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java中ReentrantLock类的tryLock和tryLock的例子和用法
ReentrantLock类的tryLock和tryLock(时间)
马克-to-win：tryLock的方法就是试一下，如果能得到锁，就返回真，如果当时得不到，马上就返回假，绝不等。tryLock(时间)的用法就是在规定的时间内设法得到锁。如果在规定的时间内最终不能得到锁，就返回假。注意，这个方法是可以被打断的，打断后的处理方法和上面的例子 lockInterruptibly的处理一样。马克- to-win：马克 java社区：防盗版实名手机尾号： 73203。
例1.9.8_a:
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;
class A {
private ReentrantLock lock = new ReentrantLock();
int ticketNum = 10;
public void buyOne() {
System.out.println("just before lock.lockInterruptibly();");
boolean succeed = lock.tryLock();
if (succeed) {
System.out.println(Thread.currentThread().getName()
+ "ticketNum is" + ticketNum);
if (ticketNum > 0) {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
ticketNum--;
System.out
.println("模仿select * from table for update,执行的很慢,买了一张"
+ Thread.currentThread().getName()
+ "ticketNum is" + ticketNum);
}
lock.unlock();
}else{
System.out.println("没获得锁，一张");
}
}
public void buyBatch(int num) throws InterruptedException {
System.out.println("just before lock.lockInterruptibly();");
boolean succeed = false;
boolean sleepSucceed = false;
succeed = lock.tryLock(2, TimeUnit.SECONDS);
if (succeed) {
System.out.println("Thread.currentThread().getName()+ticketNum is"
+ ticketNum);
try {
Thread.sleep(5000);
sleepSucceed=true;
} catch (InterruptedException e) {
System.out.println("已经获得了锁了，几张的睡觉被打断,表示预备工作没做好，什么也不买");
}
if (ticketNum >= num&&sleepSucceed) {
ticketNum = ticketNum - num;
System.out
.println("模仿select * from table for update,执行的很慢，买了几张"
+ Thread.currentThread().getName()
+ "ticketNum is" + ticketNum);
}
lock.unlock();
}else{
System.out.println("没获得锁，几张");
}
}
}
class MyThread1 extends Thread {
A a;
public MyThread1(A a) {
this.a = a;
}
public void run() {
a.buyOne();
}
}
class MyThread2 extends Thread {
A a;
public MyThread2(A a) {
this.a = a;
}
public void run() {
try {
a.buyBatch(3);
} catch (InterruptedException e) {
System.out.println("正要获得锁的过程当中呢，几张的线程被打断");
}
}
}
public class TestMark_to_win {
public static void main(String[] args) throws InterruptedException {
MyThread1[] threads = new MyThread1[2];
A a = new A();
for (int i = 0; i < 2; i++) {
threads[i] = new MyThread1(a);
}
MyThread2 myThread2 = new MyThread2(a);
threads[0].start();
Thread.sleep(50);
myThread2.start();
threads[1].start();
// Thread.sleep(50);
// myThread2.interrupt();
}
}
（这种情况是tryLock(2, TimeUnit.SECONDS);根本没得到锁）运行结果：
just before lock.lockInterruptibly();
Thread-0ticketNum is10
just before lock.lockInterruptibly();
just before lock.lockInterruptibly();
没获得锁，一张
没获得锁，几张
模仿select * from table for update,执行的很慢,买了一张Thread-0ticketNum is9
例1.9.8_b:（只需要把上面实验的主程序改变一下就行，其他都不用变。）
public class TestMark_to_win {
public static void main(String[] args) throws InterruptedException {
MyThread1[] threads = new MyThread1[2];
A a = new A();
for (int i = 0; i < 2; i++) {
threads[i] = new MyThread1(a);
}
MyThread2 myThread2 = new MyThread2(a);
threads[0].start();
Thread.sleep(50);
myThread2.start();
threads[1].start();
Thread.sleep(50);
myThread2.interrupt();
}
}
（这种情况是tryLock(2, TimeUnit.SECONDS);正要获得锁的过程当中被打断了）运行结果：
just before lock.lockInterruptibly();
Thread-0ticketNum is10
just before lock.lockInterruptibly();
just before lock.lockInterruptibly();
没获得锁，一张
正要获得锁的过程当中呢，几张的线程被打断
模仿select * from table for update,执行的很慢,买了一张Thread-0ticketNum is9
例1.9.8_c:
public class TestMark_to_win {
public static void main(String[] args) throws InterruptedException {
MyThread1[] threads = new MyThread1[2];
A a = new A();
for (int i = 0; i < 2; i++) {
threads[i] = new MyThread1(a);
}
MyThread2 myThread2 = new MyThread2(a);
myThread2.start();
Thread.sleep(50);
threads[0].start();
threads[1].start();
// Thread.sleep(50);
// myThread2.interrupt();
}
}
（这种情况是tryLock(2, TimeUnit.SECONDS);获得了锁）运行结果：
just before lock.lockInterruptibly();
Thread.currentThread().getName()+ticketNum is10
just before lock.lockInterruptibly();
没获得锁，一张
just before lock.lockInterruptibly();
没获得锁，一张
模仿select * from table for update,执行的很慢，买了几张Thread-2ticketNum is7
例1.9.8_d:
public class TestMark_to_win {
public static void main(String[] args) throws InterruptedException {
MyThread1[] threads = new MyThread1[2];
A a = new A();
for (int i = 0; i < 2; i++) {
threads[i] = new MyThread1(a);
}
MyThread2 myThread2 = new MyThread2(a);
myThread2.start();
Thread.sleep(50);
threads[0].start();
threads[1].start();
Thread.sleep(50);
myThread2.interrupt();
}
}
（这种情况是tryLock(2, TimeUnit.SECONDS);已经获得了锁，【注意虽然写的是2秒，但是myThread2是排在队列里第一个的，根本不用等，更别说是等2秒，2秒是真正等时的极限。】正在干事的的过程当中被打断了）运行结果：
just before lock.lockInterruptibly();
Thread.currentThread().getName()+ticketNum is10
just before lock.lockInterruptibly();
没获得锁，一张
just before lock.lockInterruptibly();
没获得锁，一张
已经获得了锁了，几张的睡觉被打断,表示预备工作没做好，什么也不买
volatile关键字与原子操作
volatile关键字到底是干什么的？（新手可略过）
我们先简要了解一下java虚拟机的内存模型。就像数据从物理内存拷贝到cpu高速缓存，进行操作完，再把数据返回到内存一样，为了屏蔽CPU高速缓存和内存复杂细节且赢得跨平台的效果，java把所有的变量都存在主存（相当于物理内存）当中，每个线程都有自己的工作内存（相当于CPU高速缓存）。线程在自己的工作内存做操作，不能直接对主存进行操作，最后把结果返回到主存。如果一个变量有volatile（易变的意思）修饰词，这意味着当有一个线程修改了这个变量，系统会把工作内存当中的变化强制立刻反应在主存当中。其他线程要想读这个变量，也会被强迫读变化了的新值。volatile其实就保证了此变量无论怎么变，任何线程看都是最新的。当两个线程，根据一个共同的信号，做互动时，一定要加volatile，保证这个信号是最新的。
volatile，有人说能够控制程序的语句有序性，但jvm并不能保证在所有的平台上都能够做到这一点，所以我的书中就不赘述了。既然volatile控制程序语句的有序性不能保证所有的平台都正确运行，基于它的技术“双重检查锁”创建单态对象也就变得不可靠了，本书也就不覆盖了。
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