JAVA启动后，经过JVM各级ClassLoader来加载各个类到内存。

JVM的ClassLoader分三层，分别为Bootstrap ClassLoader，Extension ClassLoader，System ClassLoader。

Bootstrap ClassLoader是启动类加载器，它是用C++编写的，从%jre%/lib目录中加载类。

Extension ClassLoader是扩展类加载器，从%jre%/lib/ext目录加载类。

System ClassLoader，系统类加载器，它会从系统环境变量配置的classpath来查找路径。

情形一  重新jre/lib中的jar包

在windows系统上，我们可以安装多个JDK和JRE。设想一种情形： 我们设定当前JVM启动的是C:\Program Files (x86)\Java\jre1.7.0_85， 但是我们想读取在C:\Program Files (x86)\Java\jre1.8.0_60目录下\lib下deploy.jar，并通过com.sun.deploy.util.SecurityBaseline.getDeployVersion()输出当前的deployVersion。

在了解类加载机制后，我们知道lib/deploy.jar是通过启动类加载器加载的，在JVM启动后，如何去加载另一个指定目录中的deply.jar呢？

方法之一是利用写一个简单的main方法，利用命令行机制，java -cp ;C:/Program Files (x86)/Java/jre1.8.0_60/lib/deploy.jar  去加载这个main方法，具体如下

package H1;

import com.sun.deploy.util.SecurityBaseline;
public class GetDeploymentVersionMain {
    public static void main(String args[]) {
        System.out.println(com.sun.deploy.util.SecurityBaseline.getDeployVersion());
　　如何去用java调用shell命令呢？
package H1;
import java.io.BufferedReader;
import java.io.File;
import java.io.InputStreamReader;
public class TestGetDeployVersionMain{
    public static String getDeployVersion(String testJREHome) {
        File javaFile = new File(testJREHome, "/bin/java");
        File deployJar = new File(testJREHome, "/lib/deploy.jar");
        Process process = null;
        String deployVersion = null;
        String classpath = System.getProperty("user.dir");
        try {
            process = Runtime.getRuntime().exec(new String[] {javaFile.getAbsolutePath(), "-cp", ";" + deployJar.getAbsolutePath()+";"+classpath+"/bin/", "H1.GetDeploymentVersionMain"});
        } catch (Exception e) {
            throw new RuntimeException("fail to execute command");
        System.out.println("before read deploy version...");
        System.out.println("the stream is: " + process.getInputStream());
        try (BufferedReader in = new BufferedReader(new InputStreamReader(process.getInputStream()))) {
            String line = null;
            while ((line = in.readLine()) != null) {
                System.out.println(line);
                if (line.matches("^(\\d+\\.)+\\d+")) {
                    deployVersion = line;
            System.out.println("deployVersion=" + deployVersion);
            return deployVersion;
        } catch (Exception e) {
            return "error_version";
    public static void main(String args[]) {
        TestGetDeployVersionMain.getDeployVersion("c:/PROGRA~2/Java/JRE18~1.0_6");
执行结果:
c:\ws\TestProject\src>java -cp ;c:/PROGRA~2/Java/JRE18~1.0_6/lib/deploy.jar H1.TestGetDeployVersionMain
before read deploy version...
11.60.2.19
deployVersion=11.60.2.19
情形二 加载自定义类中的jar包
 在程序运行的时候，如何动态加载自定义的jar包中的类呢？
 步骤一 定义一个类
package com.test;
public class Utils {
    public void printUtils(){
        System.out.println("hello world");
使用命令jar cvf Utils.jar com/test/Utils.class 将Utils.class构建jar包
步骤二 定义加载方法 
 public static void loadUtilSJAR() throws MalformedURLException, ClassNotFoundException, InstantiationException, IllegalAccessException{
        String class_path = "com.test.Utils";// Jar中的所需要加载的类的类名
        String jar_path = "file://c:/Utils.jar";// jar所在的文件的URL
        ClassLoader cl = new URLClassLoader(new URL[] { new URL(jar_path) });// 从Jar文件得到一个Class加载器
        Class<?> c = cl.loadClass(class_path);// 从加载器中加载Class
        Utils util = (Utils) c.newInstance();// 从Class中实例出一个对象
        util.printUtils();
　使用main调用这个方法，类名可以为TestDeployVersion
c:\ws\TestProject\src>java -cp ;c:/Utils.jar TestDeployVersion
hello world
方法三 使用自定义的classloader 
具体可参考 http://kalanir.blogspot.jp/2010/01/how-to-write-custom-class-loader-to.html
public class JarClassLoader extends ClassLoader {  
    private String jarFile = "jar/test.jar"; //Path to the jar file  
    private Hashtable classes = new Hashtable(); //used to cache already defined classes  
    public JarClassLoader() {  
        super(JarClassLoader.class.getClassLoader()); //calls the parent class loader's constructor  
    public Class loadClass(String className) throws ClassNotFoundException {  
        return findClass(className);  
    public Class findClass(String className) {  
        byte classByte[];  
        Class result = null;  
        result = (Class) classes.get(className); //checks in cached classes  
        if (result != null) {  
            return result;  
        try {  
            return findSystemClass(className);  
        } catch (Exception e) {  
        try {  
            JarFile jar = new JarFile(jarFile);  
            JarEntry entry = jar.getJarEntry(className + ".class");  
            InputStream is = jar.getInputStream(entry);  
            ByteArrayOutputStream byteStream = new ByteArrayOutputStream();  
            int nextValue = is.read();  
            while (-1 != nextValue) {  
                byteStream.write(nextValue);  
                nextValue = is.read();  
            classByte = byteStream.toByteArray();  
            result = defineClass(className, classByte, 0, classByte.length, null);  
            classes.put(className, result);  
            return result;  
        } catch (Exception e) {  
            return null;