13- different class loaders loading operation and effect analysis

Different class loaders loading operation and effect analysis

• Java Virtual Machine that comes with the loader (loop through output!)

  public class MyTest13 {
      public static void main(String[] args) throws Exception {
          ClassLoader loader = ClassLoader.getSystemClassLoader();
          System.out.println(loader);
          while (null!=loader){
              loader = loader.getParent();
              System.out.println(loader);
          }
      }
  }
  运行结果：
        sun.misc.Launcher$AppClassLoader@18b4aac2
        sun.misc.Launcher$ExtClassLoader@6d6f6e28
        null
  

• Example one:

  • Gets the full path to the corresponding class file.
  • Mainly for Jar hell problems and solutions:
    • When multiple jar a class or a resource file exists, it will exist jar hell problem.
    • You can diagnose a problem by the following code:

  public class MyTest14 {
      public static void main(String[] args) throws Exception {
          ClassLoader loader = Thread.currentThread().getContextClassLoader();
          String resoureName = "Jvm/MyTest14.class";
          Enumeration<URL> urls = loader.getResources(resoureName);
          while (urls.hasMoreElements()){
              URL url = urls.nextElement();
              System.out.println(url);
          }
      }
  }
  运行结果：
  file:/E:/Program%20Files%20(x86)/IdeaProject/Javalearn/.../out/production/IdeaProject/Jvm/MyTest14.class
  

• Ways to get the ClassLoader

  //获取当前类的ClassLoaer
  clazz.getClassLoader();
  //获取当前线程上下文的ClassLoader
  Thread.currentThread().getContextClassLoader();
  //获得系统的ClassLoader
  ClassLoader.getSystemClassLoader();
  //获取调用者的ClassLoader
  DriverManager.getCallerClassLoader();
  

• Example 2:

  • MyTest14 custom class, the class loader loads the system; String type is located under rt.jar, the loaded class is loaded from the root

    public class MyTest14 {
        public static void main(String[] args) throws Exception {
    		  Class<?> clazz1 = MyTest14.class;
            System.out.println(clazz1.getClassLoader());
            Class<?> clazz2 = String.class;
            System.out.println(clazz2.getClassLoader());
        }
    }
    运行结果：
       sun.misc.Launcher$AppClassLoader@18b4aac2
       null