知否知否，应知ReentrantLock和synchronized的区别

在应用开发中总是会遇到需要对方法进行加锁的场景，java中为我们提供了两种加锁方式，一是synchronized，二是lock方式。那么在实际开发中我们应该选取哪种加锁方式呢

写在前面

通过阅读本篇文章，你将了解到：

synchronized和ReentrantLock的不同点、相同点
ReentrantLock、synchronized响应中断测试
ReentrantLock#tryLock定时获取锁测试
ReentrantLock公平锁、非公平锁测试
ReentrantLock和synchronized性能测试
ReentrantLock应用场景介绍

相同点

互斥 => 同时只有一个线程获取锁
内存可见性 => 对共享变量的修改对另一个线程立即可见
可重入 => 同一个线程在外层方法获取锁的时候，再进入该线程的内层方法会自动获取锁（前提锁对象得是同一个对象或者class），不会因为之前已经获取过还没释放而阻塞

不同点

锁	synchronize	ReentrantLock
加锁释放锁方式	使用者无需关心，自动加锁、释放锁	显式加锁、释放锁，必须调用lock方法获取锁，调用unlock方法释放锁
中断	不可响应中断	可响应中断
超时获取锁	不允许	允许
是否可以实现公平锁	否，默认就为非公平锁	是，通过构造函数指定是否为公平锁，默认为非公平锁，传入true为公平锁
实现方式	JVM级别	API级别

ReentrantLock相比synchronized更灵活一些

ReentrantLock方法测试

ReentrantLock是Lock接口的其中一个实现类，Lock接口中定义的方法有：

void lock() => 立即获取锁
boolean tryLock() => 立即获取锁
boolean tryLock(long time, TimeUnit unit) throws InterruptedException => 在指定时间内获取锁，获得锁返回true，获取不到锁返回false
void lockInterruptibly() throws InterruptedException => 获取可中断锁
void unlock() => 释放锁
Condition newCondition() => 返回绑定到此 Lock 实例的Condition 实例

lock和tryLock方法除了返回值不一样以外，lock获取到的锁是不可响应中断的，而tryLock获取到的锁是可响应中断的。除此以外tryLock(long time, TimeUnit unit)获取到的锁也是可响应中断，即获取锁的方法中只有lock方法获取到的锁是不可以响应中断的

lockInterruptibly响应中断：ReentrantLockLockInterruptiblyTest.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42

public class ReentrantLockLockInterruptiblyTest {
public static void main(String[] args) throws InterruptedException {
Thread thread = new Thread(new Task());
thread.start();
Thread.sleep(3 * 1000); //执行三秒后中断线程
thread.interrupt();
}

public static class Task implements Runnable {
Lock lock = new ReentrantLock();

public Task () {
new Thread(() -> {
try {
lockMethod();
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
}

@Override
public void run() {
try {
lockMethod();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("End...");
}

private void lockMethod() throws InterruptedException {
lock.lockInterruptibly();
try {
//模拟长时间不释放锁
while (true) {}
} finally {
lock.unlock();
}
}
}
}

执行结果：

1
2
3
4
5
6
7
8
9
10
11
12
13
14

java.lang.InterruptedException

at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:898)

at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)

at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)

at com.h2t.study.concurrent.lock.ReentrantLockLockInterruptiblyTest$Task.lockMethod(ReentrantLockLockInterruptiblyTest.java:45)

at com.h2t.study.concurrent.lock.ReentrantLockLockInterruptiblyTest$Task.run(ReentrantLockLockInterruptiblyTest.java:37)

at java.lang.Thread.run(Thread.java:748)
End...

中断成功

synchronize响应中断：SynchronizedBlock.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

public class SynchronizedBlock {
public static void main(String[] args) throws InterruptedException {
Thread thread = new Thread(new Task());
thread.start();
Thread.sleep(3 * 1000); //执行三秒后中断线程
thread.interrupt();
System.out.println(thread.isInterrupted());
}

public static class Task implements Runnable {
public Task() {
new Thread() {
public void run() {
f();
}
}.start();
}

public synchronized void f() {
while (true) {
}
}

@Override
public void run() {
f();
System.out.println("End");
}
}
}

控制台永远不会抛出异常、打印出End
对于synchronized来说，如果一个线程在等待锁，调用中断线程的方法，不会生效即不响应中断。而lock可以响应中断

tryLock定时锁：ReentrantLockTryLockTest.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44

public class ReentrantLockTryLockTest {
public static void main(String[] args) {
ExecutorService es = Executors.newCachedThreadPool();
for (int i = 0; i < 2; i++) {
es.execute(new Task(i));
}
}

private static class Task implements Runnable {
private static Lock lock = new ReentrantLock();

private int i;

public Task(int i) {
this.i = i;
}

@Override
public void run() {
try {
lockMethod();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

//每次只允许一个线程调用
private void lockMethod() throws InterruptedException {
long start = System.currentTimeMillis();
//2s内获得锁
if (lock.tryLock(2, TimeUnit.SECONDS)) {
System.out.println(String.format("i = %d 获取到锁，耗时：%d", i, System.currentTimeMillis() - start));
try {
Thread.sleep(1000 * 60 * 1); //睡眠1分钟
} finally {
lock.unlock();
}

} else {
System.out.println(String.format("i = %d 获取到锁失败，耗时：%d", i, System.currentTimeMillis() - start));
}
}
}
}

run方法中调用了加锁的方法，加锁方法中尝试在2s内获得锁
执行结果：

1 2	i = 0 获取到锁，耗时：0 i = 1 获取到锁失败，耗时：2001

ReentrantLock公平锁与非公平锁：ReentrantLockFairTest.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58

public class ReentrantLockFairTest {
//通过传入true创建一个公平锁
private static Lock fairLock = new ReentrantLock(true);
//非公平锁，默认为非公平锁
private static Lock unfairLock = new ReentrantLock();

public static void main(String[] args) {
ExecutorService unfairEs = Executors.newCachedThreadPool();
ExecutorService fairEs = Executors.newCachedThreadPool();

for (int i = 0; i < 5; i++) {
unfairEs.execute(new UnfairTask(i));
fairEs.execute(new FairTask(i));
}
}

/**
* 非公平锁任务
* */
private static class UnfairTask implements Runnable {
private int i;

public UnfairTask(int i) {
this.i = i;
}

@Override
public void run() {
unfairLock.lock();
try {
System.out.println(String.format("unfairTask i = %d is running", i));
} finally {
unfairLock.unlock();
}
}
}

/**
* 公平锁任务
* */
private static class FairTask implements Runnable {
private int i;

public FairTask(int i) {
this.i = i;
}

@Override
public void run() {
fairLock.lock();
try {
System.out.println(String.format("fairTask i = %d is running", i));
} finally {
fairLock.unlock();
}
}
}
}

执行结果：

1
2
3
4
5
6
7
8
9
10

unfairTask i = 0 is running
fairTask i = 0 is running
unfairTask i = 1 is running
fairTask i = 1 is running
unfairTask i = 2 is running
fairTask i = 3 is running
unfairTask i = 3 is running
fairTask i = 2 is running
fairTask i = 4 is running
unfairTask i = 4 is running

公平锁先到先得，因此执行顺序是有序的。非公平锁如果后来提交的线程刚好获取释放掉的锁将获得锁先执行，因此结果执行顺序是无序的

synchronized非公平锁：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

public class SynchronizedUnfairTest {
public static void main(String[] args) {
ExecutorService unfairEs = Executors.newCachedThreadPool();

for (int i = 0; i < 5; i++) {
unfairEs.execute(new UnfairTask(i));
}
}

/**
* 非公平锁任务
* */
private static class UnfairTask implements Runnable {
private int i;

public UnfairTask(int i) {
this.i = i;
}

@Override
public synchronized void run() {
System.out.println(String.format("unfairTask i = %d is running", i));
}
}
}

执行结果：

1
2
3
4
5

unfairTask i = 1 is running
unfairTask i = 0 is running
unfairTask i = 3 is running
unfairTask i = 2 is running
unfairTask i = 4 is running

synchronized默认为非公平锁，并且只能是非公平锁，因此执行结果顺序是无序的

性能测试

ReentrantLock加锁任务：ReentrantLockTask.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26

public class ReentrantLockTask implements Runnable {
private int i;

public ReentrantLockTask(int i) {
this.i = i;
}

@Override
public void run() {
lockMethod();
}

ReentrantLock lock = new ReentrantLock();
private void lockMethod() {
int sum = 0;
lock.lock();
try {
for (int j = 0; j < 10; j++) {
sum += j;
}

} finally {
lock.unlock();
}
}
}

synchronized加锁任务：SynchronizedLockTask.java

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

public class SynchronizedLockTask implements Runnable {
private int i;

public SynchronizedLockTask(int i) {
this.i = i;
}

@Override
public void run() {
lockMethod();
}

private synchronized void lockMethod() {
int sum = 0;

for (int j = 0; j < 10; j++) {
sum += j;
}
}
}

测试类：PerformTest

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31

public class PerformTest {
public static void main(String[] args) {
for (int i = 100; i < 1000000000; i = i * 10) {
reentrantLockTest(i);
synchronizedLockTest(i);
}
}

/**
* 循环执行的次数
* */
private static void reentrantLockTest(int time) {
ExecutorService es = Executors.newCachedThreadPool();
long start = System.currentTimeMillis();
for (int i = 0; i < time; i++) {
es.execute(new ReentrantLockTask(i));
}

System.out.println(String.format("ReentrantLockTest time = %d Spend %d", time, System.currentTimeMillis() - start));
}

private static void synchronizedLockTest(int time) {
ExecutorService es = Executors.newCachedThreadPool();
long start = System.currentTimeMillis();
for (int i = 0; i < time; i++) {
es.execute(new SynchronizedLockTask(i));
}

System.out.println(String.format("SynchronizedLockTest time = %d Spend %d", time, System.currentTimeMillis() - start));
}
}

循环执行任务，统计循环任务的耗时

测试结果：

1
2
3
4
5
6
7
8
9
10
11
12
13
14

ReentrantLockTest time = 100 Spend 6
SynchronizedLockTest time = 100 Spend 2
ReentrantLockTest time = 1000 Spend 7
SynchronizedLockTest time = 1000 Spend 14
ReentrantLockTest time = 10000 Spend 42
SynchronizedLockTest time = 10000 Spend 29
ReentrantLockTest time = 100000 Spend 186
SynchronizedLockTest time = 100000 Spend 156
ReentrantLockTest time = 1000000 Spend 1428
SynchronizedLockTest time = 1000000 Spend 1006
ReentrantLockTest time = 10000000 Spend 9716
SynchronizedLockTest time = 10000000 Spend 9791
ReentrantLockTest time = 100000000 Spend 97928
SynchronizedLockTest time = 100000000 Spend 99804

synchronized和ReentrantLock性能相差不大，分不出谁好谁不好

synchronized和ReentrantLock之间如何选择

synchronized和ReentrantLock性能差不多，当且仅当synchronized无法满足的情景下使用ReentrantLock，因为ReentrantLock需要显式释放锁，同时synchronized是JVM级别的，JVM能对其进行优化，而Reentrant是API级别的不会有任何优化。synchronized无法满足的情景：

定时获取锁
响应中断
需要以公平的方式获取锁

最后附：示例代码
欢迎fork与star

附往期文章：欢迎你的阅读、点赞、评论

并发相关
1.为什么阿里巴巴要禁用Executors创建线程池？
2.自己的事情自己做，线程异常处理

设计模式相关：
1. 单例模式，你真的写对了吗？
2. (策略模式+工厂模式+map)套餐 Kill 项目中的switch case

JAVA8相关：
1. 使用Stream API优化代码
2. 亲，建议你使用LocalDateTime而不是Date哦

数据库相关：
1. mysql数据库时间类型datetime、bigint、timestamp的查询效率比较
2. 很高兴！终于踩到了慢查询的坑

高效相关：
1. 撸一个Java脚手架，一统团队项目结构风格

日志相关：
1. 日志框架，选择Logback Or Log4j2？
2. Logback配置文件这么写，TPS提高10倍

工程相关：
1. 闲来无事，动手写一个LRU本地缓存
2. Redis实现点赞功能模块
3. JMX可视化监控线程池
4. 权限管理【SpringSecurity篇】
5. Spring自定义注解从入门到精通
6. java模拟登陆优酷
7. QPS这么高，那就来写个多级缓存吧
8. java使用phantomjs进行截图

其他：
1. 使用try-with-resources优雅关闭资源
2. 老板，用float存储金额为什么要扣我工资