参考:http://www.dewen.net.cn/q/9077
http://coolxing.iteye.com/blog/1236909
lock,tryLock,lockInterruptibly
分别对应无限制锁,定时锁(通过while可以作为轮询锁),可中断锁。
Lock接口的 线程请求锁的 几个方法:
lock(), 拿不到lock就不罢休,不然线程就一直block。 比较无赖的做法。
tryLock(),马上返回,拿到lock就返回true,不然返回false。 比较潇洒的做法。
带时间限制的tryLock(),拿不到lock,就等一段时间,超时返回false。比较聪明的做法。
下面的lockInterruptibly()就稍微难理解一些。
先说说线程的打扰机制,每个线程都有一个 打扰 标志。这里分两种情况,
1. 线程在sleep或wait,join, 此时如果别的进程调用此进程的 interrupt()方法,此线程会被唤醒并被要求处理InterruptedException;(thread在做IO操作时也可能有类似行为,见java thread api)
2. 此线程在运行中, 则不会收到提醒。但是 此线程的 “打扰标志”会被设置, 可以通过isInterrupted()查看并作出处理。
lockInterruptibly()和上面的第一种情况是一样的, 线程在请求lock并被阻塞时,如果被interrupt,则“此线程会被唤醒并被要求处理InterruptedException”。
E.G. lock() & lockInterruptibly()
–lock()
package lockInterruptibly; import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock; public class MyService { public ReentrantLock lock = new ReentrantLock();
private Condition condition = lock.newCondition(); public void waitMethod() {
try {
lock.lock();
// lock.lockInterruptibly();
System.out.println("lock begin " + Thread.currentThread().getName());
for (int i = 0; i < 100000000; i++) {
String newString = new String();
Math.random();
// System.out.println(i + " ");
}
System.out.println("lock end " + Thread.currentThread().getName());
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
} }
package lockInterruptibly; public class Run { public static void main(String[] args) {
try {
final MyService service = new MyService();
Runnable runnable = new Runnable() { @Override
public void run() {
// TODO Auto-generated method stub
service.waitMethod();
}
};
Thread a = new Thread(runnable);
a.setName("A");
a.start();
Thread b = new Thread(runnable);
b.setName("B");
b.start();
Thread.sleep(500);
b.interrupt();
System.out.println("main end");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
正如lock()拿不到锁誓不罢休,线程b并未响应interrupt,一直在等待着锁。
–lockInterruptibly()
package lockInterruptibly; import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock; public class MyService { public ReentrantLock lock = new ReentrantLock();
private Condition condition = lock.newCondition(); public void waitMethod() {
try {
// lock.lock();
lock.lockInterruptibly();
System.out.println("lock begin " + Thread.currentThread().getName());
for (int i = 0; i < 100000000; i++) {
String newString = new String();
Math.random();
// System.out.println(i + " ");
}
System.out.println("lock end " + Thread.currentThread().getName());
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
if (lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
} }
package lockInterruptibly; public class Run { public static void main(String[] args) {
try {
final MyService service = new MyService();
Runnable runnable = new Runnable() { @Override
public void run() {
// TODO Auto-generated method stub
service.waitMethod();
}
};
Thread a = new Thread(runnable);
a.setName("A");
a.start();
Thread b = new Thread(runnable);
b.setName("B");
b.start();
Thread.sleep(500);
b.interrupt();
System.out.println("main end");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
线程b并没有拿到锁,此时调用interrupt方法,则线程b被唤醒并被要求处理InterruptedException,即抛出了InterruptedException
当然,线程b也并不会执行了
但是当线程b拿到锁执行时才调用interrupt,这时候线程b并不会中断
tryLock() & tryLock(long, TimeUnit)
|–void lock(): 执行此方法时, 如果锁处于空闲状态, 当前线程将获取到锁. 相反, 如果锁已经被其他线程持有, 将禁用当前线程, 直到当前线程获取到锁.
|–boolean tryLock(): 如果锁可用, 则获取锁, 并立即返回true, 否则返回false. 该方法和lock()的区别在于, tryLock()只是”试图”获取锁, 如果锁不可用, 不会导致当前线程被禁用, 当前线程仍然继续往下执行代码. 而lock()方法则是一定要获取到锁, 如果锁不可用, 就一直等待, 在未获得锁之前,当前线程并不继续向下执行. 通常采用如下的代码形式调用tryLock()方法:
|–boolean tryLock(long timeout, TimeUnit unit):如果锁定在给定等待时间内没有被另一个线程保持,且当前线程未被中断,则获取该锁定。
通常采用如下的代码形式调用tryLock()方法:
Lock lock = new ReentrantLock();
if (lock.tryLock()) {
try {
// manipulate protected state
} finally {
lock.unlock();
}
} else {
// perform alternative actions
}
此用法可确保如果获取了锁, 则会释放锁; 如果未获取锁, 则不会试图将其释放.
–tryLock()
package tryLock.copy.copy; import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock; /**
* boolean tryLock(long timeout, TimeUnit unit) 如果锁定在给定等待时间内没有被另一个线程保
* 持,且当前线程未被中断,则获取该锁定
*
* @author MicroCat
*
*/
public class MyService { public ReentrantLock lock = new ReentrantLock(); public void waitMethod() {
if (lock.tryLock()) {
try {
System.out.println(Thread.currentThread().getName() + " 获得锁");
for (int i = 0; i < 10000; i++) {
if (i % 100 == 0) {
System.out.println(Thread.currentThread().getName() + " " + i);
}
}
} finally {
lock.unlock();
}
} else {
System.out.println(Thread.currentThread().getName() + " 没有获得锁");
}
} }
package tryLock.copy.copy; public class Run { public static void main(String[] args) throws InterruptedException {
final MyService service = new MyService();
Runnable runnable = new Runnable() { @Override
public void run() {
service.waitMethod();
}
};
Thread a = new Thread(runnable);
a.setName("A");
a.start();
Thread b = new Thread(runnable);
b.setName("B");
b.start();
} }
线程b没有获得锁,a线程执行完也没有继续执行
–tryLock(long, TimeUnit)
package tryLock.copy.copy.copy; import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock; /**
* boolean tryLock(long timeout, TimeUnit unit) 如果锁定在给定等待时间内没有被另一个线程保
* 持,且当前线程未被中断,则获取该锁定
*
* @author MicroCat
*
*/
public class MyService { public ReentrantLock lock = new ReentrantLock(); public void waitMethod() {
try {
if (lock.tryLock(3, TimeUnit.SECONDS)) {
try {
System.out.println(Thread.currentThread().getName() + " 获得锁");
for (int i = 0; i < 10000; i++) {
if (i % 100 == 0) {
System.out.println(Thread.currentThread().getName() + " " + i);
}
}
} finally {
lock.unlock();
}
} else {
System.out.println(Thread.currentThread().getName() + " 没有获得锁");
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} }
package tryLock.copy.copy.copy; public class Run { public static void main(String[] args) throws InterruptedException {
final MyService service = new MyService();
Runnable runnable = new Runnable() { @Override
public void run() {
service.waitMethod();
}
};
Thread a = new Thread(runnable);
a.setName("A");
a.start();
Thread b = new Thread(runnable);
b.setName("B");
b.start();
} }
在给定时间内各线程都有拿到锁,所以线程a,b都有执行