[英]Impossible NullPointerException thrown
在幾年前編寫的一個Web服務器項目中,我偶爾會遇到一個非常奇怪的-在我看來-不可能的NullPointerException
。 它以實用程序方法發生,用於在控制台上記錄輸出。
這是該方法的錯誤摘錄:
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BufferedWriter
。 NPE被拋出在調用logWriter.close()
的第一個catch
塊中。
但是:當logWriter為!= null
,如何在try
塊中引發IOException
? Base64.getEncoder()
無法引發IOException
並且沒有其他代碼可以執行。
這是我的堆棧跟蹤:
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synchronized
,因此互斥不會造成任何困難,而且我不知道為什么會發生這種情況。
簡短答案:
重寫您的catch
塊,以避免NullPointerException
try {
if(logWriter != null) {
logWriter.close();
}
} catch (IOException e1) {
println("### Possible resource leak; unable to close log writer", Color.RED);
}
長答案:
是的,logWriter是靜態的。...不同步會阻止對方法執行並行方法嗎?
沒有。
需要證明嗎?
運行此代碼:
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) {
}
}
}
輸出是這樣的:
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)
Howsa?
同步方法將獲得對該方法的調用者的鎖定。 含義t1
不在乎t2
(當然也logWriter
)。
而已。 在這里閱讀全文 。
另外,很久以前,我為我一直使用的Util類編寫了這樣的5襯里。 它使我Closeables
編寫用於關閉Closeables
catch
塊
public static void close(Closeable closeable) {
if (closeable != null) {
try {
closeable.close();
}
catch (IOException e) {
//logging
}
}
}
無論如何,Java 1.7引入了您可能想探索的AutoCloseable
(由BufferedWriter
實現)。
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