关于classloader

jvm自带的三个classloader

1.Bootstrap classloader，是jvm的一部分，由C写成，调用的是核心库。

   sun java下，可以用以下语句查看调用类路径

  

URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs(); 

 

写道

%JAVA_HOME%/jre/lib/resources.jar
%JAVA_HOME%/jre/lib/rt.jar
%JAVA_HOME%/jre/lib/sunrsasign.jar
%JAVA_HOME%/jre/lib/jsse.jar
%JAVA_HOME%/jre/lib/jce.jar
%JAVA_HOME%/jre/lib/charsets.jar
%JAVA_HOME%/jre/lib/jfr.jar
%JAVA_HOME%/jre/classes

 

2.ExtClassLoader，调用系统扩展类

  

写道

%JAVA_HOME%/jre/lib/ext/access-bridge.jar
%JAVA_HOME%/jre/lib/ext/cldrdata.jar
%JAVA_HOME%/jre/lib/ext/dnsns.jar
%JAVA_HOME%/jre/lib/ext/jaccess.jar
%JAVA_HOME%/jre/lib/ext/jfxrt.jar
%JAVA_HOME%/jre/lib/ext/localedata.jar
%JAVA_HOME%/jre/lib/ext/nashorn.jar
%JAVA_HOME%/jre/lib/ext/sunec.jar
%JAVA_HOME%/jre/lib/ext/sunjce_provider.jar
%JAVA_HOME%/jre/lib/ext/sunmscapi.jar
%JAVA_HOME%/jre/lib/ext/sunpkcs11.jar
%JAVA_HOME%/jre/lib/ext/zipfs.jar

 3.AppClassLoader，调用classpath里的类

 

这三个类遵循双亲委派模型，也就是先每装载一个类时，先从父装载器装载，找不到才一级级往下找，这也就是classpath里的类无法覆盖jdk里自带类的原因。

 

像tomcat等应用，涉及到webapp隔离，所以在装载每个webapp里的类时，没有遵循双亲委派模型，而是优先自己所调用的类，另外这里有个关键点，就是隐式装载，也就是用关键字new创建对象时，装载这个类所使用的装载器是使用new关键字所在类的装载器。

以下代码可以模拟这个过程。

 

自定义类装载器

package jp.co.wqf;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

public class MyClassLoader extends ClassLoader {
	private String name;
	private String path = "D:/";
    private final String fileType = ".class";
	public MyClassLoader(String name){
		super();
		this.name = name;
	}
	public MyClassLoader(String name, ClassLoader cl){
		super(cl);
		this.name = name;
	}
	public String getPath() {
		return path;
	}
	public void setPath(String path) {
		this.path = path;
	}
	@Override
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		byte[] data = loaderClassData(name);
		return this.defineClass(name, data, 0, data.length);
	}
    private byte[] loaderClassData(String className){
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        String fileName = className.replace(".", "/");
        try {
            is = new FileInputStream(new File(path + fileName + fileType));
            int c = 0;
            while(-1 != (c = is.read())){
                baos.write(c);
            }
            data = baos.toByteArray();

        } catch (Exception e) {
            e.printStackTrace();
        } finally{
            try {
                is.close();
                baos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return data;
    }
}

 测试

package jp.co.wqf;

public class Test {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		MyClassLoader classLoader = new MyClassLoader("classload1");
		classLoader.setPath("/eclipse/workspace/Test/bin/");
		try {
			Class<?> clsA = classLoader.loadClass("jp.co.wqf.A");
			Object objA = clsA.newInstance();
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

}

 

以下类不要放在上面代码的classpath里

package jp.co.wqf;

public class A {
	private B b;
	public A(){
		this.b = new B();
	}
}

 

package jp.co.wqf;

public class B {
	private C c;
	public B(){
		this.c = new C();
	}
}

 

package jp.co.wqf;

public class C {
	public C(){
		System.out.println("create class c!");
	}
}