遗憾的是,在
Java中对String使用正则表达式时无法指定超时.因此,如果您没有严格控制哪些模式应用于哪个输入,您可能最终会拥有消耗大量cpu的线程,同时无休止地尝试将(不那么精心设计的)模式与(恶意？)输入匹配.
我知道为什么弃用Thread#stop()的原因(见http://download.oracle.com/javase/1.5.0/docs/guide/misc/threadPrimitiveDeprecation.html).它们以可能在ThreadDeath异常情况下被损坏的对象为中心,然后污染正在运行的JVM环境并可能导致细微的错误.
我对这个对JVM工作有更深入了解的人的问题是：如果需要停止的线程没有任何(明显的)监视器或对程序其余部分使用的对象的引用,那么可以使用Thread#stop()吗？
我创建了一个相当防御的解决方案,能够处理与超时匹配的正则表达式.我会很高兴任何评论或评论,尤其是尽管我努力避免它们,这种方法可能导致的问题.
谢谢！
import java.util.concurrent.Callable;
public class SafeRegularExpressionMatcher {
// demonstrates behavior for regular expression running into catastrophic backtracking for given input
public static void main(String[] args) {
SafeRegularExpressionMatcher matcher = new SafeRegularExpressionMatcher(
"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx","(x+x+)+y",2000);
System.out.println(matcher.matches());
}
final String stringToMatch;
final String regularExpression;
final int timeoutMillis;
public SafeRegularExpressionMatcher(String stringToMatch,String regularExpression,int timeoutMillis) {
this.stringToMatch = stringToMatch;
this.regularExpression = regularExpression;
this.timeoutMillis = timeoutMillis;
}
public Boolean matches() {
CallableThread<Boolean> thread = createSafeRegularExpressionMatchingThread();
Boolean result = tryToGetResultFromThreadWithTimeout(thread);
return result;
}
private CallableThread<Boolean> createSafeRegularExpressionMatchingThread() {
final String stringToMatchForUseInThread = new String(stringToMatch);
final String regularExpressionForUseInThread = new String(regularExpression);
Callable<Boolean> callable = createRegularExpressionMatchingCallable(stringToMatchForUseInThread,regularExpressionForUseInThread);
CallableThread<Boolean> thread = new CallableThread<Boolean>(callable);
return thread;
}
private Callable<Boolean> createRegularExpressionMatchingCallable(final String stringToMatchForUseInThread,final String regularExpressionForUseInThread) {
Callable<Boolean> callable = new Callable<Boolean>() {
public Boolean call() throws Exception {
return Boolean.valueOf(stringToMatchForUseInThread.matches(regularExpressionForUseInThread));
}
};
return callable;
}
private Boolean tryToGetResultFromThreadWithTimeout(CallableThread<Boolean> thread) {
startThreadAndApplyTimeout(thread);
Boolean result = processthreadResult(thread);
return result;
}
private void startThreadAndApplyTimeout(CallableThread<Boolean> thread) {
thread.start();
try {
thread.join(timeoutMillis);
} catch (InterruptedException e) {
throwRuntimeException("Interrupt",e);
}
}
private Boolean processthreadResult(CallableThread<Boolean> thread) {
Boolean result = null;
if (thread.isAlive()) {
killThread(thread); // do not use anything from the thread anymore,objects may be damaged!
throwRuntimeException("Timeout",null);
} else {
Exception exceptionOccurredInThread = thread.getException();
if (exceptionOccurredInThread != null) {
throwRuntimeException("Exception",exceptionOccurredInThread);
} else {
result = thread.getResult();
}
}
return result;
}
private void throwRuntimeException(String situation,Exception e) {
throw new RuntimeException(situation + " occured while applying pattern /" + regularExpression + "/ to input '"
+ stringToMatch + " after " + timeoutMillis + "ms!",e);
}
/**
* This method uses {@link Thread#stop()} to kill a thread that is running wild. Although it is ackNowledged that
* {@link Thread#stop()} is inherently unsafe,the assumption is that the thread to kill does not hold any monitors on or
* even references to objects referenced by the rest of the JVM,so it is acceptable to do this.
*
* After calling this method nothing from the thread should be used anymore!
*
* @param thread Thread to stop
*/
@SuppressWarnings("deprecation")
private static void killThread(CallableThread<Boolean> thread) {
thread.stop();
}
private static class CallableThread<V> extends Thread {
private final Callable<V> callable;
private V result = null;
private Exception exception = null;
public CallableThread(Callable<V> callable) {
this.callable = callable;
}
@Override
public void run() {
try {
V result = compute();
setResult(result);
} catch (Exception e) {
exception = e;
} catch (ThreadDeath e) {
cleanup();
}
}
private V compute() throws Exception {
return callable.call();
}
private synchronized void cleanup() {
result = null;
}
private synchronized void setResult(V result) {
this.result = result;
}
public synchronized V getResult() {
return result;
}
public synchronized Exception getException() {
return exception;
}
}
}
编辑：
感谢dawce指出我this solution我已经能够解决我原来的问题,而无需额外的线程.我在那里发布了代码.感谢所有回复的人.
