ReentrantLock学习
ReentrantLock
ReentrantLock功能
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ReentrantLock和synchronized一样是可重入的
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可重入即当线程拥有了锁时，当该线程再次请求锁资源的时候，线程是可以再次成功获得的。
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static ReentrantLock lock = new ReentrantLock(); public static void main(String[] args) { new Thread(() ->{ lock.lock(); try{ log.debug("获得了锁，再次请求锁"); lock.lock(); try{ log.debug("再次获得了锁"); }finally { log.debug("释放了锁"); lock.unlock(); } }finally { log.debug("释放了锁"); lock.unlock(); } },"t1").start(); }
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ReentrantLock是可打断的
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即请求锁资源的时候，当请求不到锁资源的时候，可以被interrupt方法打断。
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static ReentrantLock lock = new ReentrantLock(); public static void main(String[] args) { Thread t1 = new Thread(() -> { try { // 如果当lock请求锁资源的同时被其他其他线程调用了interrupt方法 // 就会进入异常状态 lock.lockInterruptibly(); } catch (InterruptedException e) { e.printStackTrace(); log.debug("请求锁被打断"); return; // 当请求锁被打断后没有获得锁，不应该在进入下面的语句 } try { log.debug("获得到了锁资源"); } finally { lock.unlock(); } }, "t1"); lock.lock(); t1.start(); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } t1.interrupt(); }
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ReentrantLock可以锁超时
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synchronized请求锁会一直地等待锁资源，而ReentrantLock请求锁不会无限制的进行下去，使用tryLock()方法，可以在一次请求锁资源或请求一段时间的锁资源后结束请求锁。
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static ReentrantLock lock = new ReentrantLock(); public static void main(String[] args) throws InterruptedException { Thread t1 = new Thread(() -> { log.debug("开始运行了"); // 该情况下只会请求一次锁资源，请求到锁资源返回true，否则返回false // 加上时间限制的trylock是一样的。 try { if (lock.tryLock(2, TimeUnit.SECONDS)) { try { log.debug("请求到了锁资源"); } finally { lock.unlock(); } } else { log.debug("没请求到锁资源"); } } catch (InterruptedException e) { e.printStackTrace(); } }, "t1"); lock.lock(); log.debug("上锁了"); t1.start(); Thread.sleep(1000); log.debug("解锁了"); lock.unlock(); }
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当一些线程一直无法获得锁资源时，使用公平锁就可以时获得锁变成先进先获得。
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static ReentrantLock lock = new ReentrantLock(true);
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条件变量，即ReentrantLock支持多条件的进入不同的WaitSet进行等待，synchronized就只有一个WaitSet队列。
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当出现需要不同条件进入等待就可以使用该条件变量。
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static ReentrantLock lock = new ReentrantLock(); static Condition waitCondition1Set = lock.newCondition(); // 然后调用condition的await方法即可进入等待状态。
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设计模式---顺序执行
保证线程执行的一致性，因为这里是同一个锁对象所以不能用join
wait-notify
@Slf4j
public class Test5 {
static boolean already = false;
public static void main(String[] args) {
final Object o = new Object();
Thread t1 = new Thread(() -> {
synchronized (o){
while (!already){
try {
o.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
log.debug("1");
}
}, "t1");
Thread t2 = new Thread(() -> {
synchronized (o){
log.debug("2");
already = true;
o.notify();
}
}, "t2");
t1.start();
t2.start();
}
}
park-unpark
@Slf4j
public class Test6 {
public static void main(String[] args) throws InterruptedException {
Thread t1 = new Thread(() -> {
LockSupport.park();
log.debug("1");
}, "t1");
Thread t2 = new Thread(() -> {
LockSupport.park();
log.debug("2");
LockSupport.unpark(t1);
}, "t2");
t1.start();
t2.start();
Thread.sleep(2000);
LockSupport.unpark(t2);
}
}
设计模式---交替输出
三个线程交替输出
wait-notify
public class Test7 {
public static void main(String[] args) {
PrintObject object = new PrintObject(5,2);
Thread t1 = new Thread(() -> {
object.print(2,1);
}, "t1");
Thread t2 = new Thread(() -> {
object.print(3,2);
}, "t2");
Thread t3 = new Thread(() -> {
object.print(1,3);
}, "t3");
t1.start();
t2.start();
t3.start();
}
}
class PrintObject{
private int loopNum;
private int flag;
public PrintObject(int loopNum,int flag) {
this.loopNum = loopNum;
this.flag = flag;
}
public synchronized void print(int next,int now){
for(int i = 0;i < loopNum;i++){
while (flag != now){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(now);
flag = next;
notifyAll();
}
}
}
await-signal
class PrintObject{
private ReentrantLock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
private int loopNum;
private int flag;
public PrintObject(int loopNum,int flag) {
this.loopNum = loopNum;
this.flag = flag;
}
public void print(int next,int now){
lock.lock();
try{
for(int i = 0;i < loopNum;i++){
while (flag != now){
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println(flag);
flag = next;
condition.signalAll();
}
}finally {
lock.unlock();
}
}
}
park-unpark
@Slf4j
public class Test8 {
static Thread t1;
static Thread t2;
static Thread t3;
public static void main(String[] args) throws InterruptedException {
t1 = new Thread(() -> {
LockSupport.park();
log.debug("1");
LockSupport.unpark(t2);
}, "t1");
t2 = new Thread(() -> {
LockSupport.park();
log.debug("2");
LockSupport.unpark(t3);
}, "t2");
t3 = new Thread(() -> {
LockSupport.park();
log.debug("3");
LockSupport.unpark(t1);
}, "t3");
t1.start();
t2.start();
t3.start();
Thread.sleep(1000);
LockSupport.unpark(t1);
}
}
