Java多线程 ReentrantLock互斥锁详解
作者：慢慢来
这篇文章主要介绍了Java多线程 ReentrantLock互斥锁详解,文中通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值,需要的朋友可以参考下
加锁和解锁
我们来看下ReentrantLock的基本用法
ThreadDomain35类
public class ThreadDomain35 {
private Lock lock = new ReentrantLock();
public void testMethod()
{
try
{
lock.lock();
for (int i = 0; i < 2; i++)
{
System.out.println("ThreadName = " + Thread.currentThread().getName() + ", i = " + i);
}
}
finally
{
lock.unlock();
}
}
}
线程和main方法
public class MyThread35 extends Thread {
private ThreadDomain35 td;
public MyThread35(ThreadDomain35 td)
{
this.td = td;
}
public void run()
{
td.testMethod();
}
public static void main(String[] args)
{
ThreadDomain35 td = new ThreadDomain35();
MyThread35 mt0 = new MyThread35(td);
MyThread35 mt1 = new MyThread35(td);
MyThread35 mt2 = new MyThread35(td);
mt0.start();
mt1.start();
mt2.start();
}
}
输出结果
ThreadName = Thread-2, i = 0 ThreadName = Thread-2, i = 1 ThreadName = Thread-0, i = 0 ThreadName = Thread-0, i = 1 ThreadName = Thread-1, i = 0 ThreadName = Thread-1, i = 1
一个线程必须执行完才能执行下一个线程，说明ReentrantLock可以加锁。
ReentrantLock持有的对象监视器和synchronized不同
ThreadDomain37类,methodB用synchronized修饰
public class ThreadDomain37 {
private Lock lock = new ReentrantLock();
public void methodA()
{
try
{
lock.lock();
System.out.println("MethodA begin ThreadName = " + Thread.currentThread().getName());
Thread.sleep(5000);
System.out.println("MethodA end ThreadName = " + Thread.currentThread().getName());
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
lock.unlock();
}
}
public synchronized void methodB()
{
System.out.println("MethodB begin ThreadName = " + Thread.currentThread().getName());
System.out.println("MethodB begin ThreadName = " + Thread.currentThread().getName());
}
}
MyThread37_0类
public class MyThread37_0 extends Thread {
private ThreadDomain37 td;
public MyThread37_0(ThreadDomain37 td)
{
this.td = td;
}
public void run()
{
td.methodA();
}
}
MyThread37_1类
public class MyThread37_1 extends Thread {
private ThreadDomain37 td;
public MyThread37_1(ThreadDomain37 td)
{
this.td = td;
}
public void run()
{
td.methodB();
}
}
MyThread37_main方法
public class MyThread37_main {
public static void main(String[] args)
{
ThreadDomain37 td = new ThreadDomain37();
MyThread37_0 mt0 = new MyThread37_0(td);
MyThread37_1 mt1 = new MyThread37_1(td);
mt0.start();
mt1.start();
}
}
运行结果如下
MethodA begin ThreadName = Thread-0 MethodB begin ThreadName = Thread-1 MethodB begin ThreadName = Thread-1 MethodA end ThreadName = Thread-0
加了synchronized依然是异步执行，说明ReentrantLock和synchronized持有的对象监视器不同。ReentrantLock需要手动加锁和释放锁。
Condition
基本用法
synchronized与wait()和nitofy()/notifyAll()方法可以实现等待/唤醒模型，ReentrantLock同样可以，需要借助Condition的await()和signal/signalAll()，await()释放锁。
ThreadDomain38类
public class ThreadDomain38 {
private Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
public void await()
{
try
{
lock.lock();
System.out.println("await时间为：" + System.currentTimeMillis());
condition.await();
System.out.println("await等待结束");
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
lock.unlock();
}
}
public void signal()
{
try
{
lock.lock();
System.out.println("signal时间为：" + System.currentTimeMillis());
condition.signal();
System.out.println("signal等待结束");
}
finally
{
lock.unlock();
}
}
}
MyThread38类，线程和main方法
public class MyThread38 extends Thread
{
private ThreadDomain38 td;
public MyThread38(ThreadDomain38 td)
{
this.td = td;
}
public void run()
{
td.await();
}
public static void main(String[] args) throws Exception
{
ThreadDomain38 td = new ThreadDomain38();
MyThread38 mt = new MyThread38(td);
mt.start();
Thread.sleep(3000);
td.signal();
}
}
运行结果如下
await时间为：1563505465346 signal时间为：1563505468345 signal等待结束 await等待结束
可以看到，ReentrantLock和Condition实现了等待/通知模型。
一个Lock可以创建多个Condition;
notify()唤醒的线程是随机的，signal()可以有选择性地唤醒。
Condition选择 唤醒/等待
现在看一个利用Condition选择等待和唤醒的例子
ThreadDomain47类，定义add和sub方法
public class ThreadDomain47 {
private final Lock lock = new ReentrantLock();
private final Condition addCondition = lock.newCondition();
private final Condition subCondition = lock.newCondition();
private static int num = 0;
private List<String> lists = new LinkedList<String>();
public void add() {
lock.lock();
try {
while(lists.size() == 10) {//当集合已满,则"添加"线程等待
addCondition.await();
}
num++;
lists.add("add Banana" + num);
System.out.println("The Lists Size is " + lists.size());
System.out.println("The Current Thread is " + "增加线程");
System.out.println("==============================");
this.subCondition.signal();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {//释放锁
lock.unlock();
}
}
public void sub() {
lock.lock();
try {
while(lists.size() == 0) {//当集合为空时,"减少"线程等待
subCondition.await();
}
String str = lists.get(0);
lists.remove(0);
System.out.println("The Token Banana is [" + str + "]");
System.out.println("The Current Thread is " + "减少线程");
System.out.println("==============================");
num--;
addCondition.signal();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
MyThread40_0类，增加线程
public class MyThread40_0 implements Runnable {
private ThreadDomain47 task;
public MyThread40_0(ThreadDomain47 task) {
this.task = task;
}
@Override
public void run() {
task.add();
}
}
MyThread40_1类，减少线程
public class MyThread40_1 implements Runnable {
private ThreadDomain47 task;
public MyThread40_1(ThreadDomain47 task) {
this.task = task;
}
@Override
public void run() {
task.sub();
}
}
main方法，启动线程
public class MyThread40_main {
public static void main(String[] args) {
ThreadDomain47 task = new ThreadDomain47();
Thread t1=new Thread(new MyThread40_0(task));
Thread t3=new Thread(new MyThread40_0(task));
Thread t7=new Thread(new MyThread40_0(task));
Thread t8=new Thread(new MyThread40_0(task));
Thread t2 = new Thread(new MyThread40_1(task));
Thread t4 = new Thread(new MyThread40_1(task));
Thread t5 = new Thread(new MyThread40_1(task));
Thread t6 = new Thread(new MyThread40_1(task));
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
t6.start();
t7.start();
t8.start();
}
}
输出结果如下
The Lists Size is 1 The Current Thread is 增加线程 ============================== The Lists Size is 2 The Current Thread is 增加线程 ============================== The Token Banana is [add Banana1] The Current Thread is 减少线程 ============================== The Token Banana is [add Banana2] The Current Thread is 减少线程 ============================== The Lists Size is 1 The Current Thread is 增加线程 ============================== The Token Banana is [add Banana1] The Current Thread is 减少线程 ============================== The Lists Size is 1 The Current Thread is 增加线程 ============================== The Token Banana is [add Banana1] The Current Thread is 减少线程 ==============================
可以看到，lists的数量不会增加太多，也不会减少太多。当集合满，使增加线程等待，唤醒减少线程；当集合空，使减少线程等待，唤醒增加线程。我们用wait()/notify()机制无法实现该效果，这里体现了Condition的强大之处。
ReentrantLock中的方法
公平锁和非公平锁
ReentrantLock可以指定公平锁和非公平锁，公平锁根据线程运行的顺序获取锁，非公平锁则通过抢占获得锁，不按线程运行顺序。synchronized是非公平锁。在ReentrantLock(boolean fair)构造函数传入true/false来指定公平锁/非公平锁。
看个例子
ThreadDomain39类和main方法
public class ThreadDomain39 {
private Lock lock = new ReentrantLock(true);
public void testMethod()
{
try
{
lock.lock();
System.out.println("ThreadName" + Thread.currentThread().getName() + "获得锁");
}
finally
{
lock.unlock();
}
}
public static void main(String[] args) throws Exception
{
final ThreadDomain39 td = new ThreadDomain39();
Runnable runnable = new Runnable()
{
public void run()
{
System.out.println("线程" + Thread.currentThread().getName() + "运行了");
td.testMethod();
}
};
Thread[] threads = new Thread[5];
for (int i = 0; i < 5; i++)
threads[i] = new Thread(runnable);
for (int i = 0; i < 5; i++)
threads[i].start();
}
}
输出结果如下
线程Thread-0运行了 ThreadNameThread-0获得锁 线程Thread-1运行了 线程Thread-2运行了 ThreadNameThread-1获得锁 线程Thread-3运行了 线程Thread-4运行了 ThreadNameThread-2获得锁 ThreadNameThread-3获得锁 ThreadNameThread-4获得锁
可以看到公平锁获得锁的顺序和线程运行的顺序相同。公平锁尽可能地让线程获取锁的顺序和线程运行顺序保持一致，再执行几次，可能不一致。
ReentrantLock构造函数传入false，输出结果如下：
线程Thread-0运行了 线程Thread-2运行了 线程Thread-4运行了 线程Thread-3运行了 ThreadNameThread-0获得锁 线程Thread-1运行了 ThreadNameThread-1获得锁 ThreadNameThread-2获得锁 ThreadNameThread-4获得锁 ThreadNameThread-3获得锁
非公平锁获得锁的顺序和线程运行的顺序不同
getHoldCount()
获取当前线程调用lock()的次数，一般debug使用。
看个例子
public class ThreadDomain40 {
private ReentrantLock lock = new ReentrantLock();
public void testMethod1()
{
try
{
lock.lock();
System.out.println("testMethod1 getHoldCount = " + lock.getHoldCount());
testMethod2();
}
finally
{
lock.unlock();
}
}
public void testMethod2()
{
try
{
lock.lock();
System.out.println("testMethod2 getHoldCount = " + lock.getHoldCount());
}
finally
{
lock.unlock();
}
}
public static void main(String[] args)
{
ThreadDomain40 td = new ThreadDomain40();
td.testMethod1();
}
}
输出结果如下
testMethod1 getHoldCount = 1 testMethod2 getHoldCount = 2
可以看到，testMethod1()被调用了一次，testMethod2()被调用了两次，ReentrantLock和synchronized一样，锁都是可重入的。
getQueueLength()和isFair()
getQueueLength()获取等待的线程数量，isFair()判断是否是公平锁。
ThreadDomain41类和main方法,Thread.sleep(2000)使第一个线程之后的线程都来不及启动，Thread.sleep(Integer.MAX_VALUE)使线程无法unlock()。
public class ThreadDomain41 {
public ReentrantLock lock = new ReentrantLock();
public void testMethod()
{
try
{
lock.lock();
System.out.println("ThreadName = " + Thread.currentThread().getName() + "进入方法！");
System.out.println("是否公平锁？" + lock.isFair());
Thread.sleep(Integer.MAX_VALUE);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
lock.unlock();
}
}
public static void main(String[] args) throws InterruptedException
{
final ThreadDomain41 td = new ThreadDomain41();
Runnable runnable = new Runnable()
{
public void run()
{
td.testMethod();
}
};
Thread[] threads = new Thread[10];
for (int i = 0; i < 10; i++)
threads[i] = new Thread(runnable);
for (int i = 0; i < 10; i++)
threads[i].start();
Thread.sleep(2000);
System.out.println("有" + td.lock.getQueueLength() + "个线程正在等待！");
}
}
输出结果如下
ThreadName = Thread-1进入方法！ 是否公平锁？false 有9个线程正在等待！
ReentrantLock默认是非公平锁，只有一个线程lock()，9个线程在等待。
hasQueuedThread()和hasQueuedThreads()
hasQueuedThread(Thread thread)查询指定线程是否在等待锁，hasQueuedThreads()查询是否有线程在等待锁。
看个例子
ThreadDomain41类和main方法，和上面例子类似，Thread.sleep(Integer.MAX_VALUE); 让线程不释放锁，Thread.sleep(2000);让第一个线程之后的线程都无法启动。
public class ThreadDomain42 extends ReentrantLock {
public void waitMethod()
{
try
{
lock();
Thread.sleep(Integer.MAX_VALUE);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
unlock();
}
}
public static void main(String[] args) throws InterruptedException
{
final ThreadDomain42 td = new ThreadDomain42();
Runnable runnable = new Runnable()
{
public void run()
{
td.waitMethod();
}
};
Thread t0 = new Thread(runnable);
t0.start();
Thread.sleep(500);
Thread t1 = new Thread(runnable);
t1.start();
Thread.sleep(500);
Thread t2 = new Thread(runnable);
t2.start();
Thread.sleep(500);
System.out.println("t0 is waiting？" + td.hasQueuedThread(t0));
System.out.println("t1 is waiting？" + td.hasQueuedThread(t1));
System.out.println("t2 is waiting？" + td.hasQueuedThread(t2));
System.out.println("Is any thread waiting？" + td.hasQueuedThreads());
}
}
输出结果如下
t0 is waiting？false t1 is waiting？true t2 is waiting？true Is any thread waiting？true
t0线程获得了锁，t0没有释放锁，导致t1,t2等待锁。
isHeldByCurrentThread()和isLocked()
isHeldByCurrentThread()判断锁是否由当前线程持有，isLocked()判断锁是否由任意线程持有。
请看示例
ThreadDomain43类和main方法
public class ThreadDomain43 extends ReentrantLock {
public void testMethod()
{
try
{
lock();
System.out.println(Thread.currentThread().getName() + "线程持有了锁！");
System.out.println(Thread.currentThread().getName() + "线程是否持有锁？" +
isHeldByCurrentThread());
System.out.println("是否任意线程持有了锁？" + isLocked());
} finally
{
unlock();
}
}
public void testHoldLock()
{
System.out.println(Thread.currentThread().getName() + "线程是否持有锁？" +
isHeldByCurrentThread());
System.out.println("是否任意线程持有了锁？" + isLocked());
}
public static void main(String[] args)
{
final ThreadDomain43 td = new ThreadDomain43();
Runnable runnable0 = new Runnable()
{
public void run()
{
td.testMethod();
}
};
Runnable runnable1 = new Runnable()
{
public void run()
{
td.testHoldLock();
}
};
Thread t0 = new Thread(runnable0);
Thread t1 = new Thread(runnable1);
t0.start();
t1.start();
}
}
输出结果如下
Thread-0线程持有了锁！ Thread-1线程是否持有锁？false Thread-0线程是否持有锁？true 是否任意线程持有了锁？true 是否任意线程持有了锁？true
Thread-0线程testMethod方法持有锁，Thread-1线程testHoldLock方法没有lock操作，所以不持有锁。
tryLock()和tryLock(long timeout, TimeUnit unit)
tryLock()有加锁的功能，获得了锁且锁没有被另外一个线程持有，此时返回true，否则返回false，可以有效避免死锁。tryLock(long timeout, TimeUnit unit)表示在给定的时间内获得了锁，锁没有被其他线程持有，且不处于中断状态。返回true,否则返回false；
看个例子
public class MyThread39 {
public static void main(String[] args) {
System.out.println("开始");
final Lock lock = new ReentrantLock();
new Thread() {
@Override
public void run() {
String tName = Thread.currentThread().getName();
if (lock.tryLock()) {
System.out.println(tName + "获取到锁！");
} else {
System.out.println(tName + "获取不到锁！");
return;
}
try {
for (int i = 0; i < 5; i++) {
System.out.println(tName + ":" + i);
}
Thread.sleep(5000);
} catch (Exception e) {
System.out.println(tName + "出错了！");
} finally {
System.out.println(tName + "释放锁！");
lock.unlock();
}
}
}.start();
new Thread() {
@Override
public void run() {
String tName = Thread.currentThread().getName();
try {
if (lock.tryLock(1,TimeUnit.SECONDS)) {
System.out.println(tName + "获取到锁！");
} else {
System.out.println(tName + "获取不到锁！");
return;
}
} catch (InterruptedException e) {
e.printStackTrace();
}
try {
for (int i = 0; i < 5; i++) {
System.out.println(tName + ":" + i);
}
} catch (Exception e) {
System.out.println(tName + "出错");
} finally {
System.out.println(tName + "释放锁！");
lock.unlock();
}
}
}.start();
System.out.println("结束");
}
}
输出结果如下
开始 Thread-0获取到锁！ Thread-0:0 Thread-0:1 Thread-0:2 Thread-0:3 Thread-0:4 结束 Thread-1获取不到锁！ Thread-0释放锁！
Thread-0先获得了锁，且sleep了5秒，导致Thread-1获取不到锁，我们给Thread-1的tryLock设置1秒，一秒内获取不到锁就会返回false。
如果Thread.sleep(0)，那么Thread-0和Thread-1都可以获得锁，园友可以自己试下。
synchronized和ReentrantLock的比较
1.synchronized关键字是语法层面的实现，ReentrantLock要手动lock()和unlock();
2.synchronized是不公平锁，ReentrantLock可以指定是公平锁还是非公平锁；
3.synchronized等待/唤醒机制是随机的，ReentrantLock借助Condition的等待/唤醒机制可以自行选择等待/唤醒；
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。