ReentrantLock学习

ReentrantLock是可打断的

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。

• 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();
  }

ReentrantLock可以锁超时

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。

• 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();
  }

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。

static ReentrantLock lock = new ReentrantLock(true);

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。

当出现需要不同条件进入等待就可以使用该条件变量。

• 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。

设计模式---顺序执行

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

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);
}
}