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.
     
      (FileInputStream.java:146)	at java.io.FileInputStream.
      
       (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)