一般是在数据表中加上一个数据版本号 version
字段,表示数据被修改的次数。当数据被修改时,version
值会加一。当线程 A 要更新数据值时,在读取数据的同时也会读取 version
值,在提交更新时,若刚才读取到的 version 值为当前数据库中的 version
值相等时才更新,否则重试更新操作,直到更新成功。
举一个简单的例子:假设数据库中帐户信息表中有一个 version 字段,当前值为 1 ;而当前帐户余额字段( balance
)为 $100 。
version
=1 ),并从其帐户余额中扣除 $50( $100-$50 )。version
=1 ),并从其帐户余额中扣除 $20 ( $100-$20 )。version
=1 ),连同帐户扣除后余额( balance
=$50 ),提交至数据库更新,此时由于提交数据版本等于数据库记录当前版本,数据被更新,数据库记录 version
更新为 2 。version
=1 )试图向数据库提交数据( balance
=$80 ),但此时比对数据库记录版本时发现,操作员 B 提交的数据版本号为 1 ,数据库记录当前版本也为 2 ,不满足 “ 提交版本必须等于当前版本才能执行更新 “ 的乐观锁策略,因此,操作员 B 的提交被驳回。这样就避免了操作员 B 用基于 version
=1 的旧数据修改的结果覆盖操作员 A 的操作结果的可能。
CAS 的全称是 Compare And Swap(比较与交换) ,用于实现乐观锁,被广泛应用于各大框架中。CAS 的思想很简单,就是用一个预期值和要更新的变量值进行比较,两值相等才会进行更新。
CAS 是一个原子操作,底层依赖于一条 CPU 的原子指令。
原子操作 即最小不可拆分的操作,也就是说操作一旦开始,就不能被打断,直到操作完成。
CAS 涉及到三个操作数:
当且仅当 V 的值等于 E 时,CAS 通过原子方式用新值 N 来更新 V 的值。如果不等,说明已经有其它线程更新了 V,则当前线程放弃更新。
举一个简单的例子:线程 A 要修改变量 i 的值为 6,i 原值为 1(V = 1,E=1,N=6,假设不存在 ABA 问题)。
当多个线程同时使用 CAS 操作一个变量时,只有一个会胜出,并成功更新,其余均会失败,但失败的线程并不会被挂起,仅是被告知失败,并且允许再次尝试,当然也允许失败的线程放弃操作。
Java 语言并没有直接实现 CAS,CAS 相关的实现是通过 C++ 内联汇编的形式实现的(JNI 调用)。因此, CAS 的具体实现和操作系统以及 CPU 都有关系。
③. 乐观锁常见问题
如果一个变量 V 初次读取的时候是 A 值,并且在准备赋值的时候检查到它仍然是 A 值,那我们就能说明它的值没有被其他线程修改过了吗?很明显是不能的,因为在这段时间它的值可能被改为其他值,然后又改回 A,那 CAS 操作就会误认为它从来没有被修改过。这个问题被称为 CAS 操作的 "ABA"问题。
ABA 问题的解决思路是在变量前面追加上版本号或者时间戳。JDK 1.5 以后的 AtomicStampedReference
类就是用来解决 ABA 问题的,其中的 compareAndSet()
方法就是首先检查当前引用是否等于预期引用,并且当前标志是否等于预期标志,如果全部相等,则以原子方式将该引用和该标志的值设置为给定的更新值。
public boolean compareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
Pair current = pair;
return
expectedReference == current.reference &&
expectedStamp == current.stamp &&
((newReference == current.reference &&
newStamp == current.stamp) ||
casPair(current, Pair.of(newReference, newStamp)));
}
CAS 经常会用到自旋操作来进行重试,也就是不成功就一直循环执行直到成功。如果长时间不成功,会给 CPU 带来非常大的执行开销。
如果 JVM 能支持处理器提供的 pause 指令那么效率会有一定的提升,pause 指令有两个作用:
CAS 只对单个共享变量有效,当操作涉及跨多个共享变量时 CAS 无效。但是从 JDK 1.5 开始,提供了AtomicReference
类来保证引用对象之间的原子性,你可以把多个变量放在一个对象里来进行 CAS 操作.所以我们可以使用锁或者利用AtomicReference
类把多个共享变量合并成一个共享变量来操作。
class Ticket {
private int number = 50;
private Lock lock = new ReentrantLock(true); //默认用的是非公平锁,分配的平均一点,=--》公平一点
public void sale() {
lock.lock();
try {
if(number > 0) {
System.out.println(Thread.currentThread().getName()+"\t 卖出第: "+(number--)+"\t 还剩下: "+number);
}
}finally {
lock.unlock();
}
}
/*Object objectLock = new Object();
public void sale(){
synchronized (objectLock)
{
if(number > 0)
{
System.out.println(Thread.currentThread().getName()+"\t 卖出第: "+(number--)+"\t 还剩下: "+number);
}
}
}*/
}
public class SaleTicketDemo {
public static void main(String[] args) {
Ticket ticket = new Ticket();
new Thread(() -> { for (int i = 1; i <=55; i++) ticket.sale(); },"a").start();
new Thread(() -> { for (int i = 1; i <=55; i++) ticket.sale(); },"b").start();
new Thread(() -> { for (int i = 1; i <=55; i++) ticket.sale(); },"c").start();
new Thread(() -> { for (int i = 1; i <=55; i++) ticket.sale(); },"d").start();
new Thread(() -> { for (int i = 1; i <=55; i++) ticket.sale(); },"e").start();
}
}
ReentrantLock
和Synchronized
都是可重入锁,可重入锁的一个优点是可在一定程度避免死锁③. 代码验证synchronized和ReentrantLock是可重入锁
//synchronized 是可重入锁
class Phone{
public synchronized void sendSms() throws Exception{
System.out.println(Thread.currentThread().getName()+"\tsendSms");
sendEmail();
}
public synchronized void sendEmail() throws Exception{
System.out.println(Thread.currentThread().getName()+"\tsendEmail");
}
}
/**
* Description:
* 可重入锁(也叫做递归锁)
* 指的是同一先生外层函数获得锁后,内层敌对函数任然能获取该锁的代码
* 在同一线程外外层方法获取锁的时候,在进入内层方法会自动获取锁
* * 也就是说,线程可以进入任何一个它已经标记的锁所同步的代码块
* **/
public class ReenterLockDemo {
/**
* t1 sendSms
* t1 sendEmail
* t2 sendSms
* t2 sendEmail
* @param args
*/
public static void main(String[] args) {
Phone phone = new Phone();
new Thread(()->{
try {
phone.sendSms();
} catch (Exception e) {
e.printStackTrace();
}
},"t1").start();
new Thread(()->{
try {
phone.sendSms();
} catch (Exception e) {
e.printStackTrace();
}
},"t2").start();
}
}
//ReentrantLock 是可重入锁
class Phone implements Runnable {
private Lock lock = new ReentrantLock();
@Override
public void run() {
get();
}
private void get() {
lock.lock();
try {
System.out.println(Thread.currentThread().getName() + "\tget");
set();
} finally {
lock.unlock();
}
}
private void set() {
lock.lock();
try {
System.out.println(Thread.currentThread().getName() + "\tset");
} finally {
lock.unlock();
}
}
}
/**
* Description:
* 可重入锁(也叫做递归锁)
* 指的是同一先生外层函数获得锁后,内层敌对函数任然能获取该锁的代码
* 在同一线程外外层方法获取锁的时候,在进入内层方法会自动获取锁
*
* 也就是说,线程可以进入任何一个它已经标记的锁所同步的代码块
**/
public class ReenterLockDemo {
/**
* Thread-0 get
* Thread-0 set
* Thread-1 get
* Thread-1 set
*/
public static void main(String[] args) {
Phone phone = new Phone();
Thread t3 = new Thread(phone);
Thread t4 = new Thread(phone);
t3.start();
t4.start();
}
}
public class DeadLockDemo{
static Object lockA = new Object();
static Object lockB = new Object();
public static void main(String[] args){
Thread a = new Thread(() -> {
synchronized (lockA) {
System.out.println(Thread.currentThread().getName() + "\t" + " 自己持有A锁,期待获得B锁");
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lockB) {
System.out.println(Thread.currentThread().getName() + "\t 获得B锁成功");
}
}
}, "a");
a.start();
new Thread(() -> {
synchronized (lockB){
System.out.println(Thread.currentThread().getName()+"\t"+" 自己持有B锁,期待获得A锁");
try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }
synchronized (lockA){
System.out.println(Thread.currentThread().getName()+"\t 获得A锁成功");
}
}
},"b").start();
}
}
D:\studySoft\Idea201903\JavaSelfStudy>jps
10048 Launcher
6276 DeadLockDemo
6332 Jps
9356
D:\studySoft\Idea201903\JavaSelfStudy>jstack 6276 (最后面有一个发现了一个死锁)
2021-07-28 16:05:36
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.111-b14 mixed mode):
"DestroyJavaVM" #16 prio=5 os_prio=0 tid=0x0000000003592800 nid=0x830 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"b" #15 prio=5 os_prio=0 tid=0x00000000253d5000 nid=0x1ba8 waiting for monitor entry [0x0000000025c8e000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.xiaozhi.juc.DeadLockDemo.lambda$main$1(DeadLockDemo.java:31)
- waiting to lock <0x0000000741404050> (a java.lang.Object)
- locked <0x0000000741404060> (a java.lang.Object)
at com.xiaozhi.juc.DeadLockDemo$$Lambda$2/2101440631.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
"a" #14 prio=5 os_prio=0 tid=0x00000000253d3800 nid=0xad8 waiting for monitor entry [0x0000000025b8e000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.xiaozhi.juc.DeadLockDemo.lambda$main$0(DeadLockDemo.java:20)
- waiting to lock <0x0000000741404060> (a java.lang.Object)
- locked <0x0000000741404050> (a java.lang.Object)
at com.xiaozhi.juc.DeadLockDemo$$Lambda$1/1537358694.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
"Service Thread" #13 daemon prio=9 os_prio=0 tid=0x000000002357b800 nid=0x1630 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C1 CompilerThread3" #12 daemon prio=9 os_prio=2 tid=0x00000000234f6000 nid=0x1fd4 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread2" #11 daemon prio=9 os_prio=2 tid=0x00000000234f3000 nid=0x5c0 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" #10 daemon prio=9 os_prio=2 tid=0x00000000234ed800 nid=0x1afc waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" #9 daemon prio=9 os_prio=2 tid=0x00000000234eb800 nid=0x2ae0 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"JDWP Command Reader" #8 daemon prio=10 os_prio=0 tid=0x0000000023464800 nid=0xc50 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"JDWP Event Helper Thread" #7 daemon prio=10 os_prio=0 tid=0x000000002345f800 nid=0x1b0c runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"JDWP Transport Listener: dt_socket" #6 daemon prio=10 os_prio=0 tid=0x0000000023451000 nid=0x2028 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Attach Listener" #5 daemon prio=5 os_prio=2 tid=0x000000002343f800 nid=0x1ea0 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" #4 daemon prio=9 os_prio=2 tid=0x00000000233eb800 nid=0x10dc runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Finalizer" #3 daemon prio=8 os_prio=1 tid=0x00000000233d3000 nid=0xafc in Object.wait() [0x000000002472f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000741008e98> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:143)
- locked <0x0000000741008e98> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:164)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:209)
"Reference Handler" #2 daemon prio=10 os_prio=2 tid=0x0000000021d0d000 nid=0x28ec in Object.wait() [0x000000002462f000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x0000000741006b40> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
- locked <0x0000000741006b40> (a java.lang.ref.Reference$Lock)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
JNI global references: 2504
Found one Java-level deadlock:
=============================
"b":
waiting to lock monitor 0x0000000021d10b58 (object 0x0000000741404050, a java.lang.Object),
which is held by "a"
"a":
waiting to lock monitor 0x0000000021d13498 (object 0x0000000741404060, a java.lang.Object),
which is held by "b"
Java stack information for the threads listed above:
===================================================
"b":
at com.xiaozhi.juc.DeadLockDemo.lambda$main$1(DeadLockDemo.java:31)
- waiting to lock <0x0000000741404050> (a java.lang.Object)
- locked <0x0000000741404060> (a java.lang.Object)
at com.xiaozhi.juc.DeadLockDemo$$Lambda$2/2101440631.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
"a":
at com.xiaozhi.juc.DeadLockDemo.lambda$main$0(DeadLockDemo.java:20)
- waiting to lock <0x0000000741404060> (a java.lang.Object)
- locked <0x0000000741404050> (a java.lang.Object)
at com.xiaozhi.juc.DeadLockDemo$$Lambda$1/1537358694.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
Found 1 deadlock.
②. 如何手写一个自选锁
//自旋锁
public class AtomicReferenceThreadDemo {
static AtomicReferenceatomicReference=new AtomicReference<>();
static Thread thread;
public static void lock(){
thread=Thread.currentThread();
System.out.println(Thread.currentThread().getName()+"\t"+"coming.....");
while(!atomicReference.compareAndSet(null,thread)){
}
}
public static void unlock(){
System.out.println(Thread.currentThread().getName()+"\t"+"over.....");
atomicReference.compareAndSet(thread,null);
}
public static void main(String[] args) {
new Thread(()->{
AtomicReferenceThreadDemo.lock();
try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) {e.printStackTrace();}
AtomicReferenceThreadDemo.unlock();
},"A").start();
new Thread(()->{
AtomicReferenceThreadDemo.lock();
AtomicReferenceThreadDemo.unlock();
},"B").start();
}
}