我一直在阅读有关ThreadLocal的文章,试图了解它的工作原理以及我们为什么需要它。
到目前为止,我已经能够学到以下内容:
一切似乎都很好,直到我尝试从javadoc运行示例之前,提供的代码如下:
import java.util.concurrent.atomic.AtomicInteger;
public class UniqueThreadIdGenerator {
private static final AtomicInteger uniqueId = new AtomicInteger(0);
private static final ThreadLocal < Integer > uniqueNum =
new ThreadLocal < Integer > () {
@Override protected Integer initialValue() {
return uniqueId.getAndIncrement();
}
};
public static int getCurrentThreadId() {
return uniqueId.get();
}
} // UniqueThreadIdGenerator
如果我正确理解此代码,则调用getCurrentThreadId()应该返回正确的自动递增线程号,因为它为我返回了0。始终为0,不考虑我已启动了多少个线程。
为了使它对我有用,我不得不将getCurrentThreadId()更改为读取
public static int getCurrentThreadId() {
return uniqueId.get();
}
在这种情况下,我会得到正确的值。
下面提供了我的代码,我缺少什么?(不是javadoc实际上是错误的,对吗?)
package org.vekslers;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
public class UniqueThreadIdGenerator extends Thread {
private static final AtomicInteger uniqueId = new AtomicInteger(0);
private static final ThreadLocal <Integer> uniqueNum =
new ThreadLocal <Integer> () {
@Override protected Integer initialValue() {
return uniqueId.getAndIncrement();
}
};
public static int getCurrentThreadId() {
return uniqueNum.get();
}
//////////////////////////////////////////////////
// Testing code...
//////////////////////////////////////////////////
private static volatile boolean halt = false;
public UniqueThreadIdGenerator(String threadName) {
super(threadName);
}
@Override
public void run() {
System.out.println(Thread.currentThread() + " PREHALT " + getCurrentThreadId());
while(!halt)
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread() + " POSTHALT " + getCurrentThreadId());
}
public static void main(String[] args) throws InterruptedException {
Thread t1 = new UniqueThreadIdGenerator("t1");
Thread t2 = new UniqueThreadIdGenerator("t2");
Thread t3 = new UniqueThreadIdGenerator("t3");
Thread t4 = new UniqueThreadIdGenerator("t4");
t3.start();
t1.start();
t2.start();
t4.start();
TimeUnit.SECONDS.sleep(10);
halt = true;
}
} // UniqueThreadIdGenerator
输出:
Thread[t3,5,main] PREHALT 0
Thread[t1,5,main] PREHALT 1
Thread[t2,5,main] PREHALT 2
Thread[t4,5,main] PREHALT 3
Thread[t4,5,main] POSTHALT 3
Thread[t2,5,main] POSTHALT 2
Thread[t1,5,main] POSTHALT 1
Thread[t3,5,main] POSTHALT 0
ps代码注释OT或至此是受欢迎的注释。
javadocs是错误的。
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6475885
import java.util.concurrent.atomic.AtomicInteger;
public class ThreadId {
// Atomic integer containing the next thread ID to be assigned
private static final AtomicInteger nextId = new AtomicInteger(0);
// Thread local variable containing each thread's ID
private static final ThreadLocal<Integer> threadId =
new ThreadLocal<Integer>() {
@Override protected Integer initialValue() {
return nextId.getAndIncrement();
}
};
// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
return threadId.get();
}
}
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