JAVA多线程入门
继承Thread父类
线程代码执行顺序和调用顺序无关，例如:
public class MyThread extends Thread {@Override
public void run(){
super.run();
System.out.println("MyThread");
}
/**运行顺序存疑
* 并没有发现随机性 */
public static void main(String[] args) {
MyThread thread = new MyThread();
thread.run();
System.out.println("mainThread");
}}
上述代码执行理论上“MyThread”和“mainThread”打印顺序是随机的，和调用顺序无关，实际情况存疑。
线程执行具有随机性，CPU的执行具有不确定性
public class MyThread1 extends Thread {@Override
public void run(){
try {
for (int i = 0;
i < 10;
i++){
int time = (int) (Math.random()*1000);
Thread.sleep(time);
System.out.println("run:"+Thread.currentThread().getName());
}
}catch (InterruptedException e) {
e.printStackTrace();
}
}public static void main(String[] args) throws InterruptedException {
MyThread1 thread = new MyThread1();
thread.setName("myThread");
thread.start();
for (int i = 0;
i<10;
i++){
int time = (int) (Math.random()*1000);
Thread.sleep(time);
System.out.println("main:"+Thread.currentThread().getName());
}}
}/*结果：
run:myThread
run:myThread
run:myThread
main:main
main:main
run:myThread
run:myThread
run:myThread
main:main
main:main
main:main
main:main
run:myThread
run:myThread
main:main
run:myThread
run:myThread
main:main
main:main
main:main
*/start方法并不代表线程启动，线程启动顺序由CPU执行顺序决定，无序性。
public class MyThread2 extends Thread {
private int i;
public MyThread2(int i){
super();
this.i = i;
}
@Override
public void run(){
System.out.println("myThread:"+i);
}public static void main(String[] args){
MyThread2 t1 = new MyThread2(1);
MyThread2 t2 = new MyThread2(2);
MyThread2 t3 = new MyThread2(3);
MyThread2 t4 = new MyThread2(4);
MyThread2 t5 = new MyThread2(5);
MyThread2 t6 = new MyThread2(6);
MyThread2 t7 = new MyThread2(7);
MyThread2 t8 = new MyThread2(8);
MyThread2 t9 = new MyThread2(9);
MyThread2 t10 = new MyThread2(10);
MyThread2 t11 = new MyThread2(11);
MyThread2 t12 = new MyThread2(12);
MyThread2 t13 = new MyThread2(13);
MyThread2 t14 = new MyThread2(14);
MyThread2 t15 = new MyThread2(15);
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
t6.start();
t7.start();
t8.start();
t9.start();
t10.start();
t11.start();
t12.start();
t13.start();
t14.start();
t15.start();
}
}/*结果：
myThread:2
myThread:1
myThread:3
myThread:4
myThread:7
myThread:8
myThread:11
myThread:12
myThread:15
myThread:13
myThread:14
myThread:5
myThread:6
myThread:9
myThread:10
*/
Runnable接口构造线程 java是单基础，继承Thread类有局限性，所以更多的是使用Runnable接口去新建线程，Thread类有构造方法使用Runnable接口新建线程。
public class RunableTest implements Runnable {@Override
public void run() {
System.out.println("Runable线程运行中:"+Thread.currentThread().getName());
}public static void main(String[] arg){
RunableTest runableTest = new RunableTest();
Thread thread = new Thread(runableTest);
thread.start();
System.out.println("mainThread:"+Thread.currentThread().getName() );
}
}实例变量与线程安全 实例变量不共享 线程间变量不共享,数据不共享情况：
public class ShareThread extends Thread{
private int count = 5;
public ShareThread(String name){
super();
this.setName(name);
}
@Override
public void run(){
super.run();
while (count >0){
count--;
System.out.println("由"+Thread.currentThread().getName()+"计算,count="+count);
}
}public static void main(String[] args){
ShareThread shareThread1 = new ShareThread("A");
ShareThread shareThread2 = new ShareThread("B");
ShareThread shareThread3 = new ShareThread("C");
shareThread1.start();
shareThread2.start();
shareThread3.start();
}
}/*
由A计算,count=4
由B计算,count=4
由B计算,count=3
由B计算,count=2
由B计算,count=1
由B计算,count=0
由A计算,count=3
由A计算,count=2
由A计算,count=1
由A计算,count=0
由C计算,count=4
由C计算,count=3
由C计算,count=2
由C计算,count=1
由C计算,count=0
*/线程数据共享 共享数据情况就是多个线程可以访问同一个变量。
public class ShareThread1 extends Thread {
private int count= 10;
@Override
synchronized public void run(){
super.run();
count--;
//不要使用for语句，因为使用同步后线程就没有运行机会了
//一直由线程进行减法运算
System.out.println("由"+Thread.currentThread().getName()+"计算，count="+count);
}public static void main(String[] args){
ShareThread1 thread1 = new ShareThread1();
Thread a = new Thread(thread1,"A");
Thread b = new Thread(thread1,"B");
Thread c = new Thread(thread1,"C");
Thread d = new Thread(thread1,"D");
Thread e = new Thread(thread1,"E");
Thread f = new Thread(thread1,"F");
a.start();
b.start();
c.start();
d.start();
e.start();
f.start();
}}/*结果：
由A计算，count=9
由D计算，count=8
由E计算，count=7
由F计算，count=6
由C计算，count=5
由B计算，count=4
*/
synchronized关键字表示执行多个线程时以排队的方式进行处理。线程执行时会上锁，执行完毕后会解锁，线程调用run()方法前会请求线程锁，若已经上锁，则会不断请求线程锁。
System.out.println()使用时可能会发生“非线程安全”问题，里面打印i--时，会先执行i--，然后打印结果,造成线程安全问题。
public class ShareThread2 extends Thread {
private int i = 5;
@Override
public void run(){
System.out.println("i="+ (i--) +",threadName="+Thread.currentThread().getName());
//i--在println之前执行，故可能发生非线程安全问题
}public static void main(String[] args) {
ShareThread2 run = new ShareThread2();
Thread t1 = new Thread(run);
Thread t2 = new Thread(run);
Thread t3 = new Thread(run);
Thread t4 = new Thread(run);
Thread t5 = new Thread(run);
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
}
}/*
i=4,threadName=Thread-1
i=5,threadName=Thread-3
i=2,threadName=Thread-5
i=5,threadName=Thread-4
i=3,threadName=Thread-2*/
常用函数 currentThread()方法
currentThread返回代码段被哪个线程调用的信息。
public class CountOpertrate extends Thread {
public CountOpertrate(){
System.out.println("CountOpertate-build-start");
System.out.println("Thread.currentThread().getName() = "+Thread.currentThread().getName());
System.out.println("this.getName()"+this.getName());
System.out.println("CountOpertrate-build-end");
}@Override
public void run(){
System.out.println("run-start");
System.out.println("Thread.currentThread().getName() = "+Thread.currentThread().getName());
System.out.println("this.getName()"+this.getName());
System.out.println("run-end");
}public static void main(String[] args) {
CountOpertrate countOpertrate = new CountOpertrate();
Thread t = new Thread(countOpertrate);
t.setName("TEST");
t.start();
}/*result:
CountOpertate-build-start
Thread.currentThread().getName() = main
this.getName()Thread-0
CountOpertrate-build-end
run-start
Thread.currentThread().getName() = TEST
this.getName()Thread-0
run-end
*/
【JAVA多线程入门】上述代码显示，Count构建时时用的main线程，run是跑在TEST线程上。
isAlive()方法
isAlive方法是判断当前线程是处于活动状态。
public class IsAliveTest extends Thread {
@Override
public void run() {
System.out.println("run = "+this.isAlive());
}public static void main(String[] args) throws InterruptedException {
IsAliveTest i = new IsAliveTest();
System.out.println("start =="+i.isAlive());
i.start();
Thread.sleep(1000);
System.out.println("end =="+i.isAlive());
}
}/*result:
start ==false
run = true
end ==false
*/
若将线程对象以构造参数传递给Thread对象进行start，结果会有差异。
public class IsAliveTest1 extends Thread {public IsAliveTest1(){
System.out.println("IsAliveTest1-Start");
System.out.println("Thread.currentThread().getName() = "+Thread.currentThread().getName());
System.out.println("Thread.currentThread().isAlive() = "+Thread.currentThread().isAlive());
System.out.println("this.getName() = "+this.getName());
System.out.println("this.isAlive() = "+this.isAlive());
System.out.println("IsAliveTest1-end");
}
@Override
public void run(){
System.out.println("run-Start");
System.out.println("Thread.currentThread().getName() = "+Thread.currentThread().getName());
System.out.println("Thread.currentThread().isAlive() = "+Thread.currentThread().isAlive());
System.out.println("this.getName() = "+this.getName());
System.out.println("this.isAlive() = "+this.isAlive());
System.out.println("run-end");
}public static void main(String[] args) throws InterruptedException {
IsAliveTest1 test1 = new IsAliveTest1();
Thread t1 = new Thread(test1);
System.out.println("main bigin t1 isAlive = "+t1.isAlive());
t1.setName("AAA");
t1.start();
Thread.sleep(1000);
System.out.println("main end t1 isAlive = "+t1.isAlive());
}
}/*result:
IsAliveTest1-Start
Thread.currentThread().getName() = main
Thread.currentThread().isAlive() = true
this.getName() = Thread-0
this.isAlive() = false
IsAliveTest1-end
main bigin t1 isAlive = false
run-Start
Thread.currentThread().getName() = AAA
Thread.currentThread().isAlive() = true
this.getName() = Thread-0
this.isAlive() = false
run-end
main end t1 isAlive = false
*/sleep()方法
sleep()方法是在括号中毫秒内使正在执行的线程暂停执行的方法，正在执行的线程是this.currentThread()返回的线程。
public class sleepTest extends Thread {
@Override
public void run() {
try {
System.out.println("run threadName = "+this.getName()+"-begin");
Thread.sleep(2000);
System.out.println("run ThreadName = "+this.getName()+"-end");
} catch (InterruptedException e) {
e.printStackTrace();
}
}public static void main(String[] args) {
sleepTest test = new sleepTest();
System.out.println("begin = "+ System.currentTimeMillis());
test.run();
//test.start();
System.out.println("end = "+System.currentTimeMillis());
}
}/*直接用run()方法
begin = 1521341785159
run threadName = Thread-0-begin
run ThreadName = Thread-0-end
end = 1521341787160
*/
/*使用start()方法
begin = 1521341902632
end = 1521341902632
run threadName = Thread-0-begin
run ThreadName = Thread-0-endmain和sleepTest线程是异步的，所以先打印时间
*/
停止线程 interrupt()方法
interrupt方法并不是立刻停止线程。而是在当前线程中打一个停止标记。
public class InterruptTest extends Thread {@Override
public void run() {
super.run();
for (int i = 0;
i<50000;
i++){
System.out.println("i = "+ (i+1));
}
}public static void main(String[] args) {
try {
InterruptTest test = new InterruptTest();
test.start();
Thread.sleep(2000);
Thread.interrupted();
} catch (InterruptedException e) {
System.out.println("main-catch");
e.printStackTrace();
}
}
}
/*
无法停止，打印50000条记录
*/
判断线程是否是停止状态
interrupted()方法，测试当前线程是否已经是中断状态，执行后将状态标志改为false。
isInterrupted()方法，测试线程对象是否已经为中断状态，但不清除状态标志。
异常法停止线程
可以使用isInterrupted方法判断线程停止标志状态并抛出InterruptedException，使用interrupt()方法停止线程后，因为接收到停止状态码，抛出异常进入catch分支，继而终止线程。
public class StopThreadTest extends Thread {@Override
public void run() {
super.run();
try {
for (int i=0;
i<1000000;
i++){
if (this.isInterrupted()){
System.out.println("已是停止状态，线程退出！");
throw new InterruptedException();
}
System.out.println("i = "+(i+1));
}
System.out.println("for下面的");
} catch (InterruptedException e) {
System.out.println("线程run()方法catch！线程异常终止");
e.printStackTrace();
}}public static void main(String[] args) {try {
StopThreadTest test = new StopThreadTest();
test.start();
Thread.sleep(1000);
test.interrupt();
} catch (InterruptedException e) {
System.out.println("maincatch");
e.printStackTrace();
}
System.out.println("end!");
}}/*result:
... ...
i = 274280
i = 274281
i = 274282
i = 274283
i = 274284
end!
已是停止状态，线程退出！
线程run()方法catch！线程异常终止
java.lang.InterruptedException
at com.tz.StopThread.StopThreadTest.run(StopThreadTest.java:15)*/
沉睡中停止进程
线程在sleep状态下停止，会直接报异常，并进入catch退出，有两种情况，一个是先sleep再interrupt，还有就是先interrupt再停止。
//先sleeppublic class StopSleep1 extends Thread{@Override
public void run() {
super.run();
try {
System.out.println("run-begin");
Thread.sleep(200000);
System.out.println("run-end");
} catch (InterruptedException e) {
System.out.println("在沉睡中停止，run()进入catch"+this.isInterrupted());
e.printStackTrace();
}
}public static void main(String[] args) {
try {
StopSleep1 sleep1 = new StopSleep1();
sleep1.start();
Thread.sleep(200);
sleep1.interrupt();
} catch (InterruptedException e) {
System.out.println("main-catch");
e.printStackTrace();
}
System.out.println("end!");
}
}/*result：
run-begin
end!
在沉睡中停止，run()进入catchfalse
java.lang.InterruptedException: sleep interrupted
at java.lang.Thread.sleep(Native Method)
at com.tz.StopThread.StopSleep1.run(StopSleep1.java:13)
*/
//后sleeppublic class StopSleep2 extends Thread {@Override
public void run() {
super.run();
try {
for (int i = 0;
i<100000;
i++){
System.out.println("i = "+(i+1));
}
System.out.println("run-begin");
Thread.sleep(200000);
System.out.println("run-end");
} catch (InterruptedException e) {
System.out.println("先停止再遇到sleep，run()进入catch"+this.isInterrupted());
e.printStackTrace();
}
}public static void main(String[] args) {StopSleep2 sleep2 = new StopSleep2();
sleep2.start();
sleep2.interrupt();
System.out.println("end!");
}}/*result:
i = 99997
i = 99998
i = 99999
i = 100000
run-begin
先停止再遇到sleep，run()进入catchfalse
java.lang.InterruptedException: sleep interrupted
at java.lang.Thread.sleep(Native Method)
at com.tz.StopThread.StopSleep2.run(StopSleep2.java:16)
*/
暴力停止线程
使用stop方法停止线程，这个方法很暴力。
public class StopThread extends Thread {private int i = 0;
@Override
public void run() {
try {
while (true){
i++;
System.out.println("i=" +i);
Thread.sleep(1000);
}} catch (InterruptedException e) {
e.printStackTrace();
}
}public static void main(String[] args) {
try {
StopThread thread = new StopThread();
thread.start();
Thread.sleep(8000);
thread.stop();
System.out.println("stop暴力停止");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
stop方法已经作废，尽量不使用！！！
stop方法释放锁，会造成数据不一致的结果。
public class StopThread1 extends Thread {private SynchronizedObject object;
public StopThread1(SynchronizedObject object){
super();
this.object = object;
}@Override
public void run() {
object.printString("b","bb");
}public static void main(String[] args) {try {
SynchronizedObject object = new SynchronizedObject();
StopThread1 thread1 = new StopThread1(object);
thread1.start();
Thread.sleep(500);
thread1.stop();
System.out.println("object.getUsername()="+object.getUsername());
System.out.println("object.getPassword()="+object.getPassword());
} catch (InterruptedException e) {
e.printStackTrace();
}}
}/*result:
object.getUsername()=b
object.getPassword()=aa
*/
return 停止线程
可以将interrupt()方法与return结合实现停止线程。
public class ReturnStopThread extends Thread{@Override
public void run() {
while(true){
if (this.isInterrupted()){
System.out.println("停止！");
return;
}
System.out.println("timer = "+System.currentTimeMillis());
}
}public static void main(String[] args) throws InterruptedException {
ReturnStopThread thread = new ReturnStopThread();
thread.start();
Thread.sleep(2000);
thread.interrupt();
}
}/*result:
... ...
timer = 1521358261861
timer = 1521358261861
timer = 1521358261861
timer = 1521358261861
timer = 1521358261861
timer = 1521358261861
timer = 1521358261861
停止！
*/
建议还是使用抛异常来停止进程，因为抛异常可以通过catch语句将线程停止事件上抛，是线程停止事件得以传播。
暂停线程 暂停线程意味着次现场可以恢复运行，在Java多线程中可以使用suspend()方法暂停线程，使用resume()方法恢复线程的执行。
public class SuspendTestThread extends Thread {
private long i = 0;
public long getI(){
return i;
}public void setI(long i) {
this.i = i;
}@Override
public void run() {
while(true){
i++;
}
}public static void main(String[] args) {
try {
SuspendTestThread thread = new SuspendTestThread();
thread.start();
Thread.sleep(1000);
//A段
thread.suspend();
System.out.println("线程暂停！");
System.out.println("A= " +System.currentTimeMillis()+" i="+thread.getI());
Thread.sleep(1000);
System.out.println("A= " +System.currentTimeMillis()+" i="+thread.getI());
//B段
thread.resume();
Thread.sleep(1000);
System.out.println("线程唤醒！");
System.out.println("B= " +System.currentTimeMillis()+" i="+thread.getI());
Thread.sleep(1000);
System.out.println("B= " +System.currentTimeMillis()+" i="+thread.getI());
//c段
thread.suspend();
System.out.println("线程又暂停");
System.out.println("C= " +System.currentTimeMillis()+" i="+thread.getI());
Thread.sleep(1000);
System.out.println("C= " +System.currentTimeMillis()+" i="+thread.getI());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/*result:
线程暂停！
A= 1521516350803 i=620113660
A= 1521516351803 i=620113660
线程唤醒！
B= 1521516352804 i=1259895883
B= 1521516353804 i=1901121177
线程又暂停
C= 1521516353804 i=1901176584
C= 1521516354804 i=1901176584*/
明显线程在A和C段暂停执行了，在B段唤醒之后又能重新执行。
suspend和rusume的缺点
- 独占
//model.class
public class SynchronizedObject {
synchronized public void printString(){
System.out.println("begin");
if (Thread.currentThread().getName().equals("a")){
System.out.println("a线程永久陷入沉睡！");
Thread.currentThread().suspend();
}
System.out.println("end");
}
}public class SuspendTestThread1 extends Thread {
public static void main(String[] args) {
try {
final SynchronizedObject object = new SynchronizedObject();
Thread thread1 = new Thread(){
@Override
public void run() {
object.printString();
}
};
thread1.setName("a");
thread1.start();
Thread.sleep(1000);
Thread thread2 = new Thread(){
@Override
public void run() {
System.out.println("thraed2启动，但进入不了printString()方法");
System.out.println("因为printString()方法被a线程锁定并独占了");
object.printString();
}
};
thread2.start();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/*result:
begin
a线程永久陷入沉睡！
thraed2启动，但进入不了printString()方法
因为printString()方法被a线程锁定并独占了
*/- 不同步
因为线程暂停可能会导致数据不同步的情况。
public class MyObject {
private String username = "l";
private String password = "ll";
public void setValue(String username,String password){
this.username = username;
if (Thread.currentThread().getName().equals("a")){
System.out.println("停止a线程！");
Thread.currentThread().suspend();
}
this.password = password;
}
public void printUsernamePassword(){
System.out.println(username+""+password);
}
}public class SuspendTestThread2 extends Thread {
public static void main(String[] args) throws InterruptedException {
final MyObject myObject = new MyObject();
Thread thread1 = new Thread(){
@Override
public void run() {
myObject.setValue("a","aa");
}
};
thread1.setName("a");
thread1.start();
Thread.sleep(500);
Thread thread2 = new Thread(){
@Override
public void run(){
myObject.printUsernamePassword();
}
};
thread2.start();
}
}/*result:
停止a线程！
all
*/
suspend()和resume()方法已经废弃，不建议使用，可以研究。
yield()方法 yield()方法是让当前线程放弃cpu资源，但放弃的时间不确定，可能刚刚放弃就立刻获得cpu资源。
public class YieldTestThread extends Thread {
@Override
public void run() {
long beginTime = System.currentTimeMillis();
int count = 0;
for (int i = 0;
i < 50000000;
i++){
//Thread.yield();
count = count + (i+1);
}
long endTime = System.currentTimeMillis();
System.out.println("用时："+(endTime-beginTime)+"毫秒");
}public static void main(String[] args) {
YieldTestThread thread = new YieldTestThread();
thread.start();
}
}/*result1：(不加yield)
用时：18毫秒
*//*result2：(加yield)
用时：3362毫秒
*/
线程优先级 线程可以划分优先级，从1-10级，其他会报错。
线程的优先级具有承继性。
优先级规则，总是大部分先执行优先级高的线程。
//线程1
public class PriorityTestThread extends Thread{
@Override
public void run() {
long beginTime = System.currentTimeMillis();
long addResult = 0;
for (int j = 0;
j < 10;
j++){
for(int i = 0;
i<50000;
i++){
Random random = new Random();
random.nextInt();
addResult = addResult+1;
}
}
long endTime = System.currentTimeMillis();
System.out.println("* * * * * * thread 1 use time="+(endTime - beginTime));
}
}//线程2
public class PriorityTestThread1 extends Thread {@Override
public void run() {
long beginTime = System.currentTimeMillis();
long addResult = 0;
for (int j = 0;
j < 10;
j++){
for(int i = 0;
i<50000;
i++){
Random random = new Random();
random.nextInt();
addResult = addResult+1;
}
}
long endTime = System.currentTimeMillis();
System.out.println("* * * * * * thread 2 use time="+(endTime - beginTime));
}
}public class Run {
public static void main(String[] args) {
for (int i = 0;
i < 100;
i++){
PriorityTestThread thread1 = new PriorityTestThread();
thread1.setPriority(10);
thread1.start();
PriorityTestThread1 thread2 = new PriorityTestThread1();
thread2.setPriority(1);
thread2.start();
}
}}/*result：
... ...
* * * * * * thread 1 use time=6197
* * * * * * thread 1 use time=6207
* * * * * * thread 1 use time=6252
* * * * * * thread 1 use time=6270
* * * * * * thread 2 use time=6870
* * * * * * thread 2 use time=6524
* * * * * * thread 2 use time=7036
* * * * * * thread 1 use time=7522
* * * * * * thread 1 use time=6448
* * * * * * thread 2 use time=7035
* * * * * * thread 2 use time=7223
* * * * * * thread 2 use time=7025
* * * * * * thread 1 use time=7776
* * * * * * thread 1 use time=6747
* * * * * * thread 2 use time=7261
* * * * * * thread 1 use time=7939
... ...
*/
优先级高的不是一定先执行。
守护线程 守护线程是一种特殊线程，特性有“陪伴”的含义，当进程中不存在非守护进程时，守护进程就自动销毁了。典型的守护进程就是垃圾回收线程(垃圾回收器 GC)
public class DaemonTestThread extends Thread {
private int i = 0;
@Override
public void run() {
try {
while (true){
i++;
System.out.println("i = "+ i);
Thread.sleep(1000);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}public static void main(String[] args) {
try {
DaemonTestThread thread = new DaemonTestThread();
thread.setDaemon(true);
thread.start();
Thread.sleep(5000);
System.out.println("我离开Thread对象也不再打印了，也就是停止了！");
} catch (InterruptedException e) {
e.printStackTrace();
}
}}
//线程thread为主线程的守护进程，主线程停止守护进程也结束。/*result:
i = 1
i = 2
i = 3
i = 4
i = 5
我离开Thread对象也不再打印了，也就是停止了！
*/
对象及变量的并发访问 synchronzed同步方法 "非线程安全"会在多个线程对同一个对象中的实例变量进行并发访问时发生产生"脏读",也就是取到的数据其实是被更改过的。而线程安全就是以获得的实例变量的值是经过同步处理的，不会出现脏读现象。
方法内数据为线程安全
"非线程安全"问题存在于"实例变量"中，如果是方法内部私有变量则不存在"非线程安全问题"。
public class HasSelfPrivateNum {
public void addI(String username){
try {
int num = 0;
if (username.equals("a")){
num = 100;
System.out.println("a set over!");
Thread.sleep(2000);
}else {
num = 200;
System.out.println("b set over!");
Thread.sleep(2000);
}
System.out.println(username + " num = "+num);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}public class ThreadA extends Thread {
private HasSelfPrivateNum numRef;
public ThreadA(HasSelfPrivateNum numRef){
super();
this.numRef = numRef;
}@Override
public void run() {
super.run();
numRef.addI("a");
}
}public class ThreadB extends Thread {
private HasSelfPrivateNum numRef;
public ThreadB(HasSelfPrivateNum numRef){
super();
this.numRef = numRef;
}@Override
public void run() {
super.run();
numRef.addI("b");
}}public class Run {
public static void main(String[] args) {
HasSelfPrivateNum numRef = new HasSelfPrivateNum();
ThreadA threadA = new ThreadA(numRef);
threadA.start();
ThreadB threadB = new ThreadB(numRef);
threadB.start();
}
}/*result:
a set over!
b set over!
a num = 100
b num = 200*/
实例变量非线程安全
若多个线程访问一个对象实例中的实例变量。则可能发生“非线程安全”问题。
用线程访问的对象中如果有多个实例变量，则运行的结果有可能出现交叉的情况。
如果对象仅有一个实例变量，则有可能出现覆盖的情况。
public class HasSelfPrivateNum{
private int num = 0;
//addI()方法前加上synchronized关键字，避免“非线程安全问题”
synchronized public void addI(String username){
try {
if (username.equals("a")){
num = 100;
System.out.println("a set over!");
Thread.sleep(2000);
}else {
num = 200;
System.out.println("b set over!");
Thread.sleep(2000);
}
System.out.println(username + " num = "+num);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}public class ThreadA extends Thread {
private HasSelfPrivateNum numRef;
public ThreadA(HasSelfPrivateNum numRf){
super();
this.numRef = numRf;
}@Override
public void run() {
super.run();
numRef.addI("a");
}
}public class ThreadB extends Thread {
private HasSelfPrivateNum numRef;
public ThreadB(HasSelfPrivateNum numRf){
super();
this.numRef = numRf;
}@Override
public void run() {
super.run();
numRef.addI("b");
}}public class Run {
public static void main(String[] args) {
HasSelfPrivateNum numRef = new HasSelfPrivateNum();
ThreadA threadA = new ThreadA(numRef);
threadA.start();
ThreadB threadB = new ThreadB(numRef);
threadB.start();
}
}/*不加synchronized关键字：
a set over!
b set over!
b num = 200
a num = 200
*//*加synchronized关键字：
a set over!
a num = 100
b set over!
b num = 200
*/
多个对象多个锁
synchronized关键字取得的锁都对象锁，哪个线程先执行带有synchronized关键字的方法就先获得对象锁，其他线程只能依次等待执行完成。
public class LockTestObject {
synchronized public void methodA(){
try {
System.out.println("Begin methodA threadName = "+Thread.currentThread().getName());
Thread.sleep(5000);
System.out.println("methodA end! endTime = "+System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
}
}synchronized public void methodB(){
try {
System.out.println("Begin methodB threadName = "+Thread.currentThread().getName());
Thread.sleep(5000);
System.out.println("methodB end! endTime = "+System.currentTimeMillis());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}public class LockThreadA extends Thread {
private LockTestObject object;
public LockThreadA(LockTestObject object){
super();
this.object = object;
}@Override
public void run() {
super.run();
object.methodA();
}
}public class LockThreadB extends Thread {
private LockTestObject object;
public LockThreadB(LockTestObject object){
super();
this.object = object;
}@Override
public void run() {
super.run();
object.methodB();
}
}public class Run {
public static void main(String[] args) {
LockTestObject object = new LockTestObject();
LockThreadA threadA = new LockThreadA(object);
threadA.setName("A");
LockThreadB threadB = new LockThreadB(object);
threadB.setName("B");
threadA.start();
threadB.start();
}
}/*methodA方法不加synchronized关键字
Begin methodB threadName = B
Begin methodA threadName = A
methodB end! endTime = 1521719664578
methodA end! endTime = 1521719664579
*//*methodA方法加上synchronized关键字
Begin methodA threadName = A
methodA end! endTime = 1521719556410
Begin methodB threadName = B
methodB end! endTime = 1521719561411
*/脏读
所谓脏读是在对去实例变量时该变量已被其他线程改过，读出数据有误。
public class PublicVar {
public String userName = "A";
public String password = "AA";
synchronized public void setValue(String userName,String password){
try {
this.userName = userName;
Thread.sleep(5000);
this.password = password;
System.out.println("setValue method thread name = "+Thread.currentThread().getName()+" userName = "+userName+" password = "+password);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void getValue(){
System.out.println("setValue method thread name = "+Thread.currentThread().getName()+" userName = "+userName+" password = "+password);
}
}public class DirtyReadTestThread extends Thread{
private PublicVar publicVar;
public DirtyReadTestThread(PublicVar publicVar){
super();
this.publicVar = publicVar;
}@Override
public void run() {
super.run();
publicVar.setValue("B","BB");
}
}public class Run {
public static void main(String[] args) {
try {
PublicVar publicVar = new PublicVar();
DirtyReadTestThread thread = new DirtyReadTestThread(publicVar);
thread.start();
Thread.sleep(200);
publicVar.getValue();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}/*result:
setValue method thread name = main userName = B password = AA
setValue method thread name = Thread-0 userName = B password = BB
*/
如上所示，main线程出现了脏读，因为getValue()方法不是同步的，只需在getValue前加上synchronized关键字，即可保持数据同步性。
synchronized public void getValue(){
System.out.println("setValue method thread name = "+Thread.currentThread().getName()+" userName = "+userName+" password = "+password);
}/*result:
setValue method thread name = Thread-0 userName = B password = BB
setValue method thread name = main userName = B password = BB
*/
当线程调用对象包含的synchronized方法时获取了对象的X锁，但别的线程可以调用该实体非synchronized方法。
synchronized 锁重入
一个线程多次请求synchronized方法锁时，可以重复获得方法所在的对象实体的X锁
锁重入，即可重复获得内部锁。
public class Service {
synchronized public void service1(){
System.out.println("service1");
service2();
}synchronized public void service2(){
System.out.println("service2");
service3();
}
synchronized public void service3(){
System.out.println("service3");
}
}public class LockReentryTestThreadextends Thread {
@Override
public void run() {
Service service = new Service();
service.service1();
}
}public class Run {
public static void main(String[] args) {
LockReentryTestThread thread = new LockReentryTestThread();
thread.start();
}
}/*result:
service1
service2
service3
*/
service类中锁就重入了，三个service方法相互调用。
锁重入也支持在父子类间的锁重用。
出现异常，锁自动释放，其他线程继续调用。
synchronized方法的弊端
导致进程等待时间较长，失去多线程的意义，导致程序响应时间过长。
synchronized同步块可以解决这个问题。
synchronized同步代码块
当两个并发的线程访问同一个对象object中的synchronized(this)同步块时，一段时间内只有一个线程能访问并执行，另一个线程必须等待前一个线程执行完毕这个代码块之后，才能执行这块代码。
使用同步synchronized 代码块时，同一个object的同步代码块使用同一个对象监视器，执行一个同步块时对象中其他同步块会被阻塞。
将任意对象作为对象监视器
锁非this对象的优点是：若在一个类中有很多个synchronized方法，这时虽然能实现同步，但会收到阻塞，影响运行效率；若使用同步代码块锁非this对象，则synchronized（非this）代码块中的程序与同步方法是异步的，不与其他锁this的同步方法争抢this锁。则可提高运行效率。
静态同步synchronized方法与synchronized(class)代码块。
关键字还可以作用在static静态方法上，是对方法所在的类.class持锁，而不是对一个对象上锁。
synchronized代码块也可以对class类上锁，实现同步。synchronized(xxx.class)。
数据类型String的常量池特性
由于JVM中String数据类型的常量池特性 a==b 返回true，所以不使用String对象作为对象监视器（对象锁）。
同步synchronized方法无限等待与解决
synchronized同步方法容易造成死循环，是形成陷入死锁。同步块可以解开这个死锁问题，死锁线程依旧跳不出，但其他线程可获得锁。
多线程的死锁
synchronized嵌套代码块将带来死锁。
进入Cmd 输入 jsp 查找Run的id值 在输入 jstack -l 19560 可查看程序运行死锁情况
内置类与静态内置类
锁对象的改变
public class MyService {
private String lock = "123";
public void testMethod() {
try {
synchronized (lock) {
System.out.println(Thread.currentThread().getName() + "begin" + System.currentTimeMillis());
lock = "456";
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + "end" + System.currentTimeMillis());
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}public class ThreadA extends Thread {
private MyService service;
public ThreadA(MyService service) {
super();
this.service = service;
}@Override
public void run() {
service.testMethod();
}
}public class ThreadB extends Thread{
private MyService service;
public ThreadB(MyService service) {
super();
this.service = service;
}@Override
public void run() {
service.testMethod();
}
}public class Run1 {
public static void main(String[] args) throws InterruptedException {
MyService service = new MyService();
ThreadA threadA = new ThreadA(service);
threadA.setName("A");
ThreadB threadB = new ThreadB(service);
threadB.setName("B");
threadA.start();
//Thread.sleep(50 );
threadB.start();
}
}/*延迟50毫秒，争抢两个锁
Abegin1523325785537
Bbegin1523325785585
Aend1523325787539
Bend1523325787585
*//*不延迟，争抢一个锁
Abegin1523325815403
Aend1523325817403
Bbegin1523325817403
Bend1523325819404
*/
只要锁对象不变，即使对象属性改变依旧同步，线程还是争抢一个锁。
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