java - 抛出不可能的 NullPointerException

Question

在几年前编写的一个网络服务器项目中，我偶尔会遇到一个非常奇怪且在我看来不可能的NullPointerException。它发生在用于在控制台上记录输出的实用程序方法中。

这是该方法的错误摘录:

try {
    Encoder encoder = Base64.getEncoder();
    if(logWriter != null) {
        logWriter.write(String.valueOf(System.currentTimeMillis()));
        logWriter.write(" ");
        logWriter.write(String.valueOf(level));
        logWriter.write(" ");
        logWriter.write(encoder.encodeToString(Thread.currentThread().getName().getBytes()).replaceAll("(?:\\r\\n|\\n\\r|\\n|\\r)", ""));
        logWriter.write(" ");
        logWriter.write(encoder.encodeToString(log.getBytes()).replaceAll("(?:\\r\\n|\\n\\r|\\n|\\r)", ""));
        logWriter.write("\r\n");
        logWriter.flush();
    }

    lastWriterActivity = System.currentTimeMillis();
} catch (IOException e) {
    println("Failed to write log to file: " + e, Color.RED);

    try {
        logWriter.close();
    } catch (IOException e1) {
        println("### Possible resource leak; unable to close log writer", Color.RED);
    }

    logWriter = null;
}

其中logWriter是一个BufferedWriter。 NPE 在调用 logWriter.close() 的第一个 catch block 中抛出。

但是:当我的 logWriter 为 != null 时，如何在我的 try block 中抛出 IOException ？ Base64.getEncoder() 无法抛出 IOException 并且没有其他代码要执行。

这是我的堆栈跟踪:

Exception in thread "connection_0:0:0:0:0:0:0:1@1544725509" java.lang.NullPointerException
at org.jpuzzle.main.Logger.write(Logger.java:347)
at org.jpuzzle.main.Logger.verbose(Logger.java:187)
at org.jpuzzle.protocol.http.HttpRequest.onRequest(HttpRequest.java:1090)
at org.jpuzzle.network.ConnectionListener$Connection.proceed(ConnectionListener.java:438)
at org.jpuzzle.network.ConnectionListener$Connection.run(ConnectionListener.java:408)

我的方法是同步，因此互斥应该不会造成任何困难，而且我不知道为什么会发生这种情况。

Answer 1

简短回答:

重写您的 catch block 以避免 NullPointerException

try {
  if(logWriter != null) {
    logWriter.close();
  }
} catch (IOException e1) {
    println("### Possible resource leak; unable to close log writer", Color.RED);
}

长答案:

Yes, logWriter is static.... Doesn't synchronized prevent parralel method execution for a method?

没有。

需要证明吗？

运行此代码:

public class NowImFeelingZombified {
  static Object logWriter = new Object();

public static void main(String[] args) {
    final NowImFeelingZombified zombie1 = new NowImFeelingZombified();
    final NowImFeelingZombified zombie2 = new NowImFeelingZombified();

    Thread t1 = new Thread("zombie1 ") {
        @Override
        public void run() {
            zombie1.syncedMethod();
        }
    };
    Thread t2 = new Thread("zombie2 ") {
        @Override
        public void run() {
            zombie2.syncedMethod();
        }
    };
    t1.start();
    t2.start();
}

private synchronized void syncedMethod() {
    try {
        System.out.println(Thread.currentThread().getName() + logWriter.toString());

        Thread.sleep(2000L);

        System.out.println(Thread.currentThread().getName() + logWriter.toString());

        logWriter = null;
    }
    catch (InterruptedException e) {
    }
  }
}

输出是这样的:

zombie1 java.lang.Object@1c34796b
zombie2 java.lang.Object@1c34796b
zombie1 java.lang.Object@1c34796b
Exception in thread "zombie2 " java.lang.NullPointerException
    at NowImFeelingZombified.syncedMethod(NowImFeelingZombified.java:44)

怎么样？

同步方法会获取方法调用者的锁。这意味着 t1 不关心 t2 (当然也不关心 logWriter)。

就是这样。阅读此处了解 full story .

此外，很久以前，我为我一直使用的 Util 类编写了这样的 5 行代码。它使我无需编写用于关闭 Closeables

的 catch block

public static void close(Closeable closeable) {
    if (closeable != null) {
        try {
            closeable.close();
        }
        catch (IOException e) {
            //logging
        }
    }
}

不管怎样，Java 1.7已经推出了AutoCloseable (由 BufferedWriter 实现)您可能想探索一下。