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【深入浅出-JVM】（76）：classloader

2019-09-17 10:48:36来源：博客园阅读 ()

【深入浅出-JVM】（76）：classloader

方法

public Class<?> loadClass(String name) throws ClassNotFoundException
通过类名发挥这个类的Class实例
protected final Class<?> defineClass(byte[] b,int off,int len)
根据给定的字节码流 b，off 和 len 参数表示实际的 class 信息在byte 数组中的位置和长度，其中 byte 数组 b是 classloader 从外部获取的
protected Class<?> findClass(String name)throws ClassNotFoundException
查看一个类
protected final Class<?> findLoadedClass(String name)

分类
BootStrap ClassLoader
Extension ClassLoader
AppClassLoader

启动类加载器负责加载系统的核心类（rt.jar的java类），扩展类加载器加载 %JAVA_HOME/lib/ext/*.jar中的类，应用类加载器用于加载用户类 (classpath)，自定义类加载器加载一些特殊路径的类(自定义classloader)

双亲委托

当前 classloader 首先从自己已经加载的类中查询是否此类已经加载，如果已经加载了则直接返回原来已经加载的类
当前 classloader 的缓存中没有找到被加载的类的时候，委托父类加载器去加载，父类加载器采用同样的策略，首先查下自己的缓存，然后委托父类去加载，一直到 bootstrap classloader
当所有的父类加载器都没有加载的时候，再由当前的类加载器加载，并将其放入自己的缓存中，下次请求的时候直接返回
一直循环重复

作用

各个类加载器的基础类统一

jar -cvf test.jar HelloLoader.class 把class打包成jar

Extension ClassLoader

例子：在 ext 路径下放一个自己 jar 包并加载

package com.mousycoder.server;

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

idea 通过 structs->artifacts->jar 然后 build-> build artifacts->build 生成 helloworld.jar
放到 /Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/ext

package com.mousycoder.mycode.thinking_in_jvm;

import java.lang.reflect.Method;

/**
 * @version 1.0
 * @author: mousycoder
 * @date: 2019-09-06 10:35
 */
public class ExtClassLoader {

    public static void main(String[] args) throws ClassNotFoundException {
        System.out.println(System.getProperty("java.ext.dirs"));
        Class<?> helloClass = Class.forName("com.mousycoder.server.HelloWorld");
        System.out.println(helloClass.getClassLoader());
    }
}

输出

/Users/mousycoder/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java
sun.misc.Launcher$ExtClassLoader@610455d6

可以看出是 ExtClassLoader 加载 java.ext.dirs 目录

自定义类加载器

package com.mousycoder.mycode.thinking_in_jvm;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

/**
 * @version 1.0
 * @author: mousycoder
 * @date: 2019-09-06 11:13
 */
public class MyClassLoader extends ClassLoader {

    private final static Path DEFAULT_CLASS_PATH = Paths.get("","/Users/mousycoder/My");

    private final Path classDir;

    public MyClassLoader(){
        super();
        this.classDir = DEFAULT_CLASS_PATH;

    }

    public MyClassLoader(String classDir){
        super();
        this.classDir = Paths.get(classDir);
    }

    public MyClassLoader(String classDir, ClassLoader parent){
        super(parent);
        this.classDir = Paths.get(classDir);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        try {
            byte[] classBytes = this.readClassBytes(name);

            if (null == classBytes || 0 == classBytes.length){
                throw new ClassNotFoundException("can not load the class" + name);
            }
            return this.defineClass(name,classBytes,0,classBytes.length);
        } catch (IOException e){
            e.printStackTrace();
        }
        return null;
    }


    private byte[] readClassBytes(String name) throws ClassNotFoundException, IOException {
        String classPath = name.replace(".","/");
        Path classFullPath = classDir.resolve( "HelloWorld1.class");
        if (!classFullPath.toFile().exists()){
            throw new ClassNotFoundException("The class" + name + "mpt found");
        }

        try (ByteArrayOutputStream baos = new ByteArrayOutputStream()){
            Files.copy(classFullPath,baos);
            return baos.toByteArray();
        } catch (IOException e){
            throw new ClassNotFoundException("load the class " + name + "occur error",e);
        }

    }


    @Override
    public String toString() {
        return "My ClassLoader";
    }
}

package com.mousycoder.mycode.thinking_in_jvm;

/**
 * @version 1.0
 * @author: mousycoder
 * @date: 2019-09-06 11:34
 */
public class MyClassLoaderTest {

    public static void main(String[] args) throws ClassNotFoundException {
        MyClassLoader classLoader = new MyClassLoader();
        Class<?> class1 = classLoader.loadClass("com.mousycoder.mycode.thinking_in_jvm.HelloWorld1");
        System.out.println(class1.getClassLoader());
    }
}
package com.mousycoder.mycode.thinking_in_jvm;

/**
 * @version 1.0
 * @author: mousycoder
 * @date: 2019-09-06 11:46
 */
public class HelloWorld1 {
    public static void main(String[] args) {
        System.out.println("Hello world1 ");
    }
}

把helloword1变成class放到/Users/mousycoder/My目录下即可
输出 My ClassLoader 代表自定义类加载器加载了该类