java 的classloader机制

JVM在加载类的时候，都是通过ClassLoader的loadClass()方法来加载class的，loadClass(String name)方法：
使用的是双亲委托模式：
jvm启动时，会启动jre/rt.jar里的类加载器：bootstrap classloader，用来加载java核心api；然后启动扩展类加载器ExtClassLoader加载扩展类，并加载用户程序加载器AppClassLoader，并指定ExtClassLoader为他的父类；
当类被加载时，会先检查在内存中是否已经被加载，如果是，则不再加载，如果没有，再由AppClassLoader来加载，先从jar包里找，没有再从classpath里找；
如果自定义loader类，就会存在这命名空间的情况，不同的加载器加载同一个类时，产生的实例其实是不同的；

Java代码
public Class<?> loadClass(String name) throws ClassNotFoundException {  
return loadClass(name, false);  
   } 

public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
    }

loadClass(String name)方法再调用loadClass(String name, boolean resolve)方法:
     - name - 类的二进制名称
     - resolve - 如果该参数为 true，则分析这个类
Java代码
protected synchronized Class<?> loadClass(String name, boolean resolve)  
    throws ClassNotFoundException  
    {  
    // First, check if the class has already been loaded  
    //JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取  
    Class c = findLoadedClass(name);  
    if (c == null) {  
        try {  
        if (parent != null) {  
            c = parent.loadClass(name, false);  
        } else {  
            c = findBootstrapClass0(name);  
        }  
        } catch (ClassNotFoundException e) {  
            // If still not found, then invoke findClass in order  
            // to find the class.  
            c = findClass(name);  
        }  
    }  
    if (resolve) {  
        resolveClass(c);  
    }  
    return c;  
} 

protected synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
    {
// First, check if the class has already been loaded
//JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取
Class c = findLoadedClass(name);
if (c == null) {
    try {
if (parent != null) {
    c = parent.loadClass(name, false);
} else {
    c = findBootstrapClass0(name);
}
    } catch (ClassNotFoundException e) {
        // If still not found, then invoke findClass in order
        // to find the class.
        c = findClass(name);
    }
}
if (resolve) {
    resolveClass(c);
}
return c;
}

如果ClassLoader并没有加载这个class，则调用findBootstrapClass0:
Java代码
private Class findBootstrapClass0(String name)  
    throws ClassNotFoundException  
    {  
    check();  
    if (!checkName(name))  
        throw new ClassNotFoundException(name);  
    return findBootstrapClass(name);  
    } 

private Class findBootstrapClass0(String name)
throws ClassNotFoundException
    {
check();
if (!checkName(name))
    throw new ClassNotFoundException(name);
return findBootstrapClass(name);
    }

该方法会调用check()方法来判断这个类是否已经初始化，并且通过checkName(name)来判断由name指定的这个类是否存在
最后调用findBootstrapClass(name):
Java代码
private native Class findBootstrapClass(String name)  
    throws ClassNotFoundException; 

private native Class findBootstrapClass(String name)
throws ClassNotFoundException;

而这个findBootstrapClass方法是一个native方法，这是我们的root loader，这个载入方法并非是由JAVA所写，而是C++写的，它会最终调用JVM中的原生findBootstrapClass方法来完成类的加载。

如果上面两个都找不到，则使用findClass(name)来查找指定类名的Class:
Java代码
protected Class<?> findClass(String name) throws ClassNotFoundException {  
    throw new ClassNotFoundException(name);  
} 

protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}

JDK5.0中的说明：
使用指定的二进制名称查找类。此方法应该被类加载器的实现重写，该实现按照委托模型来加载类。在通过父类加载器检查所请求的类后，此方法将被 loadClass 方法调用。默认实现抛出一个 ClassNotFoundException。
所以，我们在自定义类中，只需要重写findClass()即可。
MyClassLoader类：
Java代码
public class MyClassLoader extends ClassLoader {  
    private String fileName;  
 
    public MyClassLoader(String fileName) {  
        this.fileName = fileName;  
    }  
 
    protected Class<?> findClass(String className) throws ClassNotFoundException {  
        Class clazz = this.findLoadedClass(className);  
        if (null == clazz) {  
            try {  
                String classFile = getClassFile(className);  
                FileInputStream fis = new FileInputStream(classFile);  
                FileChannel fileC = fis.getChannel();  
                ByteArrayOutputStream baos = new 
                        ByteArrayOutputStream();  
                WritableByteChannel outC = Channels.newChannel(baos);  
                ByteBuffer buffer = ByteBuffer.allocateDirect(1024);  
                while (true) {  
                    int i = fileC.read(buffer);  
                    if (i == 0 || i == -1) {  
                        break;  
                    }  
                    buffer.flip();  
                    outC.write(buffer);  
                    buffer.clear();  
                }  
                fis.close();  
                byte[] bytes = baos.toByteArray();  
 
                clazz = defineClass(className, bytes, 0, bytes.length);  
            } catch (FileNotFoundException e) {  
                e.printStackTrace();  
            } catch (IOException e) {  
                e.printStackTrace();  
            }  
        }  
        return clazz;  
    }  
    private byte[] loadClassBytes(String className) throws 
            ClassNotFoundException {  
        try {  
            String classFile = getClassFile(className);  
            FileInputStream fis = new FileInputStream(classFile);  
            FileChannel fileC = fis.getChannel();  
            ByteArrayOutputStream baos = new 
                    ByteArrayOutputStream();  
            WritableByteChannel outC = Channels.newChannel(baos);  
            ByteBuffer buffer = ByteBuffer.allocateDirect(1024);  
            while (true) {  
                int i = fileC.read(buffer);  
                if (i == 0 || i == -1) {  
                    break;  
                }  
                buffer.flip();  
                outC.write(buffer);  
                buffer.clear();  
            }  
            fis.close();  
            return baos.toByteArray();  
        } catch (IOException fnfe) {  
            throw new ClassNotFoundException(className);  
        }  
    }  
    private String getClassFile(String name) {  
        StringBuffer sb = new StringBuffer(fileName);  
        name = name.replace('.', File.separatorChar) + ".class";  
        sb.append(File.separator + name);  
        return sb.toString();  
    }  
} 

public class MyClassLoader extends ClassLoader {
    private String fileName;

    public MyClassLoader(String fileName) {
        this.fileName = fileName;
    }

    protected Class<?> findClass(String className) throws ClassNotFoundException {
        Class clazz = this.findLoadedClass(className);
        if (null == clazz) {
            try {
                String classFile = getClassFile(className);
                FileInputStream fis = new FileInputStream(classFile);
                FileChannel fileC = fis.getChannel();
                ByteArrayOutputStream baos = new
                        ByteArrayOutputStream();
                WritableByteChannel outC = Channels.newChannel(baos);
                ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
                while (true) {
                    int i = fileC.read(buffer);
                    if (i == 0 || i == -1) {
                        break;
                    }
                    buffer.flip();
                    outC.write(buffer);
                    buffer.clear();
                }
                fis.close();
                byte[] bytes = baos.toByteArray();

                clazz = defineClass(className, bytes, 0, bytes.length);
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return clazz;
    }
    private byte[] loadClassBytes(String className) throws
            ClassNotFoundException {
        try {
            String classFile = getClassFile(className);
            FileInputStream fis = new FileInputStream(classFile);
            FileChannel fileC = fis.getChannel();
            ByteArrayOutputStream baos = new
                    ByteArrayOutputStream();
            WritableByteChannel outC = Channels.newChannel(baos);
            ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
            while (true) {
                int i = fileC.read(buffer);
                if (i == 0 || i == -1) {
                    break;
                }
                buffer.flip();
                outC.write(buffer);
                buffer.clear();
            }
            fis.close();
            return baos.toByteArray();
        } catch (IOException fnfe) {
            throw new ClassNotFoundException(className);
        }
    }
    private String getClassFile(String name) {
        StringBuffer sb = new StringBuffer(fileName);
        name = name.replace('.', File.separatorChar) + ".class";
        sb.append(File.separator + name);
        return sb.toString();
    }
}

该类中通过调用defineClass(String name, byte[] b, int off, int len)方法来定义一个类：
Java代码
protected final Class<?> defineClass(String name, byte[] b, int off, int len)  
throws ClassFormatError  
   {  
return defineClass(name, b, off, len, null);  
   } 

protected final Class<?> defineClass(String name, byte[] b, int off, int len)
throws ClassFormatError
    {
return defineClass(name, b, off, len, null);
    }

注：MyClassLoader加载类时有一个局限，必需指定.class文件，而不能指定.jar文件。该类中的大部分代码是从网上搜索到的，是出自一牛人之笔，只是不知道原帖在哪，希望不会被隐藏。
MainClassLoader类：
Java代码
public class MainClassLoader {  
    public static void main(String[] args) {  
        try {  
            MyClassLoader tc = new MyClassLoader("F:\\OpenLib\\");  
            Class c = tc.findClass("Test");  
            c.newInstance();  
        } catch (ClassNotFoundException e) {  
            e.printStackTrace();   
        } catch (IllegalAccessException e) {  
            e.printStackTrace();  
        } catch (InstantiationException e) {  
            e.printStackTrace();   
        }  
    }  
} 

public class MainClassLoader {
    public static void main(String[] args) {
        try {
            MyClassLoader tc = new MyClassLoader("F:\\OpenLib\\");
            Class c = tc.findClass("Test");
            c.newInstance();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        }
    }
}

最后是一个简单的Test测试类：
Java代码
public class Test  
{  
    public Test() {  
        System.out.println("Test");  
    }  
    public static void main(String[] args) {  
        System.out.println("Hello World");  
    }  
}