本章讲Interceptor其实在分享怎样的设计才是具备良好的拓展性的程序，注重内功修为的童靴可能更感兴趣。Mybatis中的插件允许你针对核心组件接口Executor 、StatementHandler、ParameterHandler、ResultSetHandler中任何一个方法进行拦截调用。而每个Interceptor（拦截的接口）其实是通过JDK的动态代理技术生成的代理类，每当执行这4种接口中的方法时，就会进入拦截方法（具体就是InvocationHandler的invoke()方法）。
关于Mybatis的系统架构及Executor、ParameterHandler、ResultSetHandler、StatementHandler可参阅之前的相关的分享。
责任链模式
责任链模式是一种对象行为模式，插件用的是它，很多对象由每一个对象对其下家的引用而连接起来形成一条链，请求在这个链上传递，直到链上的某一个对象决定处理此请求。
用法
用法一点也不复杂：只需实现 Interceptor 接口，并指定想要拦截的方法签名即可，更多用法可参阅官方教程
除了用插件来修改 MyBatis 核心行为之外，还可以通过完全覆盖配置类来达到目的。只需继承后覆盖其中的每个方法，再把它传递到 SqlSessionFactoryBuilder.build(myConfig) 方法即可。再次重申，这可能会严重影响 MyBatis 的行为，务请慎之又慎。
<!-- mybatis-config.xml -->
<plugins>
<plugin interceptor="org.mybatis.example.ExamplePlugin">
<property name="someProperty" value="100"/>
</plugin>
</plugins>
//执行Executor.update时拦截
@Intercepts({@Signature(type= Executor.class, method = "update", args = {MappedStatement.class,Object.class})})
public class ExamplePlugin implements Interceptor {
public Object intercept(Invocation invocation) throws Throwable {//干你想干的事情
return invocation.proceed();
}
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
public void setProperties(Properties properties) {
}
}
源码
在上面的源码中Plugin.wrap()，它就是当前拦截器（ExamplePlugin）的代理类。而InterceptorChain.pluginAll()则定义了一个组件接口（比如：ParameterHandler）执行的责任链。
public interface Interceptor {
//拦截器具体实现
Object intercept(Invocation invocation) throws Throwable;
//拦截器的代理类
Object plugin(Object target);
//添加属性
void setProperties(Properties properties);
}
//源码Plugin.wrap其实是一个代理
public class Plugin implements InvocationHandler {
private Object target;
private Interceptor interceptor;//拦截器
private Map<Class<?>, Set<Method>> signatureMap;
//为拦截器生成代理类
public static Object wrap(Object target, Interceptor interceptor) {
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {//目标列的接口与拦截器匹配，则生成代理类
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;//原生类返回
}
//代理类执行入口
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {//执行插件类的拦截规则
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}
//处理拦截器类上的各种注解
private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
// issue #251
if (interceptsAnnotation == null) {
throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
}
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
try {
Method method = sig.type().getMethod(sig.method(), sig.args());
methods.add(method);
} catch (NoSuchMethodException e) {
throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
}
}
return signatureMap;
}
}
//拦截器的请求
public class Invocation {
private Object target;
private Method method;
private Object[] args;
//调用原先真实方法(非代理)
public Object proceed() throws InvocationTargetException, IllegalAccessException {
return method.invoke(target, args);
}
}
拦截器链执行过程
拦截器代理类对象->拦截器->目标方法，即：InvocationHandler.invoke()->Interceptor.intercept()->Invocation.proceed()。
也就是说其实在执行每个Executor、ParameterHandler、ResultSetHandler、StatementHandler都会构建一个拦截器链，如果针对每个配置了plugin，一个sql执行下来会陆续经过四个拦截器链。
//拦截器管理类
public class InterceptorChain {
private final List<Interceptor> interceptors = new ArrayList<Interceptor>();
//构建责任链
public Object pluginAll(Object target) {
for (Interceptor interceptor : interceptors) {//存在拦截器则构建
//有多个同类型的拦截器则构成链，将代理类作为target再成代理类
target = interceptor.plugin(target);
}
return target;//可以没有拦截器
}
public void addInterceptor(Interceptor interceptor) {
interceptors.add(interceptor);
}
public List<Interceptor> getInterceptors() {
return Collections.unmodifiableList(interceptors);
}
}
//重要的配置类
public class Configuration {
protected final InterceptorChain interceptorChain = new InterceptorChain();
//有匹配的拦截器则构建责任链，没有则不生成
public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
return parameterHandler;
}
//同newParameterHandler
public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
ResultHandler resultHandler, BoundSql boundSql) {
ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
return resultSetHandler;
}
//同newParameterHandler
public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
return statementHandler;
}
public Executor newExecutor(Transaction transaction) {
return newExecutor(transaction, defaultExecutorType);
}
//同newParameterHandler
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
}
//负责解析Mybatis 全局配置文件
public class XMLConfigBuilder extends BaseBuilder {
private void pluginElement(XNode parent) throws Exception {//解析interceptor
if (parent != null) {
for (XNode child : parent.getChildren()) {
String interceptor = child.getStringAttribute("interceptor");
Properties properties = child.getChildrenAsProperties();
Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).newInstance();
interceptorInstance.setProperties(properties);
configuration.addInterceptor(interceptorInstance);
}
}
}
}
public class SimpleExecutor extends BaseExecutor {
@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
Configuration configuration = ms.getConfiguration();
//生成责任链
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
return handler.<E>query(stmt, resultHandler);//可拦截query()
} finally {
closeStatement(stmt);
}
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());//可拦截prepare()
handler.parameterize(stmt);//可拦截parameterize(http://www.my516.com)
return stmt;
}
}
mybatis用拦截器的设计非常巧妙的利用了jdk的动态代理，其实复杂度并不高。
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