1. 环境搭配
保证三个1.8:
2. JUC概述
介绍
java.util.concurrent工具包!
线程和进程
-
Java默认只有2个进程:main和GC
-
Java语言实际上无法真正开启进程:
public synchronized void start() { /** * This method is not invoked for the main method thread or "system" * group threads created/set up by the VM. Any new functionality added * to this method in the future may have to also be added to the VM. * * A zero status value corresponds to state "NEW". */ if (threadStatus != 0) throw new IllegalThreadStateException(); /* Notify the group that this thread is about to be started * so that it can be added to the group's list of threads * and the group's unstarted count can be decremented. */ group.add(this); boolean started = false; try { //调用的此方法是个本地方法 start0(); started = true; } finally { try { if (!started) { group.threadStartFailed(this); } } catch (Throwable ignore) { /* do nothing. If start0 threw a Throwable then it will be passed up the call stack */ } } } //这是个本地方法,java无法直接操作硬件,底层是C++ private native void start0();
并发
- 并发:多线程操作同一个资源
- 并行编程的本质:充分利用CPU的资源
3. 线程和进程
线程状态
6个:创建、运行、阻塞、等待、超时等待、终止
wait和sleep的区别
区别 | wait | sleep |
---|---|---|
所属类 | Object | thread |
锁的释放 | 释放锁 | 不释放锁 |
使用范围 | 必须在同步代码块中 | 任何地方 |
异常捕获 | 不需捕获 | 必须捕获异常 |
线程就是一个单独的资源类,没有任何附属的操作!
4. Lock锁
Lock实现类
- ReentrantLock:可重入锁(最常用),分为两类!
- 公平锁:十分公平,可以先来后到
- 非公平锁:十分不公平,可以插队(默认 3h 3s)
- ReadLock:读锁
- WriteLock:写锁
synchronized锁和lock锁的区别
- synchronized 是内置的Java关键字,lock 是一个Java类
- synchronized 无法获取锁的状态,lock 可以判断是否获取到了锁
- synchronized 会自动释放锁,lock 必须手动释放,如果不释放,会死锁!
- synchronized 线程1(获得锁,然后阻塞),线程2会一直等待锁的释放,lock锁不一定(trylock方法尝试获取锁)
- synchronized 可重入锁,不可以中断,非公平;lock 可重入锁,可以判断锁,非公平(可以自己设置是否公平)
- synchronized 适合锁少量的代码同步问题,lock 适合锁大量的同步代码
package com.kuang;
public class SaleTicketDemo1 {
public static void main(String[] args) {
Ticket ticket = new Ticket();
new Thread(()->{
for (int i = 0; i < 20 ; i++) {
try {
ticket.inc();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.dec();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"B").start();
}
}
class Ticket {
private int number = 0;
public synchronized void inc() throws InterruptedException {
if(number!=0){
wait();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
public synchronized void dec() throws InterruptedException {
if(number!=1){
wait();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
}
问题?如果多加两个进程参与加减呢?
package com.kuang;
public class SaleTicketDemo1 {
public static void main(String[] args) {
Ticket ticket = new Ticket();
new Thread(()->{
for (int i = 0; i < 20 ; i++) {
try {
ticket.inc();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.dec();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"B").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.inc();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"C").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.dec();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"D").start();
}
}
class Ticket {
private int number = 0;
public synchronized void inc() throws InterruptedException {
while (number!=0){
wait();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
public synchronized void dec() throws InterruptedException {
while (number!=1){
wait();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
}
结果:会出现虚假唤醒!
解决方案:将wait()放在while中! 或者将wait放在if判断外面
//方法2:
class Ticket {
private int number = 0;
public synchronized void inc() throws InterruptedException {
if(number==0){
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
wait();
}
public synchronized void dec() throws InterruptedException {
if(number==1){
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
wait();
}
}
//方法1:
class Ticket {
private int number = 0;
public synchronized void inc() throws InterruptedException {
while (number!=0){
wait();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
public synchronized void dec() throws InterruptedException {
while (number!=1){
wait();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
notifyAll();
}
}
Lock版生产者消费者
package com.kuang;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class SaleTicketDemo1 {
public static void main(String[] args) {
Ticket ticket = new Ticket();
new Thread(()->{
for (int i = 0; i < 20 ; i++) {
try {
ticket.inc();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.dec();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"B").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.inc();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"C").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.dec();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"D").start();
}
}
class Ticket {
private int number = 0;
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
// condition.await() 等待
// condition.signalAll() 唤醒全部
public void inc() throws InterruptedException {
lock.lock();
try {
while (number!=0){
condition.await();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
condition.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void dec() throws InterruptedException {
lock.lock();
try {
while (number!=1){
condition.await();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
condition.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
condition:精准通知和唤醒
package com.kuang;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class SaleTicketDemo1 {
public static void main(String[] args) {
Ticket ticket = new Ticket();
new Thread(()->{
for (int i = 0; i < 20 ; i++) {
try {
ticket.incA();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.decB();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"B").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.incC();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"C").start();
new Thread(()->{
for (int i = 0; i < 40 ; i++) {
try {
ticket.decD();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"D").start();
}
}
class Ticket {
private int number = 0;
Lock lock = new ReentrantLock();
Condition condition1 = lock.newCondition();
Condition condition2 = lock.newCondition();
Condition condition3 = lock.newCondition();
Condition condition4 = lock.newCondition();
// condition.await() 等待
// condition.signalAll() 唤醒全部
public void incA() throws InterruptedException {
lock.lock();
try {
while (number!=0){
condition1.await();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
condition2.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void decB() throws InterruptedException {
lock.lock();
try {
while (number!=1){
condition2.await();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
condition3.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void incC() throws InterruptedException {
lock.lock();
try {
while (number!=0){
condition3.await();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
condition4.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void decD() throws InterruptedException {
lock.lock();
try {
while (number!=1){
condition4.await();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
condition1.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
5. 八锁现象
介绍
8锁,就是关于锁的八个问题!
-
标准情况下,两个线程执行,先打印发短信还是打电话? 1/发短信 2/打电话
解释:在主线程main中,B线程是延迟一秒创建的,在这一秒中,A已经拿到锁了!synchronized锁的对象是方法的调用者
public class SaleTicketDemo1 { public static void main(String[] args) { Ticket ticket = new Ticket(); new Thread(()->{ ticket.message();},"A").start(); try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } new Thread(()->{ ticket.call();},"B").start(); } } class Ticket { public synchronized void message(){ System.out.println("发消息"); } public synchronized void call(){ System.out.println("打电话"); } }
-
在message中添加5秒的延时,是怎样?1/发短信 2/打电话
解释:还是因为A线程先拿到这个锁,就算发短信延迟,但是还是没有释放锁啊!
-
新增一个普通非同步方法hello(),message中延迟5秒,让B线程调用hello,请问执行结果? 1/hello 2/发短信
解释:hello方法没有synchronized锁,不是同步方法,不受锁的影响,主线程1s延迟后,创建了B,B执行hello,它不会管此刻仍然在延迟中的线程中的A手中的锁!
-
两个ticket,分别在A、B中调用message和call,message中延迟5秒,请问执行结果? 1/打电话 2/发短信
解释:都不是同一把锁,A中拿到的是第一个ticket的锁,它延迟跟1s后B拿到第二个ticket的锁没任何关系
-
将call和message都设置成static,message中有延迟5秒,让A和B分别调用message和call,请问执行结果?1/发短信 2/打电话
解释:都设置成static,其实都是随着类的加载而产生的,这个时候synchronized锁的是这个类的class实例对象,而并不是锁的调用者对象,因此还是A先拿到锁,但拿的是类Class实例的锁。
-
在5的基础上,分别用两个ticket分别在A和B中调用static和message,请问执行结果? 1/发短信 2/打电话
解释:只要加了static,不管是多少个ticket,锁的都是共有的唯一的那个class实例!因此还是A先拿到这个实例的锁!
-
一个延迟4秒的静态同步方法message被A中的ticket调用,一个普通同步方法call被B中的同一个ticket调用,请问执行结果? 1/打电话 2/发短信
解释:A拿到的是Class实例的锁,B拿到的是调用对象ticket的锁,两个锁不是一个东西,因此B先执行。
-
2个不同的ticket,分别在A和B中调用一个延迟4秒的静态同步方法message和一个普通同步方法call,请问执行结果? 1/打电话 2/发短信
解释:同7!
小结
synchronized只会锁2个东西:
- 类的Class实例对象
- 调用者实例本身
6. 集合类不安全
6.1 CopyOnWriteArrayList
// java.util.ConcurrentModificationException 并发修改异常
public class SaleTicketDemo1 {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
for (int i = 1; i <= 10 ; i++) {
new Thread(()->{
list.add(UUID.randomUUID().toString().substring(0,5));
System.out.println(list);
},String.valueOf(i)).start();
}
}
}
介绍
在多线程并发的情况下,ArrayList并不安全,会报 java.util.ConcurrentModificationException并发修改异常,如何解决这个问题呢?
-
用Vector
-
List<String> list = Collections.synchronizedList(new ArrayList<>());
-
List<String> list = new CopyOnWriteArrayList<>();
CopyOnWrite
- 写入时复制,COW,是计算机设计领域的一种优化策略
- 当多个线程调用ArrayList的时候,读是固定的,而写不一定是固定的,后者的写操作可能会覆盖前者,为了避免覆盖,才使用写入时复制。
- CopyOnWrite比Vector好在哪?Vector底层的add方法加了synchronized,方法效率低!
6.2 CopyOnWriteArraySet
public class SaleTicketDemo1 {
public static void main(String[] args) {
Set<String> set = new HashSet<>();
for (int i = 1; i <=10 ; i++) {
new Thread(()->{
set.add(UUID.randomUUID().toString().substring(0,5));
System.out.println(set);
},String.valueOf(i)).start();
}
}
}
解决方式
-
Set<String> set = Collections.synchronizedSet(new HashSet<>());
-
Set<String> set = new CopyOnWriteArraySet<>();
6.3 ConcurrentHashMap
public class SaleTicketDemo1 {
public static void main(String[] args) {
Map<String,Object> map = new HashMap();
for (int i = 0; i <30 ; i++) {
new Thread(()->{
map.put(Thread.currentThread().getName(),UUID.randomUUID().toString().substring(0,5));
System.out.println(map);
},String.valueOf(i)).start();
}
}
}
解决方式
-
Map<String,Object> map = Collections.synchronizedMap(new HashMap<>());
-
Map<String,Object> map = new ConcurrentHashMap<>();
7. Callable
介绍
- 有返回值
- 可以抛出异常
- 方法不同,call()
关系
代码测试
package com.kuang;
import java.sql.Array;
import java.sql.Time;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class SaleTicketDemo1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
MyThread myThread = new MyThread();
FutureTask futureTask = new FutureTask(myThread);
new Thread(futureTask,"A").start();
new Thread(futureTask,"B").start();
Integer o = (Integer) futureTask.get();
System.out.println(o);
}
}
class MyThread implements Callable<Integer>{
@Override
public Integer call() throws Exception {
System.out.println("call()"); // 结果只会打印一次call()
return 1024;
}
}
细节:
- 有缓存
- 结果可能需要等待,会阻塞
8. 三大常用辅助类
8.1 CutDownLatch
介绍
是一个减法计数器
测试
public class SaleTicketDemo1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
// 总数是6,必须要执行任务时再使用
CountDownLatch countDownLatch = new CountDownLatch(6);
for (int i = 1; i <=6 ; i++) {
new Thread(()->{
System.out.println(Thread.currentThread().getName()+" Go out");
countDownLatch.countDown(); // 数量-1
},String.valueOf(i)).start();
}
countDownLatch.await(); //等待计数器归零,然后再向下执行
System.out.println("Close");
}
}
原理()
- countDownLatch.countDown():数量-1
- countDownLatch.await():等待计数器归零,然后再向下执行
- 每次线程调用countDown()都会使countDownLatch数量-1,当数量为0的时候,唤醒await()方法,继续往下执行
8.2 CyclicBarrier
介绍(加法计数器)
测试
public class SaleTicketDemo1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
// 集齐7颗龙珠召唤神龙
//召唤龙珠的线程
CyclicBarrier cyclicBarrier = new CyclicBarrier(7,()->{
System.out.println("召唤神龙成功");
});
for (int i = 1; i <= 7; i++) {
int temp = i;
new Thread(()->{
System.out.println(Thread.currentThread().getName()+"收集了第"+temp+"颗龙珠");
try {
cyclicBarrier.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}).start();
}
}
}
await方法会进行减法计数,计数完了之后会开启一条新的线程执行!
8.3 Semaphore
介绍
测试
public class SaleTicketDemo1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
// 线程数量:停车位 主要用来限流
Semaphore semaphore = new Semaphore(3);
for (int i = 0; i < 6; i++) {
new Thread(()->{
// acquire() 得到
try {
semaphore.acquire();
System.out.println(Thread.currentThread().getName()+"抢到车位");
TimeUnit.SECONDS.sleep(2);
System.out.println(Thread.currentThread().getName()+"离开车位");
} catch (InterruptedException e) {
e.printStackTrace();
}finally {
semaphore.release();// release() 释放
}
},String.valueOf(i)).start();
}
}
}
原理
semaphore.acquire():获得。假设车位已经满了,会等待直到被释放为止
semaphore.release():释放。会将当前的信号量释放,然后唤醒等待的线程
作用:多个共享线程互斥的使用。并发限流,控制最大的线程数!
9. 读写锁
ReadWriteLock
测试
public class SaleTicketDemo1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
MyCache myCache = new MyCache();
for (int i = 1; i <= 5 ; i++) {
int temp = i;
new Thread(()->{
myCache.put(temp+"",temp+"");
},String.valueOf(i)).start();
}
for (int i = 1; i <= 5 ; i++) {
int temp = i;
new Thread(()->{
myCache.get(temp+"");
},String.valueOf(i)).start();
}
}
}
class MyCache{
private volatile Map<String,Object> map = new HashMap<>();
//读写锁:更加细粒度的控制
private ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
//存,写入的时候只希望同时只有一个线程往里边写
public void put(String key,Object value){
readWriteLock.writeLock().lock();
try {
System.out.println(Thread.currentThread().getName()+"写入"+key);
map.put(key,value);
System.out.println(Thread.currentThread().getName()+"写入OK");
} catch (Exception e) {
e.printStackTrace();
} finally {
readWriteLock.writeLock().unlock();
}
}
//取,读
public void get(String key){
readWriteLock.readLock().lock();
try {
System.out.println(Thread.currentThread().getName()+"读取"+key);
Object o = map.get(key);
System.out.println(Thread.currentThread().getName()+"读取OK");
} catch (Exception e) {
e.printStackTrace();
} finally {
readWriteLock.readLock().unlock();
}
}
}
分析
- 独占锁(写锁)、共享锁(读锁)
10. 阻塞队列
介绍
- Queue不是新的东西,是跟list和set同级的Collection子类
- BlockingQueue的使用场景:多线程并发处理、线程池
10.1 四组API
方式 | 抛出异常 | 不会抛出异常,有返回值 | 阻塞等待 | 超时等待 |
---|---|---|---|---|
添加 | add | offer | put | offer(,,) |
移除 | remove | poll | take | poll(,,) |
判断队列首部 | element | peek | - | - |
-
抛出异常
public class SaleTicketDemo1 { public static void main(String[] args) throws ExecutionException, InterruptedException { test1(); } public static void test1(){ //队列的大小是3 ArrayBlockingQueue blockingqueue = new ArrayBlockingQueue<>(3); System.out.println(blockingqueue.add('a')); System.out.println(blockingqueue.add('a')); System.out.println(blockingqueue.add('a')); // 抛出异常 Exception in thread "main" java.lang.IllegalStateException: Queue full // System.out.println(blockingqueue.add('a')); System.out.println("========================"); System.out.println(blockingqueue.element()); //查看队首元素 System.out.println(blockingqueue.remove()); System.out.println(blockingqueue.remove()); System.out.println(blockingqueue.remove()); // 抛出异常 Exception in thread "main" java.util.NoSuchElementException // System.out.println(blockingqueue.remove()); } }
-
不会抛出异常,有返回值
public class SaleTicketDemo1 { public static void main(String[] args) throws ExecutionException, InterruptedException { test1(); } public static void test1(){ //队列的大小是3 ArrayBlockingQueue blockingqueue = new ArrayBlockingQueue<>(3); System.out.println(blockingqueue.offer('a')); // true System.out.println(blockingqueue.offer('b')); // true System.out.println(blockingqueue.offer('c')); // true System.out.println(blockingqueue.offer('d')); // 不抛出异常 false System.out.println("=============================="); System.out.println(blockingqueue.peek()); //检测队首元素 System.out.println(blockingqueue.poll()); System.out.println(blockingqueue.poll()); System.out.println(blockingqueue.poll()); System.out.println(blockingqueue.poll()); // 不抛出异常 null } }
-
阻塞等待
public class SaleTicketDemo1 { public static void main(String[] args) throws ExecutionException, InterruptedException { test1(); } public static void test1() throws InterruptedException { //队列的大小是3 ArrayBlockingQueue blockingqueue = new ArrayBlockingQueue<>(3); blockingqueue.put("a"); blockingqueue.put("b"); blockingqueue.put("c"); // blockingqueue.put("d"); // 队列没有位置 一直阻塞 System.out.println(blockingqueue.take()); System.out.println(blockingqueue.take()); System.out.println(blockingqueue.take()); System.out.println(blockingqueue.take()); // 没有这个元素,一直阻塞 } }
-
等待超时
public class SaleTicketDemo1 { public static void main(String[] args) throws ExecutionException, InterruptedException { test1(); } public static void test1() throws InterruptedException { //队列的大小是3 ArrayBlockingQueue blockingqueue = new ArrayBlockingQueue<>(3); blockingqueue.offer("a"); blockingqueue.offer("b"); blockingqueue.offer("c"); // blockingqueue.offer("d",2,TimeUnit.SECONDS); // 等待 超过2秒就退出 System.out.println("================="); blockingqueue.poll(); blockingqueue.poll(); blockingqueue.poll(); blockingqueue.poll(2,TimeUnit.SECONDS); // 等待 超过2秒就退出 } }
10.2 同步队列
SynchronousQueue
- 没有容量,进去一个元素后,必须取出来之后才能再往里面放入一个元素
- put放入,take取出
测试
public class SaleTicketDemo1 {
public static void main(String[] args) throws ExecutionException, InterruptedException {
test1();
}
public static void test1() throws InterruptedException {
SynchronousQueue<String> synchronousQueue = new SynchronousQueue<>(); // 同步队列
new Thread(()->{
System.out.println(Thread.currentThread().getName()+" put 1");
try {
synchronousQueue.put("1");
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" put 2");
try {
synchronousQueue.put("2");
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" put 3");
try {
synchronousQueue.put("3");
} catch (InterruptedException e) {
e.printStackTrace();
}
},"T1").start();
new Thread(()->{
try {
TimeUnit.SECONDS.sleep(3);
System.out.println(Thread.currentThread().getName()+" take "+synchronousQueue.take());
TimeUnit.SECONDS.sleep(3);
System.out.println(Thread.currentThread().getName()+" take "+synchronousQueue.take());
TimeUnit.SECONDS.sleep(3);
System.out.println(Thread.currentThread().getName()+" take "+synchronousQueue.take());
} catch (InterruptedException e) {
e.printStackTrace();
}
},"T2").start();
}
}
11. 线程池
11.1 池化技术及应用
池化技术
-
程序的运行,本质:占用系统的资源!需要优化资源的使用==>池化技术,例如线程池、连接池、内存池、对象池,创建和销毁十分浪费资源
-
池化技术:事先准备好一些资源,有人要用就来这里拿,用完之后还过来
-
线程池的好处:
- 降低资源的消耗
- 提高相应的速度
- 方便管理线程
即线程复用、可以控制最大并发数和管理线程
线程池
线程池分类
线程池类型 | 用途说明 | 适用场景 |
---|---|---|
newFixedThreadPool | 创建固定线程数的线程池,使用的是LinkedBlockingQueue无界队列,线程池中实际线程数永远不会变化 | 适用于可以预测线程数量的业务中,或者服务器负载较重,对线程数有严格限制的场景 |
newSingleThreadExecutor | 创建只有一个线程的线程池,使用的是LinkedBlockingQueue无界队列,线程池中实际线程数只有一个 | 适用于需要保证顺序执行各个任务,并且在任意时间点,不会同时有多个线程的场景 |
newCachedThreadPool | 创建可供缓存的线程池,该线程池中的线程空闲时间超过60s会自动销毁,使用的是SynchronousQueue特殊无界队列 | 适用于创建一个可无限扩大的线程池,服务器负载压力较轻,执行时间较短,任务多的场景 |
newScheduledThreadPool | 创建可供调度使用的线程池(可延时启动,定时启动),使用的是DelayWorkQueue无界延时队列 | 适用于需要多个后台线程执行周期任务的场景 |
newWorkStealingPool | jdk1.8提供的线程池,底层使用的是ForkJoinPool实现,创建一个拥有多个任务队列的线程池,可以减少连接数,创建当前可用cpu核数的线程来并行执行任务 | 适用于大耗时,可并行执行的场景 |
11.2 七大参数和自定义线程池
三大方法
// Executors 工具类 3大方法
public class SaleTicketDemo1 {
public static void main(String[] args) {
// ExecutorService threadPool = Executors.newSingleThreadExecutor();// 单个线程的线程池
// ExecutorService threadPool = Executors.newFixedThreadPool(5);// 固定线程数量的线程池
ExecutorService threadPool = Executors.newCachedThreadPool();// 可伸缩的线程池
try {
for (int i = 1; i <100 ; i++) {
// 使用了线程池之后,使用线程池来创建
threadPool.execute(()->{
System.out.println(Thread.currentThread().getName()+" OK");
});
}
} catch (Exception e) {
e.printStackTrace();
} finally {
// 线程池用完,程序结束,关闭线程池
threadPool.shutdown();
}
}
}
七大参数
-
源码分析
public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); } public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); } public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); } // 本质:ThreadPoolExcutor() public ThreadPoolExecutor(int corePoolSize, // 核心线程池大小 int maximumPoolSize, // 最大线程池大小 long keepAliveTime, // 超时时间,没人调用就会释放 TimeUnit unit, // 超时单位 BlockingQueue<Runnable> workQueue, // 阻塞队列 ThreadFactory threadFactory, // 线程工厂,创建线程的,一般不动 RejectedExecutionHandler handler) { // 拒绝策略 if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
-
四种拒绝策略
- AbortPolicy:超过最大承载数了还有线程进来,不处理并报错
- CallerRunsPolicy:哪来的去哪里,是main方法执行
- DiscardPolicy:超过最大承载数了就会丢掉任务,不会抛出异常
- DiscardOlderstPolicy:队列满了,尝试去和最早的竞争也不会抛出异常
手动创建线程池
public class SaleTicketDemo1 {
public static void main(String[] args) {
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(
2,
5,
3,
TimeUnit.SECONDS,
new LinkedBlockingQueue<>(3),
Executors.defaultThreadFactory(),
new ThreadPoolExecutor.DiscardOldestPolicy());
try {
// 最大承载 = queue + max
// 超出最大承载,抛出RejectedExecutionException
for (int i = 1; i <= 9 ; i++) {
threadPool.execute(()->{
System.out.println(Thread.currentThread().getName()+" ok");
});
}
} catch (Exception e) {
e.printStackTrace();
} finally {
threadPool.shutdown();
}
}
}
问题:最大线程到底该如何定义?
-
CPU密集型:几核就是几,可以保证CPU效率最高!
ThreadPoolExecutor threadPool = new ThreadPoolExecutor( 2, Runtime.getRuntime().availableProcessors(), // 最大定义为「核数」 3, TimeUnit.SECONDS, new LinkedBlockingQueue<>(3), Executors.defaultThreadFactory(), new ThreadPoolExecutor.DiscardOldestPolicy());
-
IO密集型:判断你系统中十分消耗IO的线程数量,设置数值大于这个数就行了,一般可以设置2倍!
12. 四大函数式接口
介绍
只有一个方法的接口,简化了编程模型,在新版本的框架底层大量应用!
@FunctionalInterface
public interface Runnable {
public abstract void run();
}
// foreach(消费者类型的函数式接口)
测试
-
Function函数型接口,有一个输入参数,有一个输出参数
只要是函数式接口,都能用lambda表达式简化!
public class SaleTicketDemo1 { public static void main(String[] args) { //工具类:输出输入的值 // Function function = new Function<String,String>() { // @Override // public String apply(String str) { // return str; // } // }; Function<String,String> function = (str)->{ return str;}; System.out.println(function.apply("asd")); // asd } }
-
Predicate断定型接口,有一个输入参数,返回值只能是布尔值
public class SaleTicketDemo1 { public static void main(String[] args) { Predicate<String> predicate = str -> { return str.isEmpty();}; System.out.println(predicate.test("")); // true } }
-
Consumer消费型接口,只有输入参数没有返回值
public class SaleTicketDemo1 { public static void main(String[] args) { Consumer<String> consumer = (str)->{ System.out.println(str);}; consumer.accept("aaa"); // aaa } }
-
Supplier供给型接口,只有返回值没有输入参数
public class SaleTicketDemo1 { public static void main(String[] args) { Supplier supplier = () -> { return 1024;}; supplier.get(); // aaa } }
13. Stream流式计算
介绍
// 现有五个用户,要求一行代码实现以下筛选:
// 1.ID必须是偶数
// 2.年龄必须大于23岁
// 3.用户名转为大写字母
// 4.用户名字母倒着排序
// 5.只能输入一个用户
public class SaleTicketDemo1 {
public static void main(String[] args) {
User u1 = new User(1,"a",21);
User u2 = new User(2,"b",22);
User u3 = new User(3,"c",23);
User u4 = new User(4,"d",24);
User u5 = new User(6,"e",25);
// 集合就是存储
List<User> list = Arrays.asList(u1,u2,u3,u4,u5);
// 计算交给String流
list.stream()
.filter(u->{
return u.getId()%2==0;})
.filter(u->{
return u.getAge()>23;})
.map((u)->{
return u.getName().toUpperCase();})
.sorted((uu1,uu2)->{
return uu2.compareTo(uu1);})
.limit(1)
.forEach(System.out::println);
}
}
@AllArgsConstructor
@Data
@NoArgsConstructor
class User {
private int id;
private String name;
private int age;
}
14. ForkJoin
介绍
在JDK1.7出现,将大任务拆成小任务,并行执行任务,在大数据量的背景下能提高效率。
特点:工作窃取
这个里面维护的都是双端队列,因此两个线程都可以获取工作任务进行操作从而提高效率。
使用
-
forkjoinPool:通过它来执行
-
计算任务forkjoinPool.execute(forkjointask tast)
-
forjointask:即计算类要继承forkjointask
测试
package com.kuang;
import java.util.concurrent.RecursiveTask;
public class qwe extends RecursiveTask<Long> {
private Long start;
private Long end;
private Long temp = 10000L;
public qwe(Long start,Long end){
this.start=start;
this.end=end;
}
// 计算方法
@Override
protected Long compute() {
if((end-start) > temp) {
// 分支合并计算
Long sum = 0L;
for (Long i = start; i < end; i++) {
sum +=i;
}
return sum;
}else {
// forkjoin
long middle = (start+end)/2;
qwe task1 = new qwe(start, middle);
task1.fork(); // 拆分任务,把任务压入线程队列
qwe task2 = new qwe(middle+1, end);
task2.fork(); // 拆分任务,把任务压入线程队列
return task1.join()+task2.join();
}
}
}
package com.kuang;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.stream.LongStream;
public class Test {
public static void main(String[] args) throws ExecutionException, InterruptedException {
test3();
}
public static void test1(){
Long sum = 0L;
long start = System.currentTimeMillis();
for (Long i = 1L; i < 10_0000_0000; i++) {
sum +=i;
}
long end = System.currentTimeMillis();
System.out.println("sum="+sum+" 时间:"+(end - start));
}
public static void test2() throws ExecutionException, InterruptedException {
long start = System.currentTimeMillis();
ForkJoinPool forkJoinPool = new ForkJoinPool();
ForkJoinTask<Long> task = new qwe(0L, 10_0000_0000L);
ForkJoinTask<Long> submit = forkJoinPool.submit(task);// 提交任务 有结果
Long sum = submit.get();
long end = System.currentTimeMillis();
System.out.println("sum="+sum+" 时间 "+(end - start));
}
public static void test3(){
long start = System.currentTimeMillis();
// Stream并行流
long sum = LongStream.rangeClosed(0L, 10_0000_0000L).parallel().reduce(0, Long::sum);
long end = System.currentTimeMillis();
System.out.println("sum="+"时间"+(end - start));
}
}
15. 异步回调
介绍
public class Test {
public static void main(String[] args) throws ExecutionException, InterruptedException {
// 没有返回值的runAsync异步回调
CompletableFuture<Void> completableFuture = CompletableFuture.runAsync(()->{
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+"runAsync=>Void");
});
System.out.println("1111111111111");
completableFuture.get(); //获取执行结果
}
}
public class Test {
public static void main(String[] args) throws ExecutionException, InterruptedException {
CompletableFuture<Integer> completableFuture1 = CompletableFuture.supplyAsync(()->{
return 1024;
});
System.out.println(completableFuture1.whenComplete((t, u) -> {
System.out.println("t=" + t);
System.out.println("u=" + u);
}).exceptionally((e) -> {
System.out.println(e.getMessage());
return 233; // 可以获取到错误的返回结果
}).get());
}
}
16. JMM
Volatile
- Volatile是Java虚拟机提供轻量级的同步机制
- 保证可见性
- 不保证原子性
- 禁止指令重排
JMM
-
JMM:Java内存模型,不存在的东西,是一种概念和约定!
-
关于JMM的一些同步的约定:
- 线程解锁前:必须把共享内存立刻刷回主存
- 线程加锁前:必须读取主存中的最新值到工作内存中
- 加锁和解锁是同一把锁
-
8种操作:
内存交互操作有8种,虚拟机实现必须保证每一个操作都是原子的,不可在分的(对于double和long类型的变量来说,load、store、read和write操作在某些平台上允许例外)
-
lock (锁定):作用于主内存的变量,把一个变量标识为线程独占状态
-
unlock (解锁):作用于主内存的变量,它把一个处于锁定状态的变量释放出来,释放后的变量才可以被其他线程锁定
-
read (读取):作用于主内存变量,它把一个变量的值从主内存传输到线程的工作内存中,以便随后的load动作使用
-
load (载入):作用于工作内存的变量,它把read操作从主存中变量放入工作内存中
-
use (使用):作用于工作内存中的变量,它把工作内存中的变量传输给执行引擎,每当虚拟机遇到一个需要使用到变量的值,就会使用到这个指令
-
assign (赋值):作用于工作内存中的变量,它把一个从执行引擎中接受到的值放入工作内存的变量副本中
-
store (存储):作用于主内存中的变量,它把一个从工作内存中一个变量的值传送到主内存中,以便后续的write使用
-
write (写入):作用于主内存中的变量,它把store操作从工作内存中得到的变量的值放入主内存的变量中
JMM对这八种指令的使用,制定了如下规则:
-
不允许read和load、store和write操作之一单独出现。即使用了read必须load,使用了store必须write
-
不允许线程丢弃他最近的assign操作,即工作变量的数据改变了之后,必须告知主存
-
不允许一个线程将没有assign的数据从工作内存同步回主内存
-
一个新的变量必须在主内存中诞生,不允许工作内存直接使用一个未被初始化的变量。就是怼变量实施use、store操作之前,必须经过assign和load操作
-
一个变量同一时间只有一个线程能对其进行lock。多次lock后,必须执行相同次数的unlock才能解锁
-
如果对一个变量进行lock操作,会清空所有工作内存中此变量的值,在执行引擎使用这个变量前,必须重新load或assign操作初始化变量的值
-
如果一个变量没有被lock,就不能对其进行unlock操作。也不能unlock一个被其他线程锁住的变量
-
对一个变量进行unlock操作之前,必须把此变量同步回主内存
-
问题:程序不知道主内存中的值已经被修改过了!
16.1. Volatile
保证可见性
public class Test {
// 加了volatile后,保证了可见性,循环检测到了num被修改,停止了
private static volatile int num = 0;
public static void main(String[] args) {
// main线程
new Thread(()->{
while (num==0){
}
}).start();
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
num = 1;
System.out.println(num);
}
}
不保证原子性
原子性:不可分割!线程A在执行任务的时候,不能被打扰,也不能被分割,要么同时成功要么同时失败!
public class Test {
private volatile static int num = 0;
public static void add(){
num++;
}
public static void main(String[] args) {
// main线程
//理论上num结果为2万
for (int i = 1; i <= 20 ; i++) {
new Thread(()->{
for (int j = 0; j < 1000 ; j++) {
add();
}
}).start();
}
while (Thread.activeCount()>2){
// main gc
Thread.yield();
}
System.out.println(Thread.currentThread().getName()+" "+num); // 不是20000
}
}
如果不加lock和synchronized,怎么保证原子性?
使用原子类来解决原子性问题!
原子类
public class Test {
private volatile static AtomicInteger num =new AtomicInteger();
public static void add(){
num.getAndIncrement(); // AtomicInteger + 1 CAS
}
public static void main(String[] args) {
// main线程
//理论上num结果为2万
for (int i = 1; i <= 20 ; i++) {
new Thread(()->{
for (int j = 0; j < 1000 ; j++) {
add();
}
}).start();
}
while (Thread.activeCount()>2){
// main gc
Thread.yield();
}
System.out.println(Thread.currentThread().getName()+" "+num); // 20000
}
}
这些类的底层都是和操作系统挂钩!在内存中修改值!Unsafe类是一个很特殊的存在!
指令重排
-
你写的程序,计算机并不是按照你写的顺序去执行的!
-
源代码——>编译器优化的重排——>指令并行也可能会重排——>内存系统也会重排——>执行
-
处理器在进行指令重排的时候会考虑数据之间的依赖性问题!
int x = 1; // 1
int y = 2; //2
x = x + 5; //3
y = x * x; //4
我们所期望的是 「1234」,但是可能执行的时候会编程「2134」、「1324」
可不可能是「4123」 不会!
-
可能造成的结果:a、b、x、y四个值默认都是0:
线程A 线程B x=a y=b b=1 a=2 正常结果:x=0;y=0;但是可能由于指令重排,会出现以下指令顺序结果:
线程A 线程B b=1 a=2 x=a y=b 指令重排导致的诡异结果:x=2;y=1
-
加了volatile可以避免指令重排:
内存屏障,是一个CPU指令,作用:
-
保证特定的操作的执行顺序
-
保证某些变量的内存可见性(利用这些特性,volatile实现了可见性)
-
17. 单例模式
饿汉式
package com.kuang;
public class Hungry {
private Hungry(){
}
private final static Hungry HUNGRY = new Hungry();
public static Hungry getInstance(){
return HUNGRY;
}
}
懒汉式
package com.kuang;
public class LazyMan {
private LazyMan(){
System.out.println(Thread.currentThread().getName()+" OK");
}
private volatile static LazyMan lazyMan;
public static LazyMan getInstance(){
if(lazyMan==null){
synchronized (LazyMan.class){
if(lazyMan ==null){
lazyMan = new LazyMan(); // 不是原子性操作
// 1. 分配内存空间
// 2.执行构造方法,初始化对象
// 3.这个对象指向这个空间 在多线程下,这个过程发生指令重排 例如 A 132当在3的时候,B会直接return lazyMan
}
}
}
return lazyMan;
}
public static void main(String[] args) {
for (int i = 1; i < 10 ; i++) {
new Thread(()->{
LazyMan.getInstance();
}).start();
}
}
}
内部类
package com.kuang;
import com.sun.org.apache.bcel.internal.classfile.InnerClass;
public class Holder {
private Holder(){
}
public static Holder getInstance(){
return InnerClass.HOLDER;
}
public static class InnerClass{
private static final Holder HOLDER = new Holder();
}
}
枚举方式
package com.kuang;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public enum EnumSingle {
INSTANCE;
public EnumSingle getInstance(){
return INSTANCE;
}
}
class Test{
public static void main(String[] args) throws Exception {
EnumSingle instance1 = EnumSingle.INSTANCE;
Constructor<EnumSingle> declaredConstructor = EnumSingle.class.getDeclaredConstructor(String.class,int.class);
declaredConstructor.setAccessible(true);
EnumSingle instance2 = declaredConstructor.newInstance();
System.out.println(instance1 == instance2);
//Exception in thread "main" java.lang.IllegalArgumentException: Cannot reflectively create enum objects
at java.lang.reflect.Constructor.newInstance(Constructor.java:416)
}
}
18. 深入理解CAS
CAS
- CAS:compareAndSet,比较并交换!
- CAS是CPU的并发原语
package com.kuang;
import java.util.concurrent.atomic.AtomicInteger;
public class Test {
public static void main(String[] args) {
AtomicInteger atomicInteger = new AtomicInteger(2020);
// public final boolean compareAndSet(int expect, int update) {
// 期望、更新
// 如果我期望的值达到了,那么就更新,否则就不更新
System.out.println(atomicInteger.compareAndSet(2020, 2021));
System.out.println(atomicInteger.get());
System.out.println(atomicInteger.compareAndSet(2020, 2021));
System.out.println(atomicInteger.get());
}
}
Unsafe
总结
-
CAS:比较当前工作内存中的值和主内存中的值,如果这个值是期望的,那么则执行操作,如果不是就一直循环!
-
缺点:
-
底层的自旋锁,循环耗时
-
一次性只能保证一个共享变量的原子性
-
会存在ABA问题
-
19. 原子引用
ABA问题
package com.kuang;
import java.util.concurrent.atomic.AtomicInteger;
public class Test {
public static void main(String[] args) {
AtomicInteger atomicInteger = new AtomicInteger(2020);
// 对于我们平时写的sql:乐观锁!
// 期望、更新
// 如果我期望的值达到了,那么就更新,否则就不更新
// ========================捣乱的线程============================
System.out.println(atomicInteger.compareAndSet(2020, 2021));
System.out.println(atomicInteger.get());
System.out.println(atomicInteger.compareAndSet(2021, 2020));
System.out.println(atomicInteger.get());
// ========================期望的线程============================
System.out.println(atomicInteger.compareAndSet(2020, 6666));
System.out.println(atomicInteger.get());
}
}
原子引用
带版本号的原子操作!
解决ABA问题,引入原子引用,对应的思想:乐观锁!
public class Test {
public static void main(String[] args) {
// AtomicInteger atomicInteger = new AtomicInteger(2020);
AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(1,1);
new Thread(()->{
int stamp = atomicStampedReference.getStamp();
System.out.println("a1=>"+stamp);
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(atomicStampedReference.compareAndSet(1, 2, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1));
System.out.println("a2=>"+atomicStampedReference.getStamp());
System.out.println(atomicStampedReference.compareAndSet(2, 1, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1));
System.out.println("a3=>"+atomicStampedReference.getStamp());
},"a").start();
// 乐观锁的原理相同
new Thread(()->{
int stamp = atomicStampedReference.getStamp();
System.out.println("b1=>"+stamp);
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(atomicStampedReference.compareAndSet(1, 6, stamp, stamp + 1));
System.out.println("b2=>"+atomicStampedReference.getStamp());
},"b").start();
}
}
20. 可重入锁
介绍
-
可重入锁也叫递归锁
-
Synchorized:拿一把锁里面的也拿到
public class Test { public static void main(String[] args) { Phone phone = new Phone(); new Thread(()->{ phone.message(); },"A").start(); new Thread(()->{ phone.message(); },"B").start(); } } class Phone{ public synchronized void message(){ System.out.println(Thread.currentThread().getName()+"发短信"); call(); } public synchronized void call(){ System.out.println(Thread.currentThread().getName()+"打电话"); } }
-
lock:实际是两把锁!
package com.kuang; import java.util.concurrent.TimeUnit; import java.util.concurrent.atomic.AtomicInteger; import java.util.concurrent.atomic.AtomicReference; import java.util.concurrent.atomic.AtomicStampedReference; import java.util.concurrent.locks.ReentrantLock; public class Test { public static void main(String[] args) { Phone phone = new Phone(); new Thread(()->{ phone.message(); },"A").start(); new Thread(()->{ phone.message(); },"B").start(); } } class Phone{ ReentrantLock lock = new ReentrantLock(); public void message(){ lock.lock(); try { System.out.println(Thread.currentThread().getName()+"发短信"); call(); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } public void call(){ lock.lock(); try { System.out.println(Thread.currentThread().getName()+"打电话"); } catch (Exception e) { e.printStackTrace(); } finally { lock.unlock(); } } }
21. 自旋锁
介绍
public class TestOne {
public static void main(String[] args) throws InterruptedException {
Test lock = new Test();
new Thread(()->{
lock.mylock();
try {
TimeUnit.SECONDS.sleep(3);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.myUnlock();
}
},"T1").start();
TimeUnit.SECONDS.sleep(1);
new Thread(()->{
lock.mylock();
try {
TimeUnit.SECONDS.sleep(3);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.myUnlock();
}
},"T2").start();
}
}
public class Test {
AtomicReference<Thread> atomicReference = new AtomicReference<>();
// 加锁
public void mylock(){
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName()+"===> mylock");
// 自旋锁
while (!atomicReference.compareAndSet(null,thread)){
}
}
// 解锁
public void myUnlock(){
Thread thread = Thread.currentThread();
System.out.println(Thread.currentThread().getName()+"===> myUnlock");
atomicReference.compareAndSet(thread,null);
}
}
22. 死锁排查
死锁
测试
public class Ran {
public static void main(String[] args) {
String lockA = "lockA";
String lockB = "lockB";
new Thread(new MyThread(lockA,lockB),"T1").start();
new Thread(new MyThread(lockB,lockA),"T2").start();
}
}
class MyThread implements Runnable{
private String lockA;
private String lockB;
public MyThread(String lockA,String lockB){
this.lockA = lockA;
this.lockB = lockB;
}
@SneakyThrows
@Override
public void run() {
synchronized (lockA){
System.out.println(Thread.currentThread().getName()+"lock:"+lockA+"想获取"+lockB);
TimeUnit.SECONDS.sleep(2);
synchronized (lockB){
System.out.println(Thread.currentThread().getName()+"lock:"+lockB+"想获取"+lockA);
}
}
}
}
解决
-
使用jps-l定位进程号!
-
使用jstack 进程号找到死锁问题
-
日常问题排查:
- 日志
- 堆栈信息