument(context,
19                                                           processParameterAnnotation, method);
20         }
21     }
22     if (isHttpAnnotation && data.indexToExpander().get(paramIndex) == null
23         && this.conversionService.canConvert(
24             method.getParameterTypes()[paramIndex], String.class)) {
25         data.indexToExpander().put(paramIndex, this.expander);
26     }
27     return isHttpAnnotation;
28 }

这里spring提供了三种参数上的注解解析

RequestHeaderParameterProcessor：处理@RequestHeader
PathVariableParameterProcessor：处理@PathVariable
RequestParamParameterProcessor：处理@RequestParam

代码稍微有点多，但是逻辑很清晰，先处理类上的注解，再处理方法上注解，最后处理方法参数注解，把所有注解的情况都处理到就可以了。

生成的MethodMetadata的结构如下：

 1 public final class MethodMetadata implements Serializable {
 2     //标识方法的key，接口名加方法签名：GitHub#contributors(String,String)
 3     private String configKey;
 4     //方法返回值类型
 5     private transient Type returnType;
 6     //uri参数的位置，方法中可以写个uri参数，发请求时直接使用这个参数
 7     private Integer urlIndex;
 8     //body参数的位置，只能有一个注解的参数为body，否则报错
 9     private Integer bodyIndex;
10     //headerMap参数的位置
11     private Integer headerMapIndex;
12     //@QueryMap注解参数位置
13     private Integer queryMapIndex;
14     //@QueryMap注解里面encode参数，是否已经urlEncode编码过了
15     private boolean queryMapEncoded;
16     //body的类型
17     private transient Type bodyType;
18     //RequestTemplate
19     private RequestTemplate template = new RequestTemplate();
20     //form请求参数
21     private List<String> formParams = new ArrayList<String>();
22     //方法参数位置和名称的map
23     private Map<Integer, Collection<String>> indexToName ;
24     //@Param中注解的expander方法，可以指定解析参数类
25     private Map<Integer, Class<? extends Expander>> indexToExpanderClass ;
26     //参数是否被urlEncode编码过了，@Param中encoded方法
27     private Map<Integer, Boolean> indexToEncoded ;
28     //自定义的Expander
29     private transient Map<Integer, Expander> indexToExpander;

调用

ReflectiveFeign.FeignInvocationHanlder 的 invoke 方法

 1 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
 2     //通过动态代理实现了几个通用方法，比如 equals、toString、hasCode
 3     if ("equals".equals(method.getName())) {
 4         try {
 5             Object
 6                 otherHandler =
 7                 args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
 8             return equals(otherHandler);
 9         } catch (IllegalArgumentException e) {
10             return false;
11         }
12     } else if ("hashCode".equals(method.getName())) {
13         return hashCode();
14     } else if ("toString".equals(method.getName())) {
15         return toString();
16     }
17     //找到具体的 method 的 Handler，然后调用 invoke 方法。这样就又进入了SynchronousMethodHandler对象的 invoke 方法。
18     return dispatch.get(method).invoke(args);
19 }

SynchronousMethodHandler 的 invoke 方法主要是应用 encoder，decoder 以及 retry 等配置，并执行http请求及返回结果的处理

 1 public Object invoke(Object[] argv) throws Throwable {
 2     RequestTemplate template = buildTemplateFromArgs.create(argv);
 3     Retryer retryer = this.retryer.clone();
 4     while (true) {
 5         try {
 6             return executeAndDecode(template);
 7         } catch (RetryableException e) {
 8             retryer.continueOrPropagate(e);
 9             if (logLevel != Logger.Level.NONE) {
10                 logger.logRetry(metadata.configKey(), logLevel);
11             }
12             continue;
13         }
14     }
15 }
16 
17 Object executeAndDecode(RequestTemplate template) throws Throwable {
18     //通过RequestTemplate生成Request,这里会首先执行RequestInterceptors
19     Request request = targetRequest(template);
20 
21     if (logLevel != Logger.Level.NONE) {
22         logger.logRequest(metadata.configKey(), logLevel, request);
23     }
24 
25     Response response;
26     long start = System.nanoTime();
27     try {
28         //通过 client 获得请求的返回值
29         response = client.execute(request, options);
30         // ensure the request is set. TODO: remove in Feign 10
31         response.toBuilder().request(request).build();
32     } catch (IOException e) {
33         if (logLevel != Logger.Level.NONE) {
34             logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));
35         }
36         throw errorExecuting(request, e);
37     }
38     long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);
39 
40     boolean shouldClose = true;
41     try {
42         if (logLevel != Logger.Level.NONE) {
43             response =
44                 logger.logAndRebufferResponse(metadata.configKey(), logLevel, response, elapsedTime);
45             // ensure the request is set. TODO: remove in Feign 10
46             //将request设置到response中
47             response.toBuilder().request(request).build();
48         }
49         if (Response.class == metadata.returnType()) {
50             if (response.body() == null) {
51                 return response;
52             }
53             if (response.body().length() == null ||
54                 response.body().length() > MAX_RESPONSE_BUFFER_SIZE) {
55                 shouldClose = false;
56                 return response;
57             }
58             // Ensure the response body is disconnected
59             byte[] bodyData = Util.toByteArray(response.body().asInputStream());
60             return response.toBuilder().body(bodyData).build();
61         }
62         if (response.status() >= 200 && response.status() < 300) {
63             if (void.class == metadata.returnType()) {
64                 return null;
65             } else {
66                 //解码
67                 return decode(response);
68             }
69         } else if (decode404 && response.status() == 404 && void.class != metadata.returnType()) {
70             return decode(response);
71         } else {
72             throw errorDecoder.decode(metadata.configKey(), response);
73         }
74     } 
75     //...

Client也有两种，一种直连，一种负载均衡，这里主要分析直连，负载均衡最后也要调用这里。

Client.Default#execute

调用HttpURLConnection进行http请求

  1 public Response execute(Request request, Options options) throws IOException {
  2     HttpURLConnection connection = convertAndSend(request, options);
  3     return convertResponse(connection).toBuilder().request(request).build();
  4 }
  5 
  6 HttpURLConnection convertAndSend(Request request, Options options) throws IOException {
  7     final HttpURLConnection
  8         connection =
  9         (HttpURLConnection) new URL(request.url()).openConnection();
 10     if (connection instanceof HttpsURLConnection) {
 11         HttpsURLConnection sslCon = (HttpsURLConnection) connection;
 12         if (sslContextFactory != null) {
 13             sslCon.setSSLSocketFactory(sslContextFactory);
 14         }
 15         if (hostnameVerifier != null) {
 16             sslCon.setHostnameVerifier(hostnameVerifier);
 17         }
 18     }
 19     connection.setConnectTimeout(options.connectTimeoutMillis());
 20     connection.setReadTimeout(options.readTimeoutMillis());
 21     connection.setAllowUserInteraction(false);
 22     connection.setInstanceFollowRedirects(options.isFollowRedirects());
 23     connection.setRequestMethod(request.method());
 24 
 25     Collection<String> contentEncodingValues = request.headers().get(CONTENT_ENCODING);
 26     boolean
 27         gzipEncodedRequest =
 28         contentEncodingValues != null && contentEncodingValues.contains(ENCODING_GZIP);
 29     boolean
 30         deflateEncodedRequest =
 31         contentEncodingValues != null && contentEncodingValues.contains(ENCODING_DEFLATE);
 32 
 33     boolean hasAcceptHeader = false;
 34     Integer contentLength = null;
 35     for (String field : request.headers().keySet()) {
 36         if (field.equalsIgnoreCase("Accept")) {
 37             hasAcceptHeader = true;
 38         }
 39         for (String value : request.headers().get(field)) {
 40             if (field.equals(CONTENT_LENGTH)) {
 41                 if (!gzipEncodedRequest && !deflateEncodedRequest) {
 42                     contentLength = Integer.valueOf(value);
 43                     connection.addRequestProperty(field, value);
 44                 }
 45             } else {
 46                 connection.addRequestProperty(field, value);
 47             }
 48         }
 49     }
 50     // Some servers choke on the default accept string.
 51     if (!hasAcceptHeader) {
 52         connection.addRequestProperty("Accept", "*/*");
 53     }
 54 
 55     if (request.body() != null) {
 56         if (contentLength != null) {
 57             connection.setFixedLengthStreamingMode(contentLength);
 58         } else {
 59             connection.setChunkedStreamingMode(8196);
 60         }
 61         connection.setDoOutput(true);
 62         OutputStream out = connection.getOutputStream();
 63         if (gzipEncodedRequest) {
 64             out = new GZIPOutputStream(out);
 65         } else if (deflateEncodedRequest) {
 66             out = new DeflaterOutputStream(out);
 67         }
 68         try {
 69             out.write(request.body());
 70         } finally {
 71             try {
 72                 out.close();
 73             } catch (IOException suppressed) { // NOPMD
 74             }
 75         }
 76     }
 77     return connection;
 78 }
 79 
 80 Response convertResponse(HttpURLConnection connection) throws IOException {
 81     int status = connection.getResponseCode();
 82     String reason = connection.getResponseMessage();
 83 
 84     if (status < 0) {
 85         throw new IOException(format("Invalid status(%s) executing %s %s", status,
 86                                      connection.getRequestMethod(), connection.getURL()));
 87     }
 88 
 89     Map<String, Collection<String>> headers = new LinkedHashMap<String, Collection<String>>();
 90     for (Map.Entry<String, List<String>> field : connection.getHeaderFields().entrySet()) {
 91         // response message
 92         if (field.getKey() != null) {
 93             headers.put(field.getKey(), field.getValue());
 94         }
 95     }
 96 
 97     Integer length = connection.getContentLength();
 98     if (length == -1) {
 99         length = null;
100     }
101     InputStream stream;
102     if (status >= 400) {
103         stream = connection.getErrorStream();
104     } else {
105         stream = connection.getInputStream();
106     }
107     return Response.builder()
108         .status(status)
109         .reason(reason)
110         .headers(headers)
111         .body(stream, length)
112         .build();
113 }

最后放一张读源码时画的脑图。

转载：https://blog.csdn.net/sinat_29899265/article/details/86577997

带着疑问去思考，然后串联，进而归纳总结，不断追问自己，进行自我辩证，像侦查嫌疑案件一样看待技术问题，漆黑的街道，你我一起寻找线索，你就是技术界大侦探福尔摩斯

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原文地址：https://www.cnblogs.com/cainiao-Shun666/p/14620184.html