Java实现生命周期管理机制

public abstract class AbstractLifecycle implements ILifecycle {

    private List<ILifecycleListener> listeners = new CopyOnWriteArrayList<ILifecycleListener>();

    /**
     * state 代表当前生命周期状态
     */
    private LifecycleState state = LifecycleState.NEW;

    /*
     * @see ILifecycle#init()
     */
    @Override
    public final synchronized void init() throws LifecycleException {
        if (state != LifecycleState.NEW) {
            return;
        }

        setStateAndFireEvent(LifecycleState.INITIALIZING);
        try {
            init0();
        } catch (Throwable t) {
            setStateAndFireEvent(LifecycleState.FAILED);
            if (t instanceof LifecycleException) {
                throw (LifecycleException) t;
            } else {
                throw new LifecycleException(formatString(
                        "Failed to initialize {0}， Error Msg: {1}"， toString()， t.getMessage())， t);
            }
        }
        setStateAndFireEvent(LifecycleState.INITIALIZED);
    }

    protected abstract void init0() throws LifecycleException;

    /*
     * @see ILifecycle#start()
     */
    @Override
    public final synchronized void start() throws LifecycleException {
        if (state == LifecycleState.NEW) {
            init();
        }

        if (state != LifecycleState.INITIALIZED) {
            return;
        }

        setStateAndFireEvent(LifecycleState.STARTING);
        try {
            start0();
        } catch (Throwable t) {
            setStateAndFireEvent(LifecycleState.FAILED);
            if (t instanceof LifecycleException) {
                throw (LifecycleException) t;
            } else {
                throw new LifecycleException(formatString("Failed to start {0}， Error Msg: {1}"，
                        toString()， t.getMessage())， t);
            }
        }
        setStateAndFireEvent(LifecycleState.STARTED);
    }

    protected abstract void start0() throws LifecycleException;

    /*
     * @see ILifecycle#suspend()
     */
    @Override
    public final synchronized void suspend() throws LifecycleException {
        if (state == LifecycleState.SUSPENDING || state == LifecycleState.SUSPENDED) {
            return;
        }

        if (state != LifecycleState.STARTED) {
            return;
        }

        setStateAndFireEvent(LifecycleState.SUSPENDING);
        try {
            suspend0();
        } catch (Throwable t) {
            setStateAndFireEvent(LifecycleState.FAILED);
            if (t instanceof LifecycleException) {
                throw (LifecycleException) t;
            } else {
                throw new LifecycleException(formatString("Failed to suspend {0}， Error Msg: {1}"，
                        toString()， t.getMessage())， t);
            }
        }
        setStateAndFireEvent(LifecycleState.SUSPENDED);
    }

    protected abstract void suspend0() throws LifecycleException;

    /*
     * @see ILifecycle#resume()
     */
    @Override
    public final synchronized void resume() throws LifecycleException {
        if (state != LifecycleState.SUSPENDED) {
            return;
        }

        setStateAndFireEvent(LifecycleState.RESUMING);
        try {
            resume0();
        } catch (Throwable t) {
            setStateAndFireEvent(LifecycleState.FAILED);
            if (t instanceof LifecycleException) {
                throw (LifecycleException) t;
            } else {
                throw new LifecycleException(formatString("Failed to resume {0}， Error Msg: {1}"，
                        toString()， t.getMessage())， t);
            }
        }
        setStateAndFireEvent(LifecycleState.RESUMED);
    }

    protected abstract void resume0() throws LifecycleException;

    /*
     * @see ILifecycle#destroy()
     */
    @Override
    public final synchronized void destroy() throws LifecycleException {
        if (state == LifecycleState.DESTROYING || state == LifecycleState.DESTROYED) {
            return;
        }

        setStateAndFireEvent(LifecycleState.DESTROYING);
        try {
            destroy0();
        } catch (Throwable t) {
            setStateAndFireEvent(LifecycleState.FAILED);
            if (t instanceof LifecycleException) {
                throw (LifecycleException) t;
            } else {
                throw new LifecycleException(formatString("Failed to destroy {0}， Error Msg: {1}"，
                        toString()， t.getMessage())， t);
            }
        }
        setStateAndFireEvent(LifecycleState.DESTROYED);
    }

    protected abstract void destroy0() throws LifecycleException;

    /*
     * @see
     * ILifecycle#addLifecycleListener(ILifecycleListener)
     */
    @Override
    public void addLifecycleListener(ILifecycleListener listener) {
        listeners.add(listener);
    }

    /*
     * @see
     * ILifecycle#removeLifecycleListener(ILifecycleListener)
     */
    @Override
    public void removeLifecycleListener(ILifecycleListener listener) {
        listeners.remove(listener);
    }

    private void fireLifecycleEvent(LifecycleEvent event) {
        for (Iterator<ILifecycleListener> it = listeners.iterator(); it.hasNext();) {
            ILifecycleListener listener = it.next();
            listener.lifecycleEvent(event);
        }
    }

    protected synchronized LifecycleState getState() {
        return state;
    }

    private synchronized void setStateAndFireEvent(LifecycleState newState) throws LifecycleException {
        state = newState;
        fireLifecycleEvent(new LifecycleEvent(state));
    }

    private String formatString(String pattern， Object... arguments) {
        return MessageFormat.format(pattern， arguments);
    }

    /*
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString() {
        return getClass().getSimpleName();
    }
}

　　可以看到，抽象类的骨架实现中做了几件生命周期管理中通用的事情，检查状态之间的转换是否合法(比如说start之前必须要init)，设置内部状态，以及触发相应的监听者。
　　抽象类实现了ILifeCycle定义的方法后，又留出了相应的抽象方法供其子类实现，如上面的代码所示，其留出来的抽象方法有以下这些:

protected abstract void init0() throws LifecycleException;
protected abstract void start0() throws LifecycleException;
protected abstract void suspend0() throws LifecycleException;
protected abstract void resume0() throws LifecycleException;
protected abstract void destroy0() throws LifecycleException;

　　优雅的实现
　　到目前为止，我们已经定义了接口ILifeCycle，以及其骨架实现AbstractLifeCycle，并且增加了监听者机制。貌似我们可以开始写一个类来继承AbstractLifecycle，并重写其定义的抽象方法了，so far so good。
　　但在开始之前，我们还需要考虑另外几个问题，
　　我们的实现类是否对所有的抽象方法都感兴趣？
　　是否每个实现累都需要实现init0， start0， suspend0， resume0， destroy0?
　　是否有时候，我们的那些有生命的类或者模块并不支持暂停(suspend)，恢复(resume)?
　　直接继承AbstractLifeCycle，意味着必须实现其全部的抽象方法。
　　因此，我们还需要一个默认实现，DefaultLifeCycle，让它继承AbstractLifeCycle，并实现所有抽象方法，但它并不做任何实际的事情， do nothing。只是让我们真正的实现类来继承这个默认的实现类，并重写感兴趣的方法。
　　于是，我们的DefaultLifeCycle这么诞生了:

public class DefaultLifecycle extends AbstractLifecycle {

    /*
     * @see AbstractLifecycle#init0()
     */
    @Override
    protected void init0() throws LifecycleException {
        // do nothing
    }

    /*
     * @see AbstractLifecycle#start0()
     */
    @Override
    protected void start0() throws LifecycleException {
        // do nothing
    }

    /*
     * @see AbstractLifecycle#suspend0()
     */
    @Override
    protected void suspendInternal() throws LifecycleException {
        // do nothing
    }

    /*
     * @see AbstractLifecycle#resume0()
     */
    @Override
    protected void resume0() throws LifecycleException {
        // do nothing
    }

    /*
     * @see AbstractLifecycle#destroy0()
     */
    @Override
    protected void destroy0() throws LifecycleException {
        // do nothing
    }

}

　　对于DefaultLifeCycle来说，do nothing是其职责。
　　因此接下来我们可以写一个自己的实现类，继承DefaultLifeCycle，并重写那些感兴趣的生命周期方法。
　　例如，我有一个类只需要在初始化，启动，和销毁时做一些任务，那么可以这么写:

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

public class SocketServer extends DefaultLifecycle {

    private ServerSocket acceptor = null;
    private int port = 9527;
    /* 
     * @see DefaultLifecycle#init0()
     */
    @Override
    protected void init0() throws LifecycleException {
        try {
            acceptor = new ServerSocket(port);
        } catch (IOException e) {
            throw new LifecycleException(e);
        }
    }

    /* 
     * @see DefaultLifecycle#start0()
     */
    @Override
    protected void start0() throws LifecycleException {
        Socket socket = null;
        try {
            socket = acceptor.accept();
            //do something with socket

        } catch (IOException e) {
            throw new LifecycleException(e);
        } finally {
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }

    /* 
     * @see DefaultLifecycle#destroy0()
     */
    @Override
    protected void destroy0() throws LifecycleException {
        if (acceptor != null) {
            try {
                acceptor.close();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

　　这里的ServerSocket中，init0初始化socket监听，start0开始获取socket连接， destroy0销毁socket监听。
　　在这套生命周期管理机制下，我们将会很容易地对资源进行管理，不会发生资源未关闭的情况，架构和模块化更加清晰。
　　尾声
　　到这里为止，本文已经实现了一个简易的生命周期管理机制，并给出所有的实现代码。之后会将所有源代码放到github上。请关注本文的update。