synchronized与Lock的区别与使用

原文链接
https://blog.csdn.net/u012403290/article/details/64910926

###引言：
昨天在学习别人分享的面试经验时，看到Lock的使用。想起自己在上次面试也遇到了synchronized与Lock的区别与使用。于是，我整理了两者的区别和使用情况，同时，对synchronized的使用过程一些常见问题的总结，最后是参照源码和说明文档，对Lock的使用写了几个简单的Demo。请大家批评指正。

技术点：

1、线程与进程：

在开始之前先把进程与线程进行区分一下，一个程序最少需要一个进程，而一个进程最少需要一个线程。关系是线程–>进程–>程序的大致组成结构。所以线程是程序执行流的最小单位，而进程是系统进行资源分配和调度的一个独立单位。以下我们所有讨论的都是建立在线程基础之上。

2、Thread的几个重要方法：

我们先了解一下Thread的几个重要方法。a、start()方法，调用该方法开始执行该线程；b、stop()方法，调用该方法强制结束该线程执行；c、join方法，调用该方法等待该线程结束。d、sleep()方法，调用该方法该线程进入等待。e、run()方法，调用该方法直接执行线程的run()方法，但是线程调用start()方法时也会运行run()方法，区别就是一个是由线程调度运行run()方法，一个是直接调用了线程中的run()方法！！

看到这里，可能有些人就会问啦，那wait()和notify()呢？要注意，其实wait()与notify()方法是Object的方法，不是Thread的方法！！同时，wait()与notify()会配合使用，分别表示线程挂起和线程恢复。

这里还有一个很常见的问题，顺带提一下：wait()与sleep()的区别，简单来说wait()会释放对象锁而sleep()不会释放对象锁。这些问题有很多的资料，不再赘述。

3、线程状态：

线程总共有5大状态，通过上面第二个知识点的介绍，理解起来就简单了。

新建状态：新建线程对象，并没有调用start()方法之前
就绪状态：调用start()方法之后线程就进入就绪状态，但是并不是说只要调用start()方法线程就马上变为当前线程，在变为当前线程之前都是为就绪状态。值得一提的是，线程在睡眠和挂起中恢复的时候也会进入就绪状态哦。
运行状态：线程被设置为当前线程，开始执行run()方法。就是线程进入运行状态
阻塞状态：线程被暂停，比如说调用sleep()方法后线程就进入阻塞状态
死亡状态：线程执行结束

4、锁类型

可重入锁：在执行对象中所有同步方法不用再次获得锁
可中断锁：在等待获取锁过程中可中断
公平锁：按等待获取锁的线程的等待时间进行获取，等待时间长的具有优先获取锁权利
读写锁：对资源读取和写入的时候拆分为2部分处理，读的时候可以多线程一起读，写的时候必须同步地写

###synchronized与Lock的区别

1、我把两者的区别分类到了一个表中，方便大家对比：

类别	synchronized	Lock
存在层次	Java的关键字，在jvm层面上	是一个类
锁的释放	1、以获取锁的线程执行完同步代码，释放锁 2、线程执行发生异常，jvm会让线程释放锁	在finally中必须释放锁，不然容易造成线程死锁
锁的获取	假设A线程获得锁，B线程等待。如果A线程阻塞，B线程会一直等待	分情况而定，Lock有多个锁获取的方式，具体下面会说道，大致就是可以尝试获得锁，线程可以不用一直等待
锁状态	无法判断	可以判断
锁类型	可重入不可中断非公平	可重入可判断可公平（两者皆可）
性能	少量同步	大量同步

或许，看到这里还对LOCK所知甚少，那么接下来，我们进入LOCK的深入学习。

###Lock详细介绍与Demo

以下是Lock接口的源码，笔者修剪之后的结果：

public interface Lock {/*** Acquires the lock.*/void lock();/*** Acquires the lock unless the current thread is* {@linkplain Thread#interrupt interrupted}.*/void lockInterruptibly() throws InterruptedException;/*** Acquires the lock only if it is free at the time of invocation.*/boolean tryLock();/*** Acquires the lock if it is free within the given waiting time and the* current thread has not been {@linkplain Thread#interrupt interrupted}.*/boolean tryLock(long time, TimeUnit unit) throws InterruptedException;/*** Releases the lock.*/void unlock();

从Lock接口中我们可以看到主要有个方法，这些方法的功能从注释中可以看出：

lock()：获取锁，如果锁被暂用则一直等待
unlock():释放锁
tryLock(): 注意返回类型是boolean，如果获取锁的时候锁被占用就返回false，否则返回true
tryLock(long time, TimeUnit unit)：比起tryLock()就是给了一个时间期限，保证等待参数时间
lockInterruptibly()：用该锁的获得方式，如果线程在获取锁的阶段进入了等待，那么可以中断此线程，先去做别的事

通过以上的解释，大致可以解释在上个部分中“锁类型(lockInterruptibly())”，“锁状态(tryLock())”等问题，还有就是前面子所获取的过程我所写的“大致就是可以尝试获得锁，线程可以不会一直等待”用了“可以”的原因。

下面是Lock一般使用的例子，注意ReentrantLock是Lock接口的实现。

lock()：


package com.brickworkers;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockTest {
private Lock lock = new ReentrantLock();

<span class="token comment">//需要参与同步的方法</span>
<span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span>Thread thread<span class="token punctuation">)</span><span class="token punctuation">{</span>lock<span class="token punctuation">.</span><span class="token function">lock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">try</span> <span class="token punctuation">{</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"线程名"</span><span class="token operator">+</span>thread<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"获得了锁"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token keyword">catch</span><span class="token punctuation">(</span>Exception e<span class="token punctuation">)</span><span class="token punctuation">{</span>e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"线程名"</span><span class="token operator">+</span>thread<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"释放了锁"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>lock<span class="token punctuation">.</span><span class="token function">unlock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>LockTest lockTest <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">LockTest</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//线程1</span>Thread t1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token annotation punctuation">@Override</span><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>lockTest<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">"t1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>Thread t2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token annotation punctuation">@Override</span><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>lockTest<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">"t2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>t1<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>t2<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}
//执行情况：线程名t1获得了锁
// 线程名t1释放了锁
// 线程名t2获得了锁
// 线程名t2释放了锁

tryLock():

package com.brickworkers;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockTest {
private Lock lock = new ReentrantLock();

<span class="token comment">//需要参与同步的方法</span>
<span class="token keyword">private</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span>Thread thread<span class="token punctuation">)</span><span class="token punctuation">{</span>

/* lock.lock();
try {
System.out.println(“线程名”+thread.getName() + “获得了锁”);
}catch(Exception e){
e.printStackTrace();
} finally {
System.out.println(“线程名”+thread.getName() + “释放了锁”);
lock.unlock();
}*/

 <span class="token keyword">if</span><span class="token punctuation">(</span>lock<span class="token punctuation">.</span><span class="token function">tryLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token keyword">try</span> <span class="token punctuation">{</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"线程名"</span><span class="token operator">+</span>thread<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"获得了锁"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token keyword">catch</span><span class="token punctuation">(</span>Exception e<span class="token punctuation">)</span><span class="token punctuation">{</span>e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"线程名"</span><span class="token operator">+</span>thread<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token string">"释放了锁"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>lock<span class="token punctuation">.</span><span class="token function">unlock</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"我是"</span><span class="token operator">+</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"有人占着锁，我就不要啦"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>LockTest lockTest <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">LockTest</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//线程1</span>Thread t1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token annotation punctuation">@Override</span><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>lockTest<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">"t1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>Thread t2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Thread</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token annotation punctuation">@Override</span><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>lockTest<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">,</span> <span class="token string">"t2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>t1<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>t2<span class="token punctuation">.</span><span class="token function">start</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

//执行结果：线程名t2获得了锁
// 我是t1有人占着锁，我就不要啦
// 线程名t2释放了锁

看到这里相信大家也都会使用如何使用Lock了吧，关于tryLock(long time, TimeUnit unit)和lockInterruptibly()不再赘述。前者主要存在一个等待时间，在测试代码中写入一个等待时间，后者主要是等待中断，会抛出一个中断异常，常用度不高，喜欢探究可以自己深入研究。

前面比较重提到“公平锁”，在这里可以提一下ReentrantLock对于平衡锁的定义，在源码中有这么两段：

/*** Sync object for non-fair locks*/static final class NonfairSync extends Sync {private static final long serialVersionUID = 7316153563782823691L;

    <span class="token comment">/*** Performs lock.  Try immediate barge, backing up to normal* acquire on failure.*/</span><span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">lock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">compareAndSetState</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token function">setExclusiveOwnerThread</span><span class="token punctuation">(</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">else</span><span class="token function">acquire</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token keyword">protected</span> <span class="token keyword">final</span> <span class="token keyword">boolean</span> <span class="token function">tryAcquire</span><span class="token punctuation">(</span><span class="token keyword">int</span> acquires<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">return</span> <span class="token function">nonfairTryAcquire</span><span class="token punctuation">(</span>acquires<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token comment">/*** Sync object for fair locks*/</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">FairSync</span> <span class="token keyword">extends</span> <span class="token class-name">Sync</span> <span class="token punctuation">{</span><span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> serialVersionUID <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">3000897897090466540</span>L<span class="token punctuation">;</span><span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">lock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token function">acquire</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token comment">/*** Fair version of tryAcquire.  Don't grant access unless* recursive call or no waiters or is first.*/</span><span class="token keyword">protected</span> <span class="token keyword">final</span> <span class="token keyword">boolean</span> <span class="token function">tryAcquire</span><span class="token punctuation">(</span><span class="token keyword">int</span> acquires<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">final</span> Thread current <span class="token operator">=</span> Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">int</span> c <span class="token operator">=</span> <span class="token function">getState</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">if</span> <span class="token punctuation">(</span>c <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">hasQueuedPredecessors</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span><span class="token function">compareAndSetState</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span> acquires<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token function">setExclusiveOwnerThread</span><span class="token punctuation">(</span>current<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>current <span class="token operator">==</span> <span class="token function">getExclusiveOwnerThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">int</span> nextc <span class="token operator">=</span> c <span class="token operator">+</span> acquires<span class="token punctuation">;</span><span class="token keyword">if</span> <span class="token punctuation">(</span>nextc <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token string">"Maximum lock count exceeded"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token function">setState</span><span class="token punctuation">(</span>nextc<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span><span class="token punctuation">}</span><span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span><span class="token punctuation">}</span>
<span class="token punctuation">}</span>

从以上源码可以看出在Lock中可以自己控制锁是否公平，而且，默认的是非公平锁，以下是ReentrantLock的构造函数：

   public ReentrantLock() {sync = new NonfairSync();//默认非公平锁}

###尾记录：

笔者水平一般，不过此博客在引言中的目的已全部达到。这只是笔者在学习过程中的总结与概括，如存在不正确的，欢迎大家批评指出。

延伸学习：对于LOCK底层的实现，大家可以参考：
点击Lock底层介绍博客

两种同步方式性能测试，大家可以参考：
点击查看两种同步方式性能测试博客

##博主18年3月新增：
回来看自己博客。发现东西阐述的不够完整。这里在做补充，因为这篇博客访问较大，所以为了不误导大家，尽量介绍给大家正确的表述：
1、两种锁的底层实现方式：
synchronized：我们知道java是用字节码指令来控制程序（这里不包括热点代码编译成机器码）。在字节指令中，存在有synchronized所包含的代码块，那么会形成2段流程的执行。

我们点击查看SyncDemo.java的源码SyncDemo.class，可以看到如下：

如上就是这段代码段字节码指令，没你想的那么难吧。言归正传，我们可以清晰段看到，其实synchronized映射成字节码指令就是增加来两个指令：monitorenter和monitorexit。当一条线程进行执行的遇到monitorenter指令的时候，它会去尝试获得锁，如果获得锁那么锁计数+1（为什么会加一呢，因为它是一个可重入锁，所以需要用这个锁计数判断锁的情况），如果没有获得锁，那么阻塞。当它遇到monitorexit的时候，锁计数器-1，当计数器为0，那么就释放锁。

那么有的朋友看到这里就疑惑了，那图上有2个monitorexit呀？马上回答这个问题：上面我以前写的文章也有表述过，synchronized锁释放有两种机制，一种就是执行完释放；另外一种就是发送异常，虚拟机释放。图中第二个monitorexit就是发生异常时执行的流程，这就是我开头说的“会有2个流程存在“。而且，从图中我们也可以看到在第13行，有一个goto指令，也就是说如果正常运行结束会跳转到19行执行。

这下，你对synchronized是不是了解的很清晰了呢。接下来我们再聊一聊Lock。

Lock：Lock实现和synchronized不一样，后者是一种悲观锁，它胆子很小，它很怕有人和它抢吃的，所以它每次吃东西前都把自己关起来。而Lock呢底层其实是CAS乐观锁的体现，它无所谓，别人抢了它吃的，它重新去拿吃的就好啦，所以它很乐观。具体底层怎么实现，博主不在细述，有机会的话，我会对concurrent包下面的机制好好和大家说说，如果面试问起，你就说底层主要靠volatile和CAS操作实现的。

现在，才是我真正想在这篇博文后面加的，我要说的是：尽可能去使用synchronized而不要去使用LOCK

什么概念呢？我和大家打个比方：你叫jdk，你生了一个孩子叫synchronized，后来呢，你领养了一个孩子叫LOCK。起初，LOCK刚来到新家的时候，它很乖，很懂事，各个方面都表现的比synchronized好。你很开心，但是你内心深处又有一点淡淡的忧伤，你不希望你自己亲生的孩子竟然还不如一个领养的孩子乖巧。这个时候，你对亲生的孩子教育更加深刻了，你想证明，你的亲生孩子synchronized并不会比领养的孩子LOCK差。（博主只是打个比方）

那如何教育呢？
在jdk1.6~jdk1.7的时候，也就是synchronized16、7岁的时候，你作为爸爸，你给他优化了，具体优化在哪里呢：

1、线程自旋和适应性自旋
我们知道，java’线程其实是映射在内核之上的，线程的挂起和恢复会极大的影响开销。并且jdk官方人员发现，很多线程在等待锁的时候，在很短的一段时间就获得了锁，所以它们在线程等待的时候，并不需要把线程挂起，而是让他无目的的循环，一般设置10次。这样就避免了线程切换的开销，极大的提升了性能。
而适应性自旋，是赋予了自旋一种学习能力，它并不固定自旋10次一下。他可以根据它前面线程的自旋情况，从而调整它的自旋，甚至是不经过自旋而直接挂起。

2、锁消除
什么叫锁消除呢？就是把不必要的同步在编译阶段进行移除。
那么有的小伙伴又迷糊了，我自己写的代码我会不知道这里要不要加锁？我加了锁就是表示这边会有同步呀？
并不是这样，这里所说的锁消除并不一定指代是你写的代码的锁消除，我打一个比方：
在jdk1.5以前，我们的String字符串拼接操作其实底层是StringBuffer来实现的（这个大家可以用我前面介绍的方法，写一个简单的demo，然后查看class文件中的字节码指令就清楚了），而在jdk1.5之后，那么是用StringBuilder来拼接的。我们考虑前面的情况，比如如下代码：

String str1="qwe";
String str2="asd";
String str3=str1+str2;

底层实现会变成这样：

StringBuffer sb = new StringBuffer();
sb.append("qwe");
sb.append("asd");

我们知道，StringBuffer是一个线程安全的类，也就是说两个append方法都会同步，通过指针逃逸分析（就是变量不会外泄），我们发现在这段代码并不存在线程安全问题，这个时候就会把这个同步锁消除。

3、锁粗化
在用synchronized的时候，我们都讲究为了避免大开销，尽量同步代码块要小。那么为什么还要加粗呢？
我们继续以上面的字符串拼接为例，我们知道在这一段代码中，每一个append都需要同步一次，那么我可以把锁粗化到第一个append和最后一个append（这里不要去纠结前面的锁消除，我只是打个比方）

4、轻量级锁

5、偏向锁

关于最后这两种，我希望留个有缘的读者自己去查找，我不希望我把一件事情描述的那么详细，自己动手得到才是你自己的，博主可以告诉你的是，最后两种并不难。。加油吧，各位。

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