JVM（三、双亲委派机制1）

1.获取ClassLoader

 1 public class MyTest06 {
 2 
 3     public static void main(String[] args) {
 4         String str = new String("212");
 5         C c = new C();
 6         System.out.println(str.getClass().getClassLoader());
 7         System.out.println(c.getClass().getClassLoader());
 8     }
 9 
10 }
11 class C{
12     
13 }//out: null
sun.misc.Launcher$AppClassLoader@2a139a55

第二个返回值说明自定义类C是由AppClassLoader 系统类加载器加载

第一个返回值说明String由Bootstrap根加载器加载

/** 如下是源码中对 getClassLoader() 方法的说明
     * Returns the class loader for the class.  Some implementations may use
     * null to represent the bootstrap class loader. This method will return
     * null in such implementations if this class was loaded by the bootstrap
     * class loader.
     * 如果类由bootstrap class loader加载 这个方法会返回null在一些jvm实现中
     * 
     * <p> If a security manager is present, and the caller's class loader is
     * not null and the caller's class loader is not the same as or an ancestor of
     * the class loader for the class whose class loader is requested, then
     * this method calls the security manager's {@code checkPermission}
     * method with a {@code RuntimePermission("getClassLoader")}
     * permission to ensure it's ok to access the class loader for the class.
     *
     * <p>If this object
     * represents a primitive type or void, null is returned.
     *
     * @return  the class loader that loaded the class or interface
     *          represented by this object.
     * @throws SecurityException
     *    if a security manager exists and its
     *    {@code checkPermission} method denies
     *    access to the class loader for the class.
     * @see java.lang.ClassLoader
     * @see SecurityManager#checkPermission
     * @see java.lang.RuntimePermission
     */    

2.ClassLoader.loadClass()方法与Class.forName()方法

class CL{
	static {
		System.out.println("class cl");
	}
}
public class MyTest07 {

	public static void main(String[] args) throws Exception {
		ClassLoader clazzloader = ClassLoader.getSystemClassLoader();
		Class<?> clazz = clazzloader.loadClass("jvm.CL");
		System.out.println("==================================");
		System.out.println(clazz);
		System.out.println("==================================");
		Class<?> clazz2 = Class.forName("jvm.CL");
		System.out.println("==================================");
		System.out.println(clazz2);
	}

}//out:
==================================
class jvm.CL
==================================
class cl
==================================
class jvm.CL

loadclass 只加载对应类，forName 会一并将类初始化

不过调用 clazz.newInstance（）会初始化该类:

public class MyTest07 {

	public static void main(String[] args) throws Exception {
		ClassLoader clazzloader = ClassLoader.getSystemClassLoader();
		Class<?> clazz = clazzloader.loadClass("jvm.CL");
		System.out.println("==================================");
		System.out.println(clazz);
		System.out.println("==================================");
		clazz.newInstance();
		System.out.println("==================================");
		Class<?> clazz2 = Class.forName("jvm.CL");
		System.out.println("==================================");
		System.out.println(clazz2);
	}

}out:
==================================
class jvm.CL
==================================
class cl
==================================
==================================
class jvm.CL

　　

3.