(java) 类加载器 -- 自定义类加载器

之前讲解过类加载器相关的知识，下面介绍如何自定义一个类加载器

用户自定义类加载器只需要扩展：java.lang.ClassLoader

先看一下ClassLoader的说明部分的示例，如何定义一个从网络上加载class文件

 * <p> The network class loader subclass must define the methods {@link
 * #findClass <tt>findClass</tt>} and <tt>loadClassData</tt> to load a class
 * from the network.  Once it has downloaded the bytes that make up the class,
 * it should use the method {@link #defineClass <tt>defineClass</tt>} to
 * create a class instance.  A sample implementation is:
 *
 * <blockquote><pre>
 *     class NetworkClassLoader extends ClassLoader {
 *         String host;
 *         int port;
 *
 *         public Class findClass(String name) {
 *             byte[] b = loadClassData(name);
 *             return defineClass(name, b, 0, b.length);
 *         }
 *
 *         private byte[] loadClassData(String name) {
 *             // load the class data from the connection
 *              . . .
 *         }
 *     }
 * </pre></blockquote>

 public Class findClass(String name)  这一部分是对应的ClassLoad.loadClass(String name,boolean resolve)中的调用用户自定义实现部分

 protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);//用户自定义实现部分

                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

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

分析以上代码之后，我们就清楚如何去实现自定义的类加载器了。

package com.test;

import java.io.InputStream;

/**
 * 实现一个从磁盘文件中加载Class文件的类加载器
 * @author Administrator
 *
 */
public class MyClassLoader extends ClassLoader {

	private String loaderName;
	private String loaderPath;
	
	public MyClassLoader(String loaderName){
		super();//将系统类加载器作为parent类加载器 getSystemClassLoader()
		this.loaderName=loaderName;
	}

	public void setLoaderPath(String loaderPath) {
		this.loaderPath = loaderPath;
	}

	@Override
	public Class<?> findClass(String name) throws ClassNotFoundException {
		byte[] data=loadClassData(name);
		return this.defineClass(name, data, 0, data.length);
	}
	/**
	 * 备注：在windows上测试
	 * 
	 * @param name
	 * @return
	 */
	private byte[] loadClassData(String name)  {
		//file : this.loaderPath+name+".class"
		String fullPath=this.loaderPath+name.replace(".", "//")+".class";
		InputStream in=null;
		java.io.ByteArrayOutputStream out=null;
		try{
			in=new java.io.FileInputStream(fullPath);
			out=new java.io.ByteArrayOutputStream();
			int data=0;
			while((data=in.read()) != 0 ){
				out.write(data);
			}			
		}catch(Exception ex){
			ex.printStackTrace();
		}finally{
			if( in != null){
				try{
				in.close();
				}catch(Exception ex){}
				
				try{out.close();}catch(Exception ex){}
			}
		}
		return out.toByteArray();
    }
	
	
}