synchronized关键字
锁对象。synchronized(this)和synchronized方法都是锁当前对象。
import java.util.concurrent.TimeUnit;
public class Test_01 {
private int count = 0;
private Object o = new Object();
public static void main(String[] args) {
final Test_01 t = new Test_01();
new Thread(new Runnable() {
@Override
public void run() {
t.testSync2();
}
}, "testSync2").start();
new Thread(new Runnable() {
@Override
public void run() {
t.testSync1();
}
}, "testSync1").start();
new Thread(new Runnable() {
@Override
public void run() {
t.testSync3();
}
}, "testSync3").start();
}
public void testSync1() {
synchronized (o) {
System.out.println(Thread.currentThread().getName()
+ " count = " + count++);
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public void testSync2() {
synchronized (this) {
System.out.println(Thread.currentThread().getName()
+ " count = " + count++);
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public synchronized void testSync3() {
System.out.println(Thread.currentThread().getName()
+ " count = " + count++);
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
同步方法 - static：静态同步方法，锁的是当前类型的类对象。在代码中就是类名.class
import java.util.concurrent.TimeUnit;
public class Test_02 {
private static int staticCount = 0;
public static synchronized void testSync4() {
System.out.println(Thread.currentThread().getName()
+ " staticCount = " + staticCount++);
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
public static void testSync5() {
synchronized (Test_02.class) {
System.out.println(Thread.currentThread().getName()
+ " staticCount = " + staticCount++);
}
}
}
同步方法 - 原子性
加锁的目的： 就是为了保证操作的原子性。
public class Test_03 implements Runnable {
private int count = 0;
public static void main(String[] args) {
Test_03 t = new Test_03();
for (int i = 0; i < 5; i++) {
new Thread(t, "Thread - " + i).start();
}
}
@Override
public /*synchronized*/ void run() {
System.out.println(Thread.currentThread().getName()
+ " count = " + count++);
}
}
同步方法 - 同步方法和非同步方法的调用
同步方法只影响锁定同一个锁对象的同步方法。不影响其他线程调用非同步方法，或调用其他锁资源的同步方法。
public class Test_04 {
Object o = new Object();
public static void main(String[] args) {
Test_04 t = new Test_04();
new Thread(new Test_04.MyThread01(0, t)).start();
new Thread(new Test_04.MyThread01(1, t)).start();
new Thread(new Test_04.MyThread01(-1, t)).start();
}
public synchronized void m1() { // 重量级的访问操作。
System.out.println("public synchronized void m1() start");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("public synchronized void m1() end");
}
public void m3() {
synchronized (o) {
System.out.println("public void m3() start");
try {
Thread.sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("public void m3() end");
}
}
public void m2() {
System.out.println("public void m2() start");
try {
Thread.sleep(1500);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("public void m2() end");
}
public static class MyThread01 implements Runnable {
int i;
Test_04 t;
public MyThread01(int i, Test_04 t) {
this.i = i;
this.t = t;
}
public void run() {
if (i == 0) {
t.m1();
} else if (i > 0) {
t.m2();
} else {
t.m3();
}
}
}
}
结果：
public synchronized void m1() start
public void m2() start
public void m3() start
public void m2() end
public void m3() end
public synchronized void m1() end
同步方法 - 多方法调用原子性问题（业务）
同步方法只能保证当前方法的原子性，不能保证多个业务方法之间的互相访问的原子性。
注意：在商业开发中，多方法要求结果访问原子操作，需要多个方法都加锁，且锁定统一个资源。一般来说，商业项目中，不考虑业务逻辑上的脏读问题。
import java.util.concurrent.TimeUnit;
public class Test_05 {
private double d = 0.0;
public static void main(String[] args) {
final Test_05 t = new Test_05();
new Thread(new Runnable() {
@Override
public void run() {
t.m1(100);
}
}).start();
System.out.println(t.m2());
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(t.m2());
}
public synchronized void m1(double d) {
try {
// 相当于复杂的业务逻辑代码。
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
this.d = d;
}
public double m2() {
return this.d;
}
}
锁可重入: 同一个线程，多次调用同步代码，锁定同一个锁对象，可重入。
import java.util.concurrent.TimeUnit;
public class Test_06 {
public static void main(String[] args) {
new Test_06().m1();
}
synchronized void m1() { // 锁this
System.out.println("m1 start");
try {
TimeUnit.SECONDS.sleep(2);
} catch (InterruptedException e) {
e.printStackTrace();
}
m2();
System.out.println("m1 end");
}
synchronized void m2() { // 锁this
System.out.println("m2 start");
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("m2 end");
}
}
同步方法 - 继承:：类同步方法覆盖父类同步方法。可以指定调用父类的同步方法。相当于锁的重入。
import java.util.concurrent.TimeUnit;
public class Test_07 {
public static void main(String[] args) {
new Sub_Test_07().m();
}
synchronized void m() {
System.out.println("Super Class m start");
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Super Class m end");
}
}
class Sub_Test_07 extends Test_07 {
synchronized void m() {
System.out.println("Sub Class m start");
super.m();
System.out.println("Sub Class m end");
}
}
同步方法 - 锁与异常：当同步方法中发生异常的时候，自动释放锁资源。不会影响其他线程的执行。注意同步业务逻辑中，如果发生异常如何处理。
import java.util.concurrent.TimeUnit;
public class Test_08 {
int i = 0;
public static void main(String[] args) {
final Test_08 t = new Test_08();
new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}, "t1").start();
new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}, "t2").start();
}
synchronized void m() {
System.out.println(Thread.currentThread().getName() + " - start");
while (true) {
i++;
System.out.println(Thread.currentThread().getName() + " - " + i);
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
if (i == 5) {
i = 1 / 0;
}
}
}
}
结果：
t1 - start
t1 - 1
t1 - 2
t1 - 3
t1 - 4
t1 - 5
Exception in thread "t1" java.lang.ArithmeticException: / by zero
t2 - start
at concurrent.t01.Test_08.m(Test_08.java:43)
t2 - 6
at concurrent.t01.Test_08$1.run(Test_08.java:19)
at java.base/java.lang.Thread.run(Thread.java:844)
t2 - 7
t2 - 8
t2 - 9
t2 - 10
volatile关键字
volatile的可见性：通知OS操作系统底层，在CPU计算过程中，都要检查内存中数据的有效性。保证最新的内存数据被使用。
import java.util.concurrent.TimeUnit;
public class Test_09 {
volatile boolean b = true;
public static void main(String[] args) {
final Test_09 t = new Test_09();
new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}).start();
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
t.b = false;
}
void m() {
System.out.println("start");
while (b) {
}
System.out.println("end");
}
}
volatile的非原子性问题：只能保证可见性，不能保证原子性。不是枷锁问题，只是内存数据可见。
public class Test_10 {
volatile int count = 0;
public static void main(String[] args) {
final Test_10 t = new Test_10();
List<Thread> threads = new ArrayList<>();
for (int i = 0; i < 10; i++) {
threads.add(new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}));
}
for (Thread thread : threads) {
thread.start();
}
for (Thread thread : threads) {
try {
thread.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println(t.count);
}
/*synchronized*/ void m() {
for (int i = 0; i < 10000; i++) {
count++;
}
}
}
AtomicXxx
同步类型：原子操作类型。 其中的每个方法都是原子操作。可以保证线程安全。
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger; public class Test_11 {
AtomicInteger count = new AtomicInteger(0); public static void main(String[] args) {
final Test_11 t = new Test_11();
List<Thread> threads = new ArrayList<>();
for (int i = 0; i < 10; i++) {
threads.add(new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}));
}
for (Thread thread : threads) {
thread.start();
}
for (Thread thread : threads) {
try {
thread.join();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
System.out.println(t.count.intValue());
} void m() {
for (int i = 0; i < 10000; i++) {
/*if(count.get() < 1000)*/
count.incrementAndGet();
}
}
}
同步粒度问题：尽量在商业开发中避免同步方法。使用同步代码块。 细粒度解决同步问题。可以提高效率。
public class Test_12 {
synchronized void m1() {
// 前置逻辑
System.out.println("同步逻辑");
// 后置逻辑
}
void m2() {
// 前置逻辑
synchronized (this) {
System.out.println("同步逻辑");
}
// 后置逻辑
}
}
对象变更问题：同步代码一旦加锁后，那么会有一个临时的锁引用执行锁对象，和真实的引用无直接关联。在锁未释放之前，修改锁对象引用，不会影响同步代码的执行。
public class Test_13 {
Object o = new Object();
int i = 0;
public static void main(String[] args) {
final Test_13 t = new Test_13();
new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}, "thread1").start();
try {
TimeUnit.SECONDS.sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
Thread thread2 = new Thread(new Runnable() {
@Override
public void run() {
t.m();
}
}, "thread2");
t.o = new Object();
thread2.start();
System.out.println(t.i);
System.out.println(t.a());
System.out.println(t.i);
}
int a() {
try {
/*
* return i ->
* int _returnValue = i; // 0;
* return _returnValue;
*/
return i;
} finally {
i = 10;
}
}
void m() {
System.out.println(Thread.currentThread().getName() + " start");
synchronized (o) {
while (true) {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " - " + o);
}
}
}
}
结果：
thread1 start
thread1 - java.lang.Object@6018fac7
thread1 - java.lang.Object@6018fac7
thread1 - java.lang.Object@6018fac7
0
0
10
thread2 start
thread1 - java.lang.Object@6edf18b4
thread2 - java.lang.Object@6edf18b4
thread1 - java.lang.Object@6edf18b4
thread2 - java.lang.Object@6edf18b4
thread1 - java.lang.Object@6edf18b4
常量问题：在定义同步代码块时，不要使用常量对象作为锁对象。
i1、i2会实现m1、m2方法的同步；s1、s2是不同的对象，不能实现m1、m2方法的同步。
public class Test_14 {
String s1 = "hello";
String s2 = new String("hello"); // new关键字，一定是在堆中创建一个新的对象。
Integer i1 = 1;
Integer i2 = 1;
public static void main(String[] args) {
final Test_14 t = new Test_14();
new Thread(new Runnable() {
@Override
public void run() {
t.m1();
}
}).start();
new Thread(new Runnable() {
@Override
public void run() {
t.m2();
}
}).start();
}
void m1() {
synchronized (i1) {
System.out.println("m1()");
while (true) {
}
}
}
void m2() {
synchronized (i2) {
System.out.println("m2()");
while (true) {
}
}
}
}
门闩 - CountDownLatch
可以和锁混合使用，或替代锁的功能。在门闩未完全开放之前等待。当门闩完全开放后执行。避免锁的效率低下问题。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit; public class Test_15 {
CountDownLatch latch = new CountDownLatch(5); public static void main(String[] args) {
final Test_15 t = new Test_15();
new Thread(new Runnable() {
@Override
public void run() {
t.m1();
}
}).start(); new Thread(new Runnable() {
@Override
public void run() {
t.m2();
}
}).start();
} void m1() {
try {
latch.await();// 等待门闩开放。
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("m1() method");
} void m2() {
for (int i = 0; i < 10; i++) {
if (latch.getCount() != 0) {
System.out.println("latch count : " + latch.getCount());
latch.countDown(); // 减门闩上的锁。
}
try {
TimeUnit.MILLISECONDS.sleep(500);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("m2() method : " + i);
}
} }
结果：
latch count : 5
m2() method : 0
latch count : 4
m2() method : 1
latch count : 3
m2() method : 2
latch count : 2
m2() method : 3
latch count : 1
m1() method
m2() method : 4
m2() method : 5
m2() method : 6
...
练习题
自定义容器，提供新增元素（add）和获取元素数量（size）方法。
启动两个线程。线程1向容器中新增10个数据。线程2监听容器元素数量，当容器元素数量为5时，线程2输出信息并终止。
方法一（volatile的可见性）：
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit; public class Test_01 {
public static void main(String[] args) {
final Test_01_Container t = new Test_01_Container();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println("add Object to Container " + i);
t.add(new Object());
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start(); new Thread(new Runnable() {
@Override
public void run() {
while (true) {
if (t.size() == 5) {
System.out.println("size = 5");
break;
}
}
}
}).start();
}
} class Test_01_Container {
volatile List<Object> container = new ArrayList<>(); public void add(Object o) {
this.container.add(o);
} public int size() {
return this.container.size();
}
}
方法二（synchornized）：
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit; public class Test_02 {
public static void main(String[] args) {
final Test_02_Container t = new Test_02_Container();
final Object lock = new Object(); new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
if (t.size() != 5) {
try {
lock.wait(); // 线程进入等待队列。
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("size = 5");
lock.notifyAll(); // 唤醒其他等待线程
}
}
}).start(); new Thread(new Runnable() {
@Override
public void run() {
synchronized (lock) {
for (int i = 0; i < 10; i++) {
System.out.println("add Object to Container " + i);
t.add(new Object());
if (t.size() == 5) {
lock.notifyAll();
try {
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}).start();
}
} class Test_02_Container {
List<Object> container = new ArrayList<>(); public void add(Object o) {
this.container.add(o);
} public int size() {
return this.container.size();
}
}
方法三（门闩）：
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit; public class Test_03 {
public static void main(String[] args) {
final Test_03_Container t = new Test_03_Container();
final CountDownLatch latch = new CountDownLatch(1); new Thread(new Runnable() {
@Override
public void run() {
if (t.size() != 5) {
try {
latch.await(); // 等待门闩的开放。 不是进入等待队列
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("size = 5");
}
}).start(); new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
System.out.println("add Object to Container " + i);
t.add(new Object());
if (t.size() == 5) {
latch.countDown(); // 门闩-1
}
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
}
} class Test_03_Container {
List<Object> container = new ArrayList<>(); public void add(Object o) {
this.container.add(o);
} public int size() {
return this.container.size();
}
}