在上一篇博客中介绍了一下如何书写自定义加载器.传送门
这一篇要介绍的就是自定义类加载器来打破双亲委派机制.
思路就是加载类的时候不走parent属性来找,直接从指定的classpath中找
package com.bonc.jvm;
import java.io.FileInputStream;
import java.lang.reflect.Method;
/**
* @Classname MyClassLoadTEst
* @Description TODO
* @Date 2020/6/14 16:31
* @Created by sz
*/
public class MyClassLoadTest {
static class MyClassLoad extends ClassLoader{
private String classPath;
public MyClassLoad(String classPath){
this.classPath=classPath;
}
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
//只有自己写的类走自己写的类加载器,原来的类走自己类加载机制 这行代码非常关键
if (!name.startsWith("com.bonc")){
c = super.loadClass(name,resolve);
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
private byte[] loadByte(String name) throws Exception {
name = name.replaceAll("\\.", "/");
FileInputStream fis = new FileInputStream(classPath + "/" + name
+ ".class");
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
try {
byte[] data = loadByte(name);
//defineClass将一个字节数组转为Class对象,这个字节数组是class文件读取后最终的字节 数组。
return defineClass(name, data, 0, data.length);
} catch (Exception e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
}
public static void main(String[] args) throws Exception {
MyClassLoad classLoader = new MyClassLoad("D:/tmp");
Class clazz = classLoader.loadClass("com.bonc.jvm.User1");
Object obj = clazz.newInstance();
Method method = clazz.getDeclaredMethod("print", null);
method.invoke(obj,null);
System.out.println(clazz.getClassLoader().getClass().getName());
System.out.println(clazz.getClassLoader().getParent().getClass().getName());
System.out.println(clazz.getClassLoader().getParent().getParent().getClass().getName());
}
}
代码运行效果
在不打破双亲委派机制之前的代码执行第一个的类加载器是AppClassLoader而不是自定义类加载器.因为你的classpath目录下已经存在一个Use1了.
那么我们是否能够重写一个java.lang.String呢
package com.bonc.jvm;
import java.io.FileInputStream;
import java.lang.reflect.Method;
/**
* @Classname MyClassLoadTEst
* @Description TODO
* @Date 2020/6/14 16:31
* @Created by sz
*/
public class MyClassLoadTest {
static class MyClassLoad extends ClassLoader{
private String classPath;
public MyClassLoad(String classPath){
this.classPath=classPath;
}
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
private byte[] loadByte(String name) throws Exception {
name = name.replaceAll("\\.", "/");
FileInputStream fis = new FileInputStream(classPath + "/" + name
+ ".class");
int len = fis.available();
byte[] data = new byte[len];
fis.read(data);
fis.close();
return data;
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
try {
byte[] data = loadByte(name);
//defineClass将一个字节数组转为Class对象,这个字节数组是class文件读取后最终的字节 数组。
return defineClass(name, data, 0, data.length);
} catch (Exception e) {
e.printStackTrace();
throw new ClassNotFoundException();
}
}
}
public static void main(String[] args) throws Exception {
MyClassLoad classLoader = new MyClassLoad("D:/tmp");
Class clazz = classLoader.loadClass("java.lang.String");
Object obj = clazz.newInstance();
Method method = clazz.getDeclaredMethod("print", null);
method.invoke(obj,null);
System.out.println(clazz.getClassLoader().getClass().getName());
System.out.println(clazz.getClassLoader().getParent().getClass().getName());
System.out.println(clazz.getClassLoader().getParent().getParent().getClass().getName());
}
}
运行结果
结论:java为了保护自己的核心类库被篡权,核心的包下面的类不能被重写