动态加载和卸载Java类

在开发Java服务器应用时，我们最希望开发的应用能够支持热部署，即不需要重启系统就可以用新的应用替换旧的应用。
如果使用动态语言，这些功能比较容易实现。但Java是静态语言，是否也可实现动态热部署呢？

首先，我们要深入了解一下Java的类装载（Class Loading）机制，和垃圾回收(Garbage Collection）机制。其中class loading 将负责装载新的应用包；GC将负责卸载旧的应用包。
装载新应用包的方法比较简单，只需要定制一个ClassLoader，从指定路径装载.jar文件即可。
要卸载一个ClassLoader，则必须要同时卸载通过该ClassLoader 载入的类的所有实例, 简单将ClassLoader的引用置为null并希望GC回收的做法是无效的。然而，要想统计并记录所有该ClassLoader载入的类实例是不现实的。而且，应用的装载和卸载功能，是服务器Framework的一部分，而不是应用业务功能的一部分。因此，framework也无法知道业务应用中何时载入和创建了多少对象。

解决方法还是有的。因为GC回收Heap中那些unreachable的对象，追根溯源，所有对象的创建都是由活动线程中发起的, 即所谓的Root set of references. 因此一旦活动线程结束运行，则可以说，线程中所有对象的根引用不可用了，则由根应用创建的所有对象也变为unreachable.

因此可以做如下设计：


其中Server负责service的deploy和undeploy。
其中deploy的过程可简单描述为：创建一个daemon线程，设置其context classloader为Service Jar包的ClassLoader，起动该Daemon线程。
undeploy的过程可简单描述为：停止服务（service daemon thread），设置线程context classloader为null, 等带线程彻底结束后手动执行GC来回收对象和ClassLoader.

Service接口如下：

public interface Service {

	public final static String ATTR_SERVICE_ID = "SERVICE-ID";
	public final static String ATTR_SERVICE_CLASS = "SERVICE-CLASS";

	public void install() throws ServiceException;

	public void start() throws ServiceException;

	public void stop() throws ServiceException;

	public void uninstall() throws ServiceException;

	public String getId();
}

服务管理器如下：

/**
 * 
 * @author less
 * Responsible for deploy and undeploy Services.
 */
public class ServiceManager {

	private final static HashMap<String, ServiceThread> installedServices = new HashMap<String, ServiceThread>();

	public final synchronized static String install(File jarService) throws Exception {
		MyJarLoader loader = new MyJarLoader(jarService, MyJarLoader.class.getClassLoader());
		Manifest manifest = loader.getManifest();
		Attributes attrs = manifest.getAttributes("Service");
		String svcId = attrs.getValue(Service.ATTR_SERVICE_ID);
		if (installedServices.containsKey(svcId)) {
			uninstall(svcId);
		}
		ServiceThread t = new ServiceThread();
		t.setContextClassLoader(loader);
		t.setDaemon(true);
		t.start();
		loader = null;
		return svcId;
	}

	public final static void uninstall(File jarService) throws Exception {
		MyJarLoader loader = new MyJarLoader(jarService, MyJarLoader.class.getClassLoader());
		Manifest manifest = loader.getManifest();
		Attributes attrs = manifest.getAttributes("Service");
		String svcId = attrs.getValue(Service.ATTR_SERVICE_ID);
		loader = null;
		uninstall(svcId);
	}

	public final static void uninstall(String svcId) throws Exception {
		ServiceThread t = installedServices.get(svcId);
		if (t.getStatus() == ServiceThread.Status.RUNNING) {
			t.stopService();
		}
		ServiceManager.getInstalledServices().remove(svcId);
		t.destroyService();
		t.setContextClassLoader(null);
		while (t.isAlive()) {
			try {
				Thread.sleep(200);
			} catch (InterruptedException e) {
			}
		}
		t = null;
		System.gc();
		try {
			Thread.sleep(200);
		} catch (InterruptedException e) {
		}
		System.gc();
	}

	static HashMap<String, ServiceThread> getInstalledServices() {
		return installedServices;
	}

}

/**
 * 
 * @author less
 * A Customer Service carrier.
 */
class ServiceThread extends Thread {

	public static enum Status {
		RUNNING, STOPPED
	}

	private Status status = Status.STOPPED;
	private Service service = null;

	public Status getStatus() {
		return this.status;
	}

	@Override
	public void run() {
		try {
			Manifest manifest = ((MyJarLoader) getContextClassLoader()).getManifest();
			Attributes attrs = manifest.getAttributes("Service");
			String svcId = attrs.getValue(Service.ATTR_SERVICE_ID);
			this.setName(svcId);
			String svcClass = attrs.getValue(Service.ATTR_SERVICE_CLASS);
			Class<Service> c = (Class<Service>) getContextClassLoader().loadClass(svcClass);
			service = c.newInstance();
			c = null;
			service.install();
			ServiceManager.getInstalledServices().put(svcId, this);
			startService();
			stopService();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public void stopService() {
		if (this.status != Status.STOPPED) {
			try {
				service.stop();
				this.status = Status.STOPPED;
			} catch (Exception e) {
				e.printStackTrace();
				this.status = Status.RUNNING;
			}
		}
	}

	public void startService() {
		if (this.status == Status.STOPPED) {
			this.status = Status.RUNNING;
			try {
				service.start();
			} catch (Exception e) {
				e.printStackTrace();
				this.status = Status.STOPPED;
			}
		}
	}

	public void destroyService() {
		try {
			service.uninstall();
			service = null;
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

/**
 * 
 * @author less
 * 
 */
class MyJarLoader extends JarLoader {

	public MyJarLoader(File file, ClassLoader parent) throws Exception {
		super(file, parent);
		System.out.println(this + " is created.");
	}

	@Override
	protected void finalize() throws Throwable {
		destroy();
		System.out.println(this + " is finalized.");
		super.finalize();
	}
}

示例服务代码，测试修改编译后重新部署：

/**
 * @author less
 * 
 */
public class ExampleService1 implements Service {
	private boolean bRun = true;

	@Override
	public void install() throws ServiceException {
		System.out.println("== " + this + " is installed.");
	}

	@Override
	public void start() throws ServiceException {
		System.out.println("== " + this + " is started.");

		//int i = 10000;
		int i=0;
		while (bRun) {
			//System.out.println(this + "  " + i--);
			System.out.println(this + "  " + i++);
			try {
				Thread.sleep(5000);
			} catch (InterruptedException ie) {
			}
		}
		System.out.println("== " + this + " is stopped.");
	}

	@Override
	public void stop() throws ServiceException {
		System.out.println("== Trying to stop " + this);
		bRun = false;
		Thread.currentThread().interrupt();
	}

	@Override
	public void uninstall() throws ServiceException {
		System.out.println("== " + this + " is uninstalled.");
	}

	@Override
	public String getId() {
		// TODO Auto-generated method stub
		return null;
	}

	@Override
	protected void finalize() throws Throwable {
		System.out.println(this + " - is finalized.");
		super.finalize();
	}
}

控制台输出如下：
Server started.
org.lucky.server.MyJarLoader@27b15692 is created.
== org.lucky.service.example.ExampleService1@4e76fba0 is installed.
== org.lucky.service.example.ExampleService1@4e76fba0 is started.
org.lucky.service.example.ExampleService1@4e76fba0  10000
org.lucky.service.example.ExampleService1@4e76fba0  9999
org.lucky.service.example.ExampleService1@4e76fba0  9998
org.lucky.service.example.ExampleService1@4e76fba0  9997
org.lucky.server.MyJarLoader@2f833eca is created.
== Trying to stop org.lucky.service.example.ExampleService1@4e76fba0
== org.lucky.service.example.ExampleService1@4e76fba0 is uninstalled.
== org.lucky.service.example.ExampleService1@4e76fba0 is stopped.
org.lucky.service.example.ExampleService1@4e76fba0 - is finalized.
org.lucky.server.MyJarLoader@27b15692 is finalized.
== org.lucky.service.example.ExampleService1@7b36a43c is installed.
== org.lucky.service.example.ExampleService1@7b36a43c is started.
org.lucky.service.example.ExampleService1@7b36a43c  0
org.lucky.service.example.ExampleService1@7b36a43c  1
org.lucky.service.example.ExampleService1@7b36a43c  2
org.lucky.service.example.ExampleService1@7b36a43c  3

由上述测试可见，通过这种方式，可以实现类动态加载和卸载以及热部署。

这个方法为Java类的动态加载／卸载提供了一个思路。由于它需要为每一个需要部署的服务起动一个线程，虽然该线程只负责装载和卸载服务，服务运行时并不消耗CPU，但会固定占用一定大小的内存。测试值为每线程约占用350k内存。因此服务多时，存在StackOverflowError的风险。

在JDK7中据说提供了anonymous classloading 机制来支持动态类加载／卸载。需要使用的同学可以关注一下。