Java - 如何让记录器在关闭挂钩中工作？

Question

我有一个专门的记录器类，它使用java.util.logging.Logger 类。我希望能够在另一个类的关闭挂钩中使用这个记录器。但是，它似乎没有在关机时登录。根据我的阅读，可能已经为记录器本身激活了一个关闭挂钩，这导致了问题。

我怎样才能让它工作？理想情况下，我希望在日志文件中看到我实际上在进程终止时执行了关闭挂钩。

Answer 1

再次查看源代码，解决方案似乎是定义一个系统属性 java.util.logging.manager，它是 LogManager 的子类，它覆盖了 reset(); 方法，因此 Loggers 在关闭时继续工作。

import java.util.logging.LogManager;
import java.util.logging.Logger;

public class Main {
    static {
        // must be called before any Logger method is used.
        System.setProperty("java.util.logging.manager", MyLogManager.class.getName());
    }

    public static class MyLogManager extends LogManager {
        static MyLogManager instance;
        public MyLogManager() { instance = this; }
        @Override public void reset() { /* don't reset yet. */ }
        private void reset0() { super.reset(); }
        public static void resetFinally() { instance.reset0(); }
    }

    public static void main(String... args) {
        Logger logger1 = Logger.getLogger("Main1");
        logger1.info("Before shutdown");
        Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Logger logger2 = Logger.getLogger("Main2");
                    logger2.info("Shutting down 2");

                } finally {
                    MyLogManager.resetFinally();
                }
            }
        }));
    }
}

打印

Dec 11, 2012 5:56:55 PM Main main
INFO: Before shutdown
Dec 11, 2012 5:56:55 PM Main$1 run
INFO: Shutting down 2

---

从 LogManager 的这段代码中，您可以看到有一个关闭挂钩，它可以拆除处理程序并关闭它们。记录器只有在之前没有使用过的情况下才能在关机状态下工作，因此这段代码不会运行。

// This private class is used as a shutdown hook.
// It does a "reset" to close all open handlers.
private class Cleaner extends Thread {

    private Cleaner() {
        /* Set context class loader to null in order to avoid
         * keeping a strong reference to an application classloader.
         */
        this.setContextClassLoader(null);
    }

    public void run() {
        // This is to ensure the LogManager.<clinit> is completed
        // before synchronized block. Otherwise deadlocks are possible.
        LogManager mgr = manager;

        // If the global handlers haven't been initialized yet, we
        // don't want to initialize them just so we can close them!
        synchronized (LogManager.this) {
            // Note that death is imminent.
            deathImminent = true;
            initializedGlobalHandlers = true;
        }

        // Do a reset to close all active handlers.
        reset();
    }
}


/**
 * Protected constructor.  This is protected so that container applications
 * (such as J2EE containers) can subclass the object.  It is non-public as
 * it is intended that there only be one LogManager object, whose value is
 * retrieved by calling Logmanager.getLogManager.
 */
protected LogManager() {
    // Add a shutdown hook to close the global handlers.
    try {
        Runtime.getRuntime().addShutdownHook(new Cleaner());
    } catch (IllegalStateException e) {
        // If the VM is already shutting down,
        // We do not need to register shutdownHook.
    }
}

来自我自己的测试

Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            Logger logger2 = Logger.getLogger("Main2");
            logger2.info("Shutting down 2");
        } catch (Throwable t) {
            t.printStackTrace();
        }
    }
}));

打印

Dec 11, 2012 5:40:15 PM Main$1 run
INFO: Shutting down 2

但如果你添加

Logger logger1 = Logger.getLogger("Main1");

在这个街区之外你什么也得不到。

Answer 2

关闭钩子有什么好处吗？

最好在 main 的最后一行添加 Log 输出： finally 块将在所有情况下执行， 除非 VM 崩溃但随后关闭挂钩将 也不会被执行。

static void main(String[] argv) {

  try {
    startApp();
  } finally {
    LOGGER.info("shudown");
  }
}