Java实现热替换

package test;

import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.channels.WritableByteChannel;

public class DynamicLoader extends ClassLoader {
    
    private String baseDir;
    
    public DynamicLoader(String baseDir) {
        super();
        this.baseDir = baseDir;
    }

    private String getClassFile(String className){
        return baseDir+className.replace(".", "/")+".class";
    }
    
    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);
                int count = 0;
                while ((count = fileC.read(buffer)) > 0) {
                    buffer.flip();
                    outC.write(buffer);
                    buffer.clear();
                }
                fis.close();
                byte[] bytes = baos.toByteArray();
                clazz = defineClass(className, bytes, 0, bytes.length);
            } catch (Exception e) {
                System.out.println("can not load class "+className +" from DynamicLoader.");
            }
        }
        return clazz;
    }

    protected synchronized Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
        // First, check if the class has already been loaded
        Class re=findClass(name);
        if(re==null){
            return super.loadClass(name,resolve);
        }
        return re;
    }

}

package test;

public class Worker {

    public void doit(){
        System.out.println("I am version 3");
    }
}

package test;

import java.lang.reflect.Method;

public class HelloMain {

    public static void main(String[] args) throws Exception {
        
        while(true)
        {
            DynamicLoader loader =    new DynamicLoader("F:\workspace\HibernateSrc\bin\");
            Class clazz = loader.loadClass("test.Worker");
            Object instance = clazz.newInstance();
            Method doit = clazz.getDeclaredMethod("doit",null);
            doit.invoke(instance, null);
            Thread.sleep(2000);
        }

    }

}

思路:

在HelloMain里面定时的创建新的自定义ClassLoader,然后指定加载某个目录的class文件.加载的时候不是父类优先，而是子类优先模式.

自定义的ClassLoader找到Worker类后，反射穿件实例. 

这里不能用new关键字在HelloMain类里面创建Worker实例，也能让反射生成的实例转型成Worker类型,因为那样会导致AppliationClassLoader加载Worker类.

如果被AppliationClassLoader加载了Worker类,那么新版本的Worker就不能再被Application ClassLoader加载了，一个ClassLoader里面同名的class只能有一个.

如果这个时候让AppliationClassLoader加载了老的Worker类，在替换的时候让自定义ClassLoader加载新版本的Woker类，则会出现ClassCastException.

因为这些Worker类来之不同的ClassLoader，比如下面代码会报ClassCastException.

 

DynamicLoader loader = new DynamicLoader("F:workspaceHibernateSrcbin");

Class clazz = loader.loadClass("test.Worker");

Worker instance = (Worker)clazz.newInstance();//ClassCastException error
instance.doit();

所以Worker类只能是让自定义的ClassLoader加载.同时下次要运行的时候，也要在创建一个新的自定义ClassLoader来加载.

参考 http://www.cnblogs.com/princessd8251/articles/3967569.html