一个例子深入理解ClassLoader

文件类加载器，该加载器重载了loadClass方法，逻辑是只读取文件来加载类，不委托给父类加载器进行加载

package com.ydd.study.hello.classloader;

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

public class FileSystemClassLoader extends ClassLoader {
    private String rootDir; 

    public FileSystemClassLoader(String rootDir) {
        this.rootDir = rootDir;
    } 

    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] classData = getClassData(name);
        if (classData == null) {
            throw new ClassNotFoundException();
        }
        else {
            return defineClass(name, classData, 0, classData.length);
        }
    }
    @Override
    public Class<?> loadClass(String name)
    {
        try {
            return  findClass(name);
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return null;
        }

    }

    private byte[] getClassData(String className) {
        String path = classNameToPath(className);
        try {
            InputStream ins = new FileInputStream(path);
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            int bufferSize = 4096;
            byte[] buffer = new byte[bufferSize];
            int bytesNumRead = 0;
            while ((bytesNumRead = ins.read(buffer)) != -1) {
                baos.write(buffer, 0, bytesNumRead);
            }
            return baos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    } 

    private String classNameToPath(String className) {
        return rootDir + File.separatorChar
                + className.replace('.', File.separatorChar) + ".class";
    }
}

测试代码

package com.ydd.study.hello.classloader;

import org.junit.Test;

public class FileSystemClassLoaderTest   {
    @Test
    public void test()
    {
        FileSystemClassLoader fileSystemClassLoader=new FileSystemClassLoader("D:\\");
        try {
            Class<?>c=fileSystemClassLoader.loadClass("TestClass");
            System.out.println(c);
            Object testClass=(c.newInstance());
            System.out.println(c+"|"+testClass);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}

从下面的输出，可以看出：

1、在加载TestClass类时就已经引起其他类的加载了。

2、不要轻易重载loadClass()，双亲制很重要，否则会引起一个类存在多个Class对象。

3、类A依赖的其他类的加载，全部都是由类A的ClassLoader进行初始发起加载

java.io.FileNotFoundException: D:\java\lang\Object.class (系统找不到指定的路径。)
	at java.io.FileInputStream.open(Native Method)
	at java.io.FileInputStream.<init>(FileInputStream.java:146)
	at java.io.FileInputStream.<init>(FileInputStream.java:101)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.getClassData(FileSystemClassLoader.java:42)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.findClass(FileSystemClassLoader.java:17)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass(FileSystemClassLoader.java:29)
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:643)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.findClass(FileSystemClassLoader.java:22)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass(FileSystemClassLoader.java:29)
	at com.ydd.study.hello.classloader.FileSystemClassLoaderTest.test(FileSystemClassLoaderTest.java:12)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
java.lang.ClassNotFoundException
	at com.ydd.study.hello.classloader.FileSystemClassLoader.findClass(FileSystemClassLoader.java:19)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass(FileSystemClassLoader.java:29)
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:643)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.findClass(FileSystemClassLoader.java:22)
	at com.ydd.study.hello.classloader.FileSystemClassLoader.loadClass(FileSystemClassLoader.java:29)
	at com.ydd.study.hello.classloader.FileSystemClassLoaderTest.test(FileSystemClassLoaderTest.java:12)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)