sleep和wait的区别是面试中一个非常常见的问题，因为从表象来看，好像sleep和wait都能使线程处于阻塞状态，但是却有着本质上的却别。
- sleep是线程类Thread的方法，但是wait是java对象老祖宗Object中的方法。
- sleep方法不会释放lock，但是wait会释放，而且会加入到等待队列中。
- sleep方法不依赖于同步器synchronized，但是wait需要依赖synchronized关键字。
- sleep不需要被唤醒（休眠之后推出阻塞），但是wait需要（不指定时间需要被别人中断）。
区别一 : sleep是线程方法，wait是Object方法
通过一些代码来看看区别二：
package com.xy.Multithreading;
import java.util.stream.Stream;
public class Demo1 {
private final static Object sign = new Object();
public static void TestSleep(){
synchronized(sign){
try {
System.out.println(Thread.currentThread().getName()+"执行TestSleep方法");
Thread.sleep(5000);
System.out.println(Thread.currentThread().getName()+"休眠结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args){
Stream.of("线程1","线程2").forEach(n->new Thread(n){
@Override
public void run() {
Demo1.TestSleep();
}
}.start());
}
}
package com.xy.Multithreading;
import java.util.stream.Stream;
public class Demo2 {
private final static Object sign = new Object();
public static void TestWait(){
synchronized(sign){
try {
System.out.println(Thread.currentThread().getName()+"执行TestWait方法");
sign.wait(10000);
System.out.println(Thread.currentThread().getName()+"TestWait结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args){
Stream.of("线程1","线程2").forEach(n->new Thread(n){
@Override
public void run() {
Demo2.TestWait();
}
}.start());
}
}
package com.xy.Multithreading;
import java.util.stream.Stream;
public class Demo3 {
private final static Object sign = new Object();
public static void TestWait(){
synchronized(sign){
try {
System.out.println(Thread.currentThread().getName()+"执行TestWait方法");
Thread.sleep(5000);
System.out.println(Thread.currentThread().getName()+"sleep结束");
sign.wait(10000);
System.out.println(Thread.currentThread().getName()+"TestWait结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args){
Stream.of("线程1","线程2").forEach(n->new Thread(n){
@Override
public void run() {
Demo3.TestWait();
}
}.start());
}
}
package com.xy.Multithreading;
import java.util.Date;
import java.util.stream.Stream;
public class Demo4 {
private final static Object sign = new Object();
public static void TestWait(){
synchronized(sign){
try {
long beginTime = System.currentTimeMillis();
System.out.println(Thread.currentThread().getName()+"执行TestWait方法，当前时间："+new Date().toString());
Thread.sleep(5000);
long time = System.currentTimeMillis();
System.out.println(Thread.currentThread().getName()+"sleep结束，"+"从线程执行run到现在花的时间："
+(time-beginTime)+"，当前时间："+new Date().toString());
sign.wait(12000);
long endTime = System.currentTimeMillis();
System.out.println(Thread.currentThread().getName()+"TestWait结束,"
+"从线程执行run到现在花的时间："+(endTime-beginTime)+"，当前时间："+new Date().toString());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args){
Stream.of("线程1","线程2","线程3").forEach(n->new Thread(n){
@Override
public void run() {
Demo4.TestWait();
}
}.start());
}
}
区别二：sleep不释放锁，wait会让当前线程释放锁
- 执行demo1代码，首先打印"线程1执行TestSleep方法",然后线程1睡眠5秒后，接着几乎同时打印出"test1休眠结束"和"test2执行TestSleep方法"，线程2睡眠5秒后，接着打印"test2休眠结束"。
- 执行demo2代码，首先几乎同时打印"线程1执行TestWait方法和线程2执行TestWait方法",然后等待5秒后几乎同时打印"线程2TestWait结束"和"线程1TestWait结束".
- 执行ddemo3代码，首先打印"线程1执行TestWait方法",5秒后几乎同时打印"线程1sleep结束"和"线程2执行TestWait方法",再过5秒后打印"线程2sleep结束",再过5秒打印"线程1TestWait结束",再过5秒打印"线程2TestWait结束".
- 执行demo4代码:
"C:\Program Files\Java\jdk1.8.0_111\bin\java.exe" "-javaagent:D:\IDEA\IntelliJ IDEA Free2020.1\IntelliJ IDEA Community Edition 2020.1\lib\idea_rt.jar=55642:D:\IDEA\IntelliJ IDEA Free2020.1\IntelliJ IDEA Community Edition 2020.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_111\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_111\jre\lib\rt.jar;E:\ideaWorkspace\target\classes;C:\Users\96935\.m2\repository\org\springframework\boot\spring-boot-starter\2.2.6.RELEASE\spring-boot-starter-2.2.6.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\boot\spring-boot\2.2.6.RELEASE\spring-boot-2.2.6.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\spring-context\5.2.5.RELEASE\spring-context-5.2.5.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\spring-aop\5.2.5.RELEASE\spring-aop-5.2.5.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\spring-beans\5.2.5.RELEASE\spring-beans-5.2.5.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\spring-expression\5.2.5.RELEASE\spring-expression-5.2.5.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\boot\spring-boot-autoconfigure\2.2.6.RELEASE\spring-boot-autoconfigure-2.2.6.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\boot\spring-boot-starter-logging\2.2.6.RELEASE\spring-boot-starter-logging-2.2.6.RELEASE.jar;C:\Users\96935\.m2\repository\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar;C:\Users\96935\.m2\repository\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar;C:\Users\96935\.m2\repository\org\apache\logging\log4j\log4j-to-slf4j\2.12.1\log4j-to-slf4j-2.12.1.jar;C:\Users\96935\.m2\repository\org\apache\logging\log4j\log4j-api\2.12.1\log4j-api-2.12.1.jar;C:\Users\96935\.m2\repository\org\slf4j\jul-to-slf4j\1.7.30\jul-to-slf4j-1.7.30.jar;C:\Users\96935\.m2\repository\jakarta\annotation\jakarta.annotation-api\1.3.5\jakarta.annotation-api-1.3.5.jar;C:\Users\96935\.m2\repository\org\springframework\spring-core\5.2.5.RELEASE\spring-core-5.2.5.RELEASE.jar;C:\Users\96935\.m2\repository\org\springframework\spring-jcl\5.2.5.RELEASE\spring-jcl-5.2.5.RELEASE.jar;C:\Users\96935\.m2\repository\org\yaml\snakeyaml\1.25\snakeyaml-1.25.jar;C:\Users\96935\.m2\repository\org\projectlombok\lombok\1.18.12\lombok-1.18.12.jar;C:\Users\96935\.m2\repository\cglib\cglib\3.2.9\cglib-3.2.9.jar;C:\Users\96935\.m2\repository\org\ow2\asm\asm\7.0\asm-7.0.jar;C:\Users\96935\.m2\repository\org\apache\ant\ant\1.10.3\ant-1.10.3.jar;C:\Users\96935\.m2\repository\org\apache\ant\ant-launcher\1.10.3\ant-launcher-1.10.3.jar;C:\Users\96935\.m2\repository\org\slf4j\slf4j-api\1.7.30\slf4j-api-1.7.30.jar" com.xy.Multithreading.Demo4
线程1执行TestWait方法，当前时间：Tue May 12 20:49:10 CST 2020
线程1sleep结束，从线程执行run到现在花的时间：5028，当前时间：Tue May 12 20:49:15 CST 2020
线程3执行TestWait方法，当前时间：Tue May 12 20:49:15 CST 2020
线程3sleep结束，从线程执行run到现在花的时间：5000，当前时间：Tue May 12 20:49:20 CST 2020
线程2执行TestWait方法，当前时间：Tue May 12 20:49:20 CST 2020
线程2sleep结束，从线程执行run到现在花的时间：5001，当前时间：Tue May 12 20:49:25 CST 2020
线程1TestWait结束,从线程执行run到现在花的时间：17028，当前时间：Tue May 12 20:49:27 CST 2020
线程3TestWait结束,从线程执行run到现在花的时间：17000，当前时间：Tue May 12 20:49:32 CST 2020
线程2TestWait结束,从线程执行run到现在花的时间：17001，当前时间：Tue May 12 20:49:37 CST 2020
Process finished with exit code 0
结论：执行wait方法的时候，当前线程会释放被synchronized锁住的代码块和对象，但并不是让synchronized失效。从demo4可以看出，线程1释放锁之后，synchronized(sign){...}代码块马上被线程3执行并加锁，当线程3执行wait方法后又会释放锁，线程2才能执行这块代码。
