java(JVM)结束时释放JNI资源(Runtime.addShutdownHook)

如下代码中cmjnidrv是一个动态库，CodeCacheManager在类加载时就自动将cmjnidrv加载进来。cmjnidrv中有多个独立的线程在运行并申请了大量的内存.
release()是个native方法,用于释放cmjnidrv中的申请资源并中止cmjnidrv中的线程。
为了保证在java应用结束的时候cmjnidrv的资源能被正确释放，就必须确保release()被调用。

public class CodeCacheManager{
	private static final Logger logger = LoggerFactory.getLogger(CodeCacheManager.class);

	static{
		System.loadLibrary("cmjnidrv");	
	}
	private static final CodeCacheManager instance=new CodeCacheManager();
	static public CodeCacheManager getInstance(){
		return instance;
	}
	private CodeCacheManager(){
	}
	/**
	 * 释放所有JNI资源
	 */
	public native static  void release();
	/* （非 Javadoc）
	 * @see java.lang.Object#finalize()
	 */
	@Override
	protected void finalize() throws Throwable {
		logger.info("release JNI resoure...");
		release();
		logger.info("release JNI resoure finished");
	}
}

原本的设计是在finalize()方法中执行release()方法。这样CodeCacheManager对象被垃圾回收器回收的时候会自动释放JNI资源，但是CodeCacheManager是个单例(singleton)的class，instance静态成员变量永远保持着对单例对象的引用，所以JAVA的垃圾回收器永远不可能去调用finalize()。
所以这个方案不可行，事实也是正是这样，执行shutdown.sh停止tomcat服务器时，执行top命令显示，java进程仍然在运行中，而且占用着大量的内存。也没有log "release JNI resoure…"输出，显示finalize()方法没有被执行。
所以如何在JVM结束时释放静态加载的动态库中的资源还是得另想办法。

这时 void java.lang.Runtime.addShutdownHook(Thread hook)就派上用场了。下面是addShutdownHook方法的原文说明(部分)：

Registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down in response to two kinds of events:
The program exits normally, when the last non-daemon thread exits or when the exit (equivalently, System.exit) method is invoked, or
The virtual machine is terminated in response to a user interrupt, such as typing ^C, or a system-wide event, such as user logoff or system shutdown.

当JVM结束时（正常结束，或被Ctrl-C中止，或因系统事件(如logoff ,shutdown)）会执行用addShutdownHook方法注册的线程。
所以确保静态加载的JNI资源释放的办法，就是在加载动态库的时候，向JVM注册一个hook线程，用于执行release()方法。

public class CodeCacheManager{
	private static final Logger logger = LoggerFactory.getLogger(CodeCacheManager.class);

	static{
		// add hook for release JNI resource on JVM shutdown 
		Runtime.getRuntime().addShutdownHook(new Thread (){
			@Override
			public void run() {
				logger.info("release JNI resoure...");
				relase();
				logger.info("release JNI resoure finished");
			}});
		System.loadLibrary("cmjnidrv");	
	}
	private static final CodeCacheManager instance=new CodeCacheManager();
	static public CodeCacheManager getInstance(){
		return instance;
	}
	private CodeCacheManager(){
	}
	/**
	 * 释放所有JNI资源
	 */
	public native static  void release();
}