目录
常见问题synchronizedaqsuqs
常见问题
Java有个抽象队列同步器（AQS），AQS是Java并发类的基石，不用AQS如何做线程同步机制呢？
Java如何做线程同步机制呢？
下面是一段业务代码，这段代码可能会被多线程执行，因此其中的计数器变量（cnt），可能会被并发读和改。
class Logic implements Runnable {
Integer cnt = Integer.valueOf(0);
public void run() {
var local = cnt;
java.util.logging.Logger.getGlobal().info(
String.format("thread:%s ts:%d \n want:%d->%d",
Thread.currentThread().getName(),
System.currentTimeMillis(),
cnt, local + 1));
cnt = ++local;
}
}
如果这段代码用于统计代码执行次数，因为缺少多线程同步机制，可能获得错误的结果。
public class Main {
public static void main(String[] args) throws InterruptedException {
var logic = new Logic();
for (int i = 0; i < 10; i++) {
new Thread(logic).start();
}
Thread.sleep(1000);
java.util.logging.Logger.getGlobal().info(
String.format("thread:%s ts:%d \n over:%d",
Thread.currentThread().getName(),
System.currentTimeMillis(),
logic.cnt));
}
10个线程，每个线程执行一次，预期结果是10，输出结果不及预期，显示只被执行了1次。
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-3 ts:1680320926475
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-9 ts:1680320926476
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-2 ts:1680320926475
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-0 ts:1680320926475
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-8 ts:1680320926476
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-5 ts:1680320926476
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-7 ts:1680320926476
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-4 ts:1680320926476
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-6 ts:1680320926476
want:0->1
Apr 01, 2023 3:48:46 AM *.example.Logic run
INFO: thread:Thread-1 ts:1680320926475
want:0->1
Apr 01, 2023 3:48:47 AM *.example.Main main
INFO: thread:main ts:1680320927477
over:1
为了规避错误的结果，引入线程同步机制，分别介绍三种线程同步机制。
synchronizeaqsuqs
synchronized
该文章专注于手搓源码，这块内容日后补充到免费内容中。
aqs
该文章专注于手搓源码，这块内容日后补充到免费内容中。
uqs
接下来，我们来实现一个互斥同步队列。
提炼aqs的原理，抽出来重要的地方去重新实现，非继承而是完完全全的重新实现这一套机制。
uqs提供了一个先进先出的队列，实现为抢占模式，抢占失败的线程重新排队。
使用util下的队列实现阻塞队列，使用util下原子引用实现占有线程。
public class Mutex {
private java.util.concurrent.atomic.AtomicReference
private java.util.Queue
}
接下来我们要实现线程阻塞的方法，使用cas操作、入队和park原语。
public class Mutex {
private java.util.concurrent.atomic.AtomicReference
private java.util.Queue
private synchronized boolean synceradd() {
return syncer.add(Thread.currentThread());
}
public void lock() {
if (state.compareAndSet(null, Thread.currentThread())) {
return;
}
synceradd();
java.util.concurrent.locks.LockSupport.park();
}
}
接下来我们要实现线程非阻塞的方法，使用出队和unpark原语，unpark为阻塞队列的第一个线程。
public class Mutex {
private java.util.concurrent.atomic.AtomicReference
private java.util.Queue
public void unlock() {
state.set(null);
var next = syncer.poll();
if (next == null) {
return;
}
java.util.concurrent.locks.LockSupport.unpark(next);
}
}
此时基本完成了同步器的实现了，此时注意队列的#unlock中的unpark和#lock中的park存在冲突，使用cas解决冲突即可。
public class Mutex {
private java.util.concurrent.atomic.AtomicReference
private java.util.Queue
private synchronized boolean synceradd() {
return syncer.add(Thread.currentThread());
}
public void lock() {
if (state.compareAndSet(null, Thread.currentThread())) {
return;
}
synceradd();
java.util.concurrent.locks.LockSupport.park();
lock();
}
}
完整版代码如下，还不到30行。
public class Mutex {
private java.util.concurrent.atomic.AtomicReference
private java.util.Queue
private synchronized boolean synceradd() {
return syncer.add(Thread.currentThread());
}
public void lock() {
if (state.compareAndSet(null, Thread.currentThread())) {
return;
}
synceradd();
java.util.concurrent.locks.LockSupport.park();
lock();
}
public void unlock() {
state.set(null);
var next = syncer.poll();
if (next == null) {
return;
}
java.util.concurrent.locks.LockSupport.unpark(next);
}
}
使用互斥同步器装饰下业务逻辑。
public class Main {
public static void main(String[] args) throws InterruptedException {
var logic = new MutexLogic();
for (int i = 0; i < 10; i++) {
new Thread(logic).start();
}
Thread.sleep(1000);
java.util.logging.Logger.getGlobal().info(
String.format("thread:%s ts:%d \n over:%d",
Thread.currentThread().getName(),
System.currentTimeMillis(),
logic.cnt));
}
class MutexLogic extends Logic {
*.syncer.Mutex mutex = new *.syncer.Mutex();
@Override
public void run() {
mutex.lock();
super.run();
mutex.unlock();
}
}
大功告成！
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-10 ts:1680320927479
want:0->1
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-12 ts:1680320927480
want:1->2
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-11 ts:1680320927480
want:2->3
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-13 ts:1680320927481
want:3->4
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-15 ts:1680320927481
want:4->5
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-14 ts:1680320927482
want:5->6
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-17 ts:1680320927482
want:6->7
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-16 ts:1680320927483
want:7->8
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-18 ts:1680320927484
want:8->9
Apr 01, 2023 3:48:47 AM *.example.Logic run
INFO: thread:Thread-19 ts:1680320927484
want:9->10
Apr 01, 2023 3:48:48 AM *.example.Main main
INFO: thread:main ts:1680320928480
over:10
完整代码这里【github】【gitee】【gitcode推荐】，如果文档对你有帮助，请帮忙点个star。