JAVA学习-多线程
多线程
Thread
- 自定义线程类继承Thread类
- 重写run()方法,编写线程执行体
- 创建线程对象,调用start()方法启动线程
注意:如果调用run()方法,则只有主线程一条执行路径,但调用start()方法,则多条执行路径,主线程和子线程并行交替执行
package com.myThread.demo01;
public class Demo01 {
public static void main(String[] args) {
MyThread myThread = new MyThread();
myThread.start();
for (int i = 0; i < 10000; i++) {
System.out.println("我是主线程--"+i);
}
}
}
///////////////////////////
package com.myThread.demo01;
public class MyThread extends Thread {
@Override
public void run() {
for (int i = 0; i < 200; i++) {
System.out.println("我是子线程--"+i);
}
}
}
**线程的执行由CPU决定 **
网络图片下载
需要用到commons io包 百度下载二进制文件 然后复制粘贴到idea新建一个lib包里即可
package com.myThread.demo02;
import org.apache.commons.io.FileUtils;
import java.io.File;
import java.io.IOException;
import java.net.URL;
public class Demo01 {
public static void main(String[] args) {
MyThread myThread1 = new MyThread("https://img2022.cnblogs.com/blog/1755845/202202/1755845-20220206182923194-2117509516.png","4.jpg");
MyThread myThread2 = new MyThread("https://img2022.cnblogs.com/blog/1755845/202203/1755845-20220303112203654-338817152.png","5.jpg");
MyThread myThread3 = new MyThread("https://img2022.cnblogs.com/blog/1755845/202202/1755845-20220223223043340-1183125272.png","6.jpg");
myThread1.start();
myThread2.start();
myThread3.start();
}
}
//子线程写下载,实现多线程下载资源
class MyThread extends Thread{
private String myUrl = null;
private String myname = null;
public MyThread(String url,String name) {
this.myname = name;
this.myUrl = url;
}
@Override
public void run() {
WebDownload webDownload = new WebDownload();
webDownload.download(myUrl,myname);
System.out.println("下载好了"+myname);
}
}
//下载器
class WebDownload{
public void download(String url,String name){
try {
FileUtils.copyURLToFile(new URL(url),new File(name));
} catch (IOException e) {
e.printStackTrace();
System.out.println("下载器方法出现异常!");
} finally {
}
}
}
再次证明了线程是由CUP分配的,人为无法干预
Runnable接口
创建线程方式2:实现runnable接口,重写run方法,执行线程需要丢入对象
package com.myThread.demo03;
public class MyRunnable implements Runnable{
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
System.out.println("我在看书--"+i);
}
}
}
////////////////////////
package com.myThread.demo03;
public class Demo01 {
public static void main(String[] args) {
new Thread(new MyRunnable()).start();//这里new一个Thread 然后把实现runnable接口的对象丢进去
for (int i = 0; i < 2000; i++) {
System.out.println("我在玩手机--"+i);
}
}
}
最好使用runnable接口实现多线程,避免单继承局限性,灵活方便,方便同一个对象被多个线程使用
初识并发问题
package com.myThread.demo04;
public class MyTicket implements Runnable{
private int ticketNums = 1;
@Override
public void run() {
while (true){
if (ticketNums>=20){
break;
}
try {
Thread.sleep(200);//模拟延时
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
}
//Thread.currentThread().getName() 可以拿到当前线程的名字 这里三线程同用一个对象
System.out.println(Thread.currentThread().getName()+"--抢到了第"+ticketNums+++"张票!");
}
}
}
////////////////////
package com.myThread.demo04;
public class Demo01 {
public static void main(String[] args) {
MyTicket ticket = new MyTicket();//对象new一个就行了
//三线程同用一个对象
new Thread(ticket,"张三").start();
new Thread(ticket,"李四").start();
new Thread(ticket,"王五").start();
}
}
运行结果:
李四和王五都抢到了第二张票,说明线程不安全,出现了数据紊乱。这就是线程并发出现的问题。
龟兔赛跑
要求跑道100,兔子乌龟一起跑,但兔子要睡觉,乌龟取得胜利
package com.myThread.demo05;
public class Race implements Runnable{
private int steps = 1;
private static String winner;
@Override
public void run() {
while (true){
if (Thread.currentThread().getName().equals("兔子") && steps%10 == 0){
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
}
}
if (theWinner(steps)){//小细节,判断要放在兔子休眠后,可以避免兔子在接受比赛后又多跑一步
break;
}
System.out.println(Thread.currentThread().getName()+"--已经跑了"+steps+++"步");
}
}
public boolean theWinner(int steps){
if (winner != null){
return true;//这句话必须写,要不然兔子或乌龟一个跑完了另一个线程不会停止!!
}
else if (steps>=100 && winner == null){
winner = Thread.currentThread().getName();
System.out.println(winner+"获得了胜利!");
return true;
}
return false;
}
}
////////////////////////////
package com.myThread.demo05;
public class Demo01 {
public static void main(String[] args) {
Race race = new Race();
new Thread(race,"乌龟").start();
new Thread(race,"兔子").start();
}
}
Callable接口
不是很明白,以后再来!
静态代理
真实对象与代理对象实现同一个接口,代理对象要代理真实对象。真实对象通过构造器传给代理对象,代理对象就可以不仅完成真实对象的方法,还可以做很多真实对象没有做的事情,真实对象只需要专注自己的事情。
Lambda表达式
Lambda可以简化代码,但注意:
- 接口必须是函数式接口,也就是接口只能存在一种带实现的方法
- 方法内代码如果只有一行,可以简化不写花括号,如果有多行,必须使用代码块包裹,也就是花括号
- 接口待实现方法含参,可以去掉参数类型,多个参数也能去掉参数类型,要去掉就都去掉,必须加上括号(参数只有一个,括号也能去掉)
- 格式为 接口 名字 = (参数) -> {代码};
package com.myThread.demo06;
public class Demo01 {
public static void main(String[] args) {
Marry marry = null;
marry = ()-> System.out.println("lalla");
marry.makeMarry();
}
}
interface Marry{
void makeMarry();
}
线程停止
线程停止,自己在while循环里用一个flag标志位来控制
.sleep
线程休眠
利用休眠做一个定时器,每一秒刷新一次时间
package com.myThread.demo07;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
public class Demo01 {
public static void main(String[] args) {
Runnable runnable = null;
runnable = ()->{
while (true){
LocalDateTime localDateTime = LocalDateTime.now();//获取时间
DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("HH:mm:ss");//调整格式
System.out.println(dateTimeFormatter.format(localDateTime));//按照格式输出时间
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
}
}
};
new Thread(runnable).start();
}
}
.yield
线程礼让
- 礼让线程,让当前正在执行的线程暂停,但不阻塞
- 将线程从运行状态转为就绪状态
- 让cpu重新调度,礼让不一定成功!看cpu心情
.join
合并线程,待此线程执行完成后,再执行其他线程,其他线程阻塞
可以理解为插队
package com.myThread.demo08;
public class Demo08 implements Runnable{
@Override
public void run() {
for (int i = 0; i < 1000; i++) {
System.out.println("我是vip--"+i);
}
}
public static void main(String[] args) {
Demo08 demo08 = new Demo08();
Thread thread = new Thread(demo08);
thread.start();
for (int i = 0; i < 2000; i++) {
System.out.println("我是主线程--"+i);
if (i == 1000){
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
}
}
}
}
}
由上,让线程插队,等主线程1000时插队,但主线程没到1000时也会跑,完全听从cpu调度,但一旦主线程到达1000就强行插队,知道线程跑完再开始主线程
观测线程
Thread.State state = thread.getState();
System.out.println(state);
线程优先级
**set priority() get priority() **
守护线程
set daemon()
守护线程会随着用户线程结束,虚拟机停止就停止了。虚拟机不会等待守护线程的运行。
package com.myThread.demo09;
public class Demo01 {
public static void main(String[] args) {
You you = new You();
God god = new God();
new Thread(you).start();
Thread thread = new Thread(god);
thread.setDaemon(true);
thread.start();
}
}
class You implements Runnable{
@Override
public void run() {
for (int i = 1; i < 36500; i++) {
System.out.println("活着的第--"+i+"天");
}
System.out.println("=====goodbye,world!=====");
}
}
class God implements Runnable{
@Override
public void run() {
while (true){
System.out.println("老天爷看着你");
}
}
}
上面的god线程虽然用了while(true)循环,但程序也会随着用户线程的停止然后虚拟机的停止而停止。(虚拟机的停止需要一小点时间)
不安全问题
银行取钱的不安全问题,代码稍微有点不理想,将就吧,也确实出现了线程不安全
package com.unsafeDemo.demo01;
//银行取钱线程不安全
public class Demo01 {
public static void main(String[] args) {
Account account = new Account(100,"全部家当");
Bank bank = new Bank(account,50,"小明");
Bank bank1 = new Bank(account,70,"小红");
new Thread(bank).start();
new Thread(bank1).start();
}
}
class Account {
int money;
String name;
public Account(int money, String name){
this.money = money;
this.name = name;
}
}
class Bank implements Runnable{
Account account ;
private int drawingMoney;
private String name;
public Bank(Account account, int drawingMoney, String name){
this.drawingMoney = drawingMoney;
this.account = account;
this.name = name;
}
public void drawingMoney(){
account.money = account.money - drawingMoney;
System.out.println(this.name+"取了"+drawingMoney);
System.out.println("余额为"+account.money);
}
@Override
public void run() {
if (drawingMoney > account.money){
System.out.println("没钱了,取鸡毛");
return;
}
try {
Thread.sleep(100);//利用延迟放大问题
} catch (InterruptedException e) {
e.printStackTrace();
}
drawingMoney();
}
}
同步块
synchronized (Obj){}
Obj可以是任何对象,要填入需要被锁的对象,也就是有变化的对象,然后再将变化过程的代码放在这个同步块里面。
Lock
可显示话的锁,一般使用ReentrantLock类(可重入锁)
先复习不加锁:
package com.unsafeDemo.demo02;
public class Demo02 {
public static void main(String[] args) {
BuyTicks ticks = new BuyTicks();
Thread t1 = new Thread(ticks,"我");
Thread t2 = new Thread(ticks,"你");
Thread t3 = new Thread(ticks,"他");
t1.start();
t2.start();
t3.start();
}
}
class BuyTicks implements Runnable{
private int ticks = 10;
boolean flag = true;
@Override
public void run() {
while(flag) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (ticks > 0) {
System.out.println(Thread.currentThread().getName() + "抢到了第" + ticks-- + "票");
}else {
flag = false;
}
}
}
}
加锁代码:
package com.unsafeDemo.demo02;
import java.util.concurrent.locks.ReentrantLock;
public class Demo02 {
public static void main(String[] args) {
BuyTicks ticks = new BuyTicks();
Thread t1 = new Thread(ticks,"我");
Thread t2 = new Thread(ticks,"你");
Thread t3 = new Thread(ticks,"他");
t1.start();
t2.start();
t3.start();
}
}
class BuyTicks implements Runnable{
private int ticks = 10;
boolean flag = true;
private final ReentrantLock lock = new ReentrantLock();
@Override
public void run() {
try {
lock.lock();
while(flag) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (ticks > 0) {
System.out.println(Thread.currentThread().getName() + "抢到了第" + ticks-- + "票");
}else {
flag = false;
}
}
} finally {
lock.unlock();
}
}
}
注意,new ReentrantLock的时候给他加修饰符private final,不懂,但是照葫芦画瓢,然后就是加锁和解锁,放在catch-finally里面,不懂,继续照葫芦画瓢。
生产者消费者
wait() notifyAll() 是只能写在synchronized里面的
package com.myPC.demo03;
public class Demo03 {
public static void main(String[] args) {
MyContainer myContainer = new MyContainer();
new Productor(myContainer).start();
new Consumer(myContainer).start();
}
}
class Productor extends Thread{
MyContainer myContainer;
public Productor(MyContainer myContainer) {
this.myContainer = myContainer;
}
@Override
public void run() {
for (int i = 1; i < 100; i++) {
myContainer.push(new Chicken(i));
System.out.println("生产了第"+i+"只鸡");
}
}
}
class Consumer extends Thread{
MyContainer myContainer;
public Consumer(MyContainer myContainer) {
this.myContainer = myContainer;
}
@Override
public void run() {
for (int i = 1; i < 100; i++) {
System.out.println("消费了第"+myContainer.del().id+"只鸡");
}
}
}
class Chicken{
int id;
public Chicken(int id) {
this.id = id;
}
}
class MyContainer{
Chicken[] chickens = new Chicken[10];
int count = 0;
public synchronized void push(Chicken chicken){
while (count >= chickens.length){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
chickens[count] = chicken;
count++;
notifyAll();//唤醒所有线程,包括没有鸡时停止的消费线程
}
public synchronized Chicken del(){
if (count <= 0){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
count--;
Chicken chicken = chickens[count];
notifyAll();//唤醒所有线程,包括鸡满了停止的生产线程
return chicken;
}
}
关于wait() 与 notifyAll() 的思考
package com.myPC.demo04;
import java.util.concurrent.locks.ReentrantLock;
public class Demo04 {
public static void main(String[] args) {
abc abc = new abc();
new Thread(abc,"a").start();
new Thread(abc,"b").start();
}
}
class abc implements Runnable{
int count = 1;
@Override
public synchronized void run() {
for (int i = 0; i < 19; i++) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
while (Thread.currentThread().getName().equals("a") && count%5==0 ){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("a被唤醒!");
}
while (Thread.currentThread().getName().equals("b") && count%4==0 ){
try {
wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("b被唤醒!");
}
System.out.println(Thread.currentThread().getName()+"--得到了"+count);
count++;
notifyAll();
}
}
}
这段代码,就是让ab去获取数字,一开始,a被cpu指派去获取,但到a走进while循环里面碰见wait的时候,a停了,这个时候,cpu会直接让b接进来继续获取数字,并且通过notifyAll把a唤醒,a会不会获取数字看cpu调度,但是一旦b走进while循环碰见wait,则会让a继续从a停止的wait那里开始往下走。只要a从休眠状态被唤醒再被cpu调度,都是继续从wait那里开始执行。
信号灯法
利用一个flag标志位来解决生产者消费者的问题
线程池
ExecutorService:真正的线程池接口。常见子类ThreadPoolExecutor
Executors:工具类,线程池的工厂类,用于创建并返回不同类型的线程池
package com.myPC.demo05;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Demo05 {
public static void main(String[] args) {
//创建一个线程池服务
ExecutorService service = Executors.newFixedThreadPool(3);//参数的意义是线程池的大小
//执行
service.execute(new MyPool());
service.execute(new MyPool());
service.execute(new MyPool());
//关闭线程池
service.shutdown();
}
}
class MyPool implements Runnable {
@Override
public void run() {
System.out.println(Thread.currentThread().getName());
}
}
这里与Callable有点相似,这是属于Runnable接口的线程池