Mybaits源码分析一之加载输入流

对mybatis源码进行一次分析，特此记录

根据mybatis框架和mybatis案例分析

第一步：会加载配置文件输入流

 String resource="mybatis-config.xml";
 InputStream inputStream=Resources.getResourceAsStream(resource);

   Resources.getResourceAsStream(resource)方法源码为:

 /*
   * Returns a resource on the classpath as a Stream object
   *
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static InputStream getResourceAsStream(String resource) throws IOException {
    return getResourceAsStream(null, resource);
  }

  /*
   * Returns a resource on the classpath as a Stream object
   *
   * @param loader   The classloader used to fetch the resource
   * @param resource The resource to find
   * @return The resource
   * @throws java.io.IOException If the resource cannot be found or read
   */
  public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
    InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
    if (in == null) {
      throw new IOException("Could not find resource " + resource);
    }
    return in;
  }

再通过这一行语句InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);获取输入流信息，其中类加载器包装类

  /*
   * Get a resource from the classpath, starting with a specific class loader
   *
   * @param resource    - the resource to find
   * @param classLoader - the first class loader to try
   * @return the stream or null
   */
  public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
    return getResourceAsStream(resource, getClassLoaders(classLoader));
  }

定义类加载器顺序

ClassLoader[] getClassLoaders(ClassLoader classLoader) {
    return new ClassLoader[]{
    //参数指定的类加载器
        classLoader,
   //系统指定的默认类加载器
        defaultClassLoader,
    //当前线程绑定的类加载器
        Thread.currentThread().getContextClassLoader(),
   //当前类使用的类加载器
        getClass().getClassLoader(),
   // 系统的类加载器
        systemClassLoader};
  }

 /*
   * Try to get a resource from a group of classloaders
   *
   * @param resource    - the resource to get
   * @param classLoader - the classloaders to examine
   * @return the resource or null
   */
  InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
//从指定的资源（resource资源的URL）  按顺序（ClassLoader[]中的顺序）加载，加载到开始返回
    for (ClassLoader cl : classLoader) {
      if (null != cl) {

        // try to find the resource as passed
        InputStream returnValue = cl.getResourceAsStream(resource);

        // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
        if (null == returnValue) {
          returnValue = cl.getResourceAsStream("/" + resource);
        }

        if (null != returnValue) {
          return returnValue;
        }
      }
    }
    return null;
  }