classloader 是负责载入类的对象。给出一个二进制的name，classloader就会根据name生产或载入组成一个类结构的数据。一种常见的策略是，把name转译成一个文件的名字，然后通过文件系统去读取这个类文件。
每个class 对象有一个getClassloader文法获得载入自己的class loader。数组对象不会由class loader载入，它会在运行时自动载入。对于数组元素是基本类型的，通过getClassloader返回的是null，其它自定义类型返回的时自定义对象的classloader
classloader使用委托模式搜索classes 和 resources。当需要调用一个class或resource时，class loader会首先靠自己去搜索它，如果没有找到，会在委托父class loader搜索。jvm 内建的class loader叫bootstrap class loader，它没有父class loader，它作为其它class loader的父class loader
测试：
package com.peaceful.jdk.demo;
import com.peaceful.common.util.Util;
import sun.reflect.Reflection;
import java.lang.ClassLoader;
/**
* Created by wangjun on 15/2/21.
*/
public class ClassLoaderTest {
public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
Util.report(Reflection.getCallerClass());
/** 载入ClassLoaderTest的classloader **/
Util.report("载入ClassLoaderTest的classloader->" + ClassLoaderTest.class.getClassLoader());
/** appache 提供的classloader */
com.sun.org.apache.bcel.internal.util.ClassLoader classLoader = new com.sun.org.apache.bcel.internal.util.ClassLoader(java.lang.ClassLoader.getSystemClassLoader());
Util.report("appache 提供的classloader->" + classLoader);
Class ClassLoaderTestClass = Class.forName("com.peaceful.jdk.demo.ClassLoaderTest", false, classLoader);
Util.report("classloader ? ->" + ClassLoaderTestClass.getClassLoader());
Util.report("apache class loader parent ->" + ClassLoaderTestClass.getClassLoader().getParent());
/** java.lang.ClassCastException: com.peaceful.jdk.demo.ClassLoaderTest cannot be cast to com.peaceful.jdk.demo.ClassLoaderTest **/
// ClassLoaderTest ClassLoaderTest = (ClassLoaderTest) ClassLoaderTestClass.newInstance();
/** 不同的classload 载入同一个类不能进行比较和转换 */
Util.report("? ->" + ClassLoaderTestClass.isInstance(ClassLoaderTest.class));
/** 通过指定classloader 载入指定类 **/
classLoader.loadClass("com.peaceful.jdk.demo.ClassLoaderTest");
/** 系统启动时设定的classloader **/
Util.report(ClassLoader.getSystemClassLoader());
int[] arr = {1, 2, 3, 4, 5};
ClassLoaderTest[] arrClassLoaderTest = {};
/**<p> <tt>Class</tt> objects for array classes are not created by class
* loaders, but are created automatically as required by the Java runtime.
* The class loader for an array class, as returned by {@link
* Class#getClassLoader()} is the same as the class loader for its element
* type; if the element type is a primitive type, then the array class has no
* class loader.
* */
Util.report(arr.getClass().getClassLoader());
Util.report(arrClassLoaderTest.getClass().getClassLoader());
/** 父classloader **/
Util.report(ClassLoaderTest.class.getClassLoader().getParent());
/** 父classloader的classloader **/
Util.report(ClassLoaderTest.class.getClassLoader().getParent().getParent());
Util.report(T2.class.getClassLoader());
Util.report(System.currentTimeMillis());
Util.report(System.nanoTime());
}
/**
* 输出结果
*
P_LOG: class com.intellij.rt.execution.application.AppMain
P_LOG: 载入ClassLoaderTest的classloader->[email protected]
P_LOG: appache 提供的classloader->[email protected]
P_LOG: classloader ? ->[email protected]
P_LOG: apache class loader parent ->[email protected]
P_LOG: ? ->false
P_LOG: [email protected]
P_LOG: null
P_LOG: [email protected]
P_LOG: [email protected]
P_LOG: null
P_LOG: [email protected]
P_LOG: 1424681861529
P_LOG: 1424681861530021000
*
*/
class T2 {
}
}