ReentrantLock详解
ReentrantLock是一种基于AQS框架的应用实现，是JDK中的一种线程并发访问的同步手段，它的功能类似于synchronized是一种互斥锁，可以保证线程安全。
相对于 synchronized， ReentrantLock具备如下特点：
- 可中断
- 可以设置超时时间
- 可以设置为公平锁
- 支持多个条件变量
- 与 synchronized 一样，都支持可重入
顺便总结了几点synchronized和ReentrantLock的区别：
- synchronized是JVM层次的锁实现，ReentrantLock是JDK层次的锁实现；
- synchronized的锁状态是无法在代码中直接判断的，但是ReentrantLock可以通过ReentrantLock#isLocked判断；
- synchronized是非公平锁，ReentrantLock是可以是公平也可以是非公平的；
- synchronized是不可以被中断的，而ReentrantLock#lockInterruptibly方法是可以被中断的；
- 在发生异常时synchronized会自动释放锁，而ReentrantLock需要开发者在finally块中显示释放锁；
- ReentrantLock获取锁的形式有多种：如立即返回是否成功的tryLock(),以及等待指定时长的获取，更加灵活；
- synchronized在特定的情况下对于已经在等待的线程是后来的线程先获得锁（回顾一下sychronized的唤醒策略），而ReentrantLock对于已经在等待的线程是先来的线程先获得锁；
ReentrantLock的使用
同步执行，类似于synchronized
ReentrantLock lock = new ReentrantLock(); //参数默认false，不公平锁
ReentrantLock lock = new ReentrantLock(true); //公平锁
//加锁
lock.lock();
try {
//临界区
} finally {
// 解锁
lock.unlock();
}
测试使用
public class ReentrantLockDemo {
private static int sum = 0;
private static Lock lock = new ReentrantLock();
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 3; i++) {
Thread thread = new Thread(()->{
//加锁
lock.lock();
try {
for (int j = 0; j < 10000; j++) {
sum++;
}
} finally {
// 解锁
lock.unlock();
}
});
thread.start();
}
Thread.sleep(2000);
System.out.println(sum);
}
可重入
@Slf4j
public class ReentrantLockDemo2 {
public static ReentrantLock lock = new ReentrantLock();
public static void main(String[] args) {
method1();
}
public static void method1() {
lock.lock();
try {
log.debug("execute method1");
method2();
} finally {
lock.unlock();
}
}
public static void method2() {
lock.lock();
try {
log.debug("execute method2");
method3();
} finally {
lock.unlock();
}
}
public static void method3() {
lock.lock();
try {
log.debug("execute method3");
} finally {
lock.unlock();
}
}
可中断
注意：在实际项目中，要配合中断标记位来进行使用（即某一个变量，一旦线程调用interrupt()，对应的变量也要设值），双重判断，保证安全
@Slf4j
public class ReentrantLockDemo3 {
public static void main(String[] args) {
ReentrantLock lock = new ReentrantLock();
Thread t1 = new Thread(() -> {
log.debug("t1启动...");
try {
lock.lockInterruptibly();
try {
log.debug("t1获得了锁");
} finally {
lock.unlock();
}
} catch (InterruptedException e) {
e.printStackTrace();
log.debug("t1等锁的过程中被中断");
}
}, "t1");
lock.lock();
try {
log.debug("main线程获得了锁");
t1.start();
//先让线程t1执行
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
t1.interrupt();
log.debug("线程t1执行中断");
} finally {
lock.unlock();
}
}
锁超时
立即失败
@Slf4j
public class ReentrantLockDemo4 {
public static void main(String[] args) {
ReentrantLock lock = new ReentrantLock();
Thread t1 = new Thread(() -> {
log.debug("t1启动...");
// 注意： 即使是设置的公平锁，此方法也会立即返回获取锁成功或失败，公平策略不生效
if (!lock.tryLock()) {
log.debug("t1获取锁失败，立即返回false");
return;
}
try {
log.debug("t1获得了锁");
} finally {
lock.unlock();
}
}, "t1");
lock.lock();
try {
log.debug("main线程获得了锁");
t1.start();
//先让线程t1执行
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
} finally {
lock.unlock();
}
}
超时失败
@Slf4j
public class ReentrantLockDemo4 {
public static void main(String[] args) {
ReentrantLock lock = new ReentrantLock();
Thread t1 = new Thread(() -> {
log.debug("t1启动...");
//超时
try {
if (!lock.tryLock(1, TimeUnit.SECONDS)) {
log.debug("等待 1s 后获取锁失败，返回");
return;
}
} catch (InterruptedException e) {
e.printStackTrace();
return;
}
try {
log.debug("t1获得了锁");
} finally {
lock.unlock();
}
}, "t1");
lock.lock();
try {
log.debug("main线程获得了锁");
t1.start();
//先让线程t1执行
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
} finally {
lock.unlock();
}
}
公平锁（ReentrantLock 默认是不公平的）
@Slf4j
public class ReentrantLockDemo5 {
public static void main(String[] args) throws InterruptedException {
ReentrantLock lock = new ReentrantLock(true); //公平锁
for (int i = 0; i < 500; i++) {
new Thread(() -> {
lock.lock();
try {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
log.debug(Thread.currentThread().getName() + " running...");
} finally {
lock.unlock();
}
}, "t" + i).start();
}
// 1s 之后去争抢锁
Thread.sleep(1000);
for (int i = 0; i < 500; i++) {
new Thread(() -> {
lock.lock();
try {
log.debug(Thread.currentThread().getName() + " running...");
} finally {
lock.unlock();
}
}, "强行插入" + i).start();
}
}
条件变量
java.util.concurrent类库中提供Condition类来实现线程之间的协调。调用Condition.await() 方法使线程等待，其他线程调用Condition.signal() 或 Condition.signalAll() 方法唤醒等待的线程。
注意：调用Condition的await()和signal()方法，都必须在lock保护之内。
@Slf4j
public class ReentrantLockDemo6 {
private static ReentrantLock lock = new ReentrantLock();
private static Condition cigCon = lock.newCondition();
private static Condition takeCon = lock.newCondition();
private static boolean hashcig = false;
private static boolean hastakeout = false;
//送烟
public void cigratee(){
lock.lock();
try {
while(!hashcig){
try {
log.debug("没有烟，歇一会");
cigCon.await();
}catch (Exception e){
e.printStackTrace();
}
}
log.debug("有烟了，干活");
}finally {
lock.unlock();
}
}
//送外卖
public void takeout(){
lock.lock();
try {
while(!hastakeout){
try {
log.debug("没有饭，歇一会");
takeCon.await();
}catch (Exception e){
e.printStackTrace();
}
}
log.debug("有饭了，干活");
}finally {
lock.unlock();
}
}
public static void main(String[] args) {
ReentrantLockDemo6 test = new ReentrantLockDemo6();
new Thread(() ->{
test.cigratee();
}).start();
new Thread(() -> {
test.takeout();
}).start();
new Thread(() ->{
lock.lock();
try {
hashcig = true;
//唤醒送烟的等待线程
cigCon.signal();
}finally {
lock.unlock();
}
},"t1").start();
new Thread(() ->{
lock.lock();
try {
hastakeout = true;
//唤醒送饭的等待线程
takeCon.signal();
}finally {
lock.unlock();
}
},"t2").start();
}
具体执行流程示意图如下：
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