1. CountDownLatch的介绍
CountDownLatch是一个同步工具,它主要用线程执行之间的协作。CountDownLatch 的作用和 Thread.join() 方法类似,让一些线程阻塞直到另一些线程完成一系列操作后才被唤醒。在直接创建线程的年代(Java 5.0 之前),我们可以使用 Thread.join()。在线程池出现后,因为线程池中的线程不能直接被引用,所以就必须使用 CountDownLatch 了。
CountDownLatch主要有两个方法,当一个或多个线程调用await方法时,这些线程会阻塞。其它线程调用countDown方法会将计数器减1(调用countDown方法的线程不会阻塞),当计数器的值变为0时,因await方法阻塞的线程会被唤醒,继续执行。
实现原理:计数器的值由构造函数传入,并用它初始化AQS的state值。当线程调用await方法时会检查state的值是否为0,如果是就直接返回(即不会阻塞);如果不是,将表示该节点的线程入列,然后将自身阻塞。当其它线程调用countDown方法会将计数器减1,然后判断计数器的值是否为0,当它为0时,会唤醒队列中的第一个节点,由于CountDownLatch使用了AQS的共享模式,所以第一个节点被唤醒后又会唤醒第二个节点,以此类推,使得所有因await方法阻塞的线程都能被唤醒而继续执行。
从源代码和实现原理中可以看出一个CountDownLatch对象,只能使用一次,不能重复使用。
await方法源码
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public
void
await()
throws
InterruptedException {
sync.acquireSharedInterruptibly(
1
);
}
public
final
void
acquireSharedInterruptibly(
int
arg)
throws
InterruptedException {
if
(Thread.interrupted())
throw
new
InterruptedException();
if
(tryAcquireShared(arg) <
0
)
doAcquireSharedInterruptibly(arg);
}
protected
int
tryAcquireShared(
int
acquires) {
return
(getState() ==
0
) ?
1
: -
1
;
}
|
doAcquireSharedInterruptibly 主要实现线程的入列与阻塞。
countDown方法
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public
void
countDown() {
sync.releaseShared(
1
);
}
public
final
boolean
releaseShared(
int
arg) {
if
(tryReleaseShared(arg)) {
doReleaseShared();
return
true
;
}
return
false
;
}
protected
boolean
tryReleaseShared(
int
releases) {
// Decrement count; signal when transition to zero
for
(;;) {
int
c = getState();
if
(c ==
0
)
return
false
;
int
nextc = c-
1
;
if
(compareAndSetState(c, nextc))
return
nextc ==
0
;
}
}
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doReleaseShared主要实现唤醒第一个节点,第一个节点有会唤醒第二个节点,……。
2. 使用示例
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package
demo;
import
java.util.Random;
import
java.util.concurrent.CountDownLatch;
import
java.util.concurrent.ExecutorService;
import
java.util.concurrent.Executors;
public
class
CountDownLatchDemo {
private
CountDownLatch cdl =
new
CountDownLatch(
2
);
private
Random rnd =
new
Random();
class
FirstTask
implements
Runnable{
private
String id;
public
FirstTask(String id){
this
.id = id;
}
@Override
public
void
run(){
System.out.println(
"Thread "
+ id +
" is start"
);
try
{
Thread.sleep(rnd.nextInt(
1000
));
}
catch
(InterruptedException e) {
e.printStackTrace();
}
System.out.println(
"Thread "
+ id +
" is over"
);
cdl.countDown();
}
}
class
SecondTask
implements
Runnable{
private
String id;
public
SecondTask(String id){
this
.id = id;
}
@Override
public
void
run(){
try
{
cdl.await();
}
catch
(InterruptedException e) {
e.printStackTrace();
}
System.out.println(
"----------Thread "
+ id +
" is start"
);
try
{
Thread.sleep(rnd.nextInt(
1000
));
}
catch
(InterruptedException e) {
e.printStackTrace();
}
System.out.println(
"----------Thread "
+ id +
" is over"
);
}
}
public
static
void
main(String[] args){
ExecutorService es = Executors.newCachedThreadPool();
CountDownLatchDemo cdld =
new
CountDownLatchDemo();
es.submit(cdld.
new
SecondTask(
"c"
));
es.submit(cdld.
new
SecondTask(
"d"
));
es.submit(cdld.
new
FirstTask(
"a"
));
es.submit(cdld.
new
FirstTask(
"b"
));
es.shutdown();
}
}
|
在这个示例中,我们创建了四个线程a、b、c、d,这四个线程几乎同时提交给了线程池。c线程和d线程会在a线程和b线程结束后开始执行。
运行结果
Thread a is start
Thread b is start
Thread b is over
Thread a is over
----------Thread c is start
----------Thread d is start
----------Thread d is over
----------Thread c is over