1.synchronized加锁
package com.bjsxt.base.sync001;
import java.util.concurrent.atomic.AtomicInteger;
/**
* 线程安全概念：当多个线程访问某一个类（对象或方法）时，这个对象始终都能表现出正确的行为，那么这个类（对象或方法）就是线程安全的。
* synchronized：可以在任意对象及方法上加锁，而加锁的这段代码称为"互斥区"或"临界区"
*
*/
public class MyThread extends Thread{
private int count = 5 ;
//synchronized加锁
public void run(){
count--;
System.out.println(this.currentThread().getName() + " count = "+ count);
}
public static void main(String[] args) {
/**
* 分析：当多个线程访问myThread的run方法时，以排队的方式进行处理（这里排对是按照CPU分配的先后顺序而定的），
* 一个线程想要执行synchronized修饰的方法里的代码：
* 1 尝试获得锁
* 2 如果拿到锁，执行synchronized代码体内容；拿不到锁，这个线程就会不断的尝试获得这把锁，直到拿到为止，
* 而且是多个线程同时去竞争这把锁。（也就是会有锁竞争的问题）
*/
MyThread myThread = new MyThread();
Thread t1 = new Thread(myThread,"t1");
Thread t2 = new Thread(myThread,"t2");
Thread t3 = new Thread(myThread,"t3");
Thread t4 = new Thread(myThread,"t4");
Thread t5 = new Thread(myThread,"t5");
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
}
}
未加synchronied
t1 count = 0 t3 count = 0 t2 count = 0 t4 count = 0 t5 count = 0
加synchronied
t1 count = 4 t4 count = 3 t5 count = 2 t3 count = 1 t2 count = 0
2.对象锁-同一个对象的不同方法
分析： t1线程先持有object对象的Lock锁，t2线程可以以异步的方式调用对象中的非synchronized修饰的方法 t1线程先持有object对象的Lock锁，t2线程如果在这个时候调用对象中的同步（synchronized）方法则需等待，也就是同步
package com.bjsxt.base.sync003;
/**
* 对象锁的同步和异步问题
*
*/
public class MyObject {
public synchronized void method1(){
try {
System.out.println(Thread.currentThread().getName()+"第一步");
Thread.sleep(4000);
System.out.println(Thread.currentThread().getName()+"第二步");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/** synchronized */
public void method2(){
System.out.println(Thread.currentThread().getName());
}
public static void main(String[] args) {
final MyObject mo = new MyObject();
/**
* 分析：
* t1线程先持有object对象的Lock锁，t2线程可以以异步的方式调用对象中的非synchronized修饰的方法
* t1线程先持有object对象的Lock锁，t2线程如果在这个时候调用对象中的同步（synchronized）方法则需等待，也就是同步
*/
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
mo.method1();
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
mo.method2();
}
},"t2");
t1.start();
t2.start();
}
}
（1）method2不加synchronized修饰
t2 t1第一步 t1第二步
（2）method2加synchronized修饰
t1第一步
t1第二步
t2
如果方法1和方法2是同一个事务里的话，需要都用synchronized修饰，保证事务的原子性
package com.bjsxt.base.sync004;
/**
* 业务整体需要使用完整的synchronized，保持业务的原子性。
*
*/
public class DirtyRead {
private String username = "bjsxt";
private String password = "123";
public synchronized void setValue(String username, String password){
this.username = username;
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
this.password = password;
System.out.println("setValue最终结果：username = " + username + " , password = " + password);
}
public void getValue(){
System.out.println("getValue方法得到：username = " + this.username + " , password = " + this.password);
}
public static void main(String[] args) throws Exception{
final DirtyRead dr = new DirtyRead();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
dr.setValue("z3", "456");
}
});
t1.start();
Thread.sleep(1000);
dr.getValue();
}
}
主线程getValue时，t1可能已经重置了username，但还未执行后后续的重置password
getValue方法得到：username = z3 , password = 123 setValue最终结果：username = z3 , password = 456
3.对象锁-不同对象的同一方法
synchronized加在静态方法上的话，会变成类锁
package com.bjsxt.base.sync002;
/**
* 关键字synchronized取得的锁都是对象锁，而不是把一段代码（方法）当做锁，
* 所以代码中哪个线程先执行synchronized关键字的方法，哪个线程就持有该方法所属对象的锁（Lock），
*
* 在静态方法上加synchronized关键字，表示锁定.class类，类一级别的锁（独占.class类）。
*
*/
public class MultiThread {
private int num = 0;
/** static */
public synchronized void printNum(String tag){
try {
if(tag.equals("a")){
num = 100;
System.out.println("tag a, set num over!");
Thread.sleep(1000);
} else {
num = 200;
System.out.println("tag b, set num over!");
}
System.out.println("tag " + tag + ", num = " + num);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
//注意观察run方法输出顺序
public static void main(String[] args) {
//俩个不同的对象
final MultiThread m1 = new MultiThread();
final MultiThread m2 = new MultiThread();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
m1.printNum("a");
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
m2.printNum("b");
}
});
t1.start();
t2.start();
}
}
（1）修饰普通方法
tag a, set num over! tag b, set num over! tag b, num = 200 tag a, num = 100
（2）修饰静态方法
tag a, set num over! tag a, num = 100 tag b, set num over! tag b, num = 200
4.synchronized遇到异常，锁自动释放
package com.bjsxt.base.sync005;
/**
* synchronized异常
*
*/
public class SyncException {
private int i = 0;
public synchronized void operation(){
while(true){
try {
i++;
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + " , i = " + i);
if(i == 2){
//Integer.parseInt("a");
throw new RuntimeException();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
final SyncException se = new SyncException();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
se.operation();
}
},"t1");
t1.start();
}
}
打印 为什么退出循环了？
t1 , i = 1 t1 , i = 2 Exception in thread "t1" java.lang.RuntimeException at com.bjsxt.base.sync005.SyncException.operation(SyncException.java:18) at com.bjsxt.base.sync005.SyncException$1.run(SyncException.java:32) at java.lang.Thread.run(Unknown Source)
例2
package com.bjsxt.base.sync005;
/**
* synchronized异常
*
*/
public class SyncException {
private int i = 0;
public synchronized void operation(){
while(i<5){
try {
i++;
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + " , i = " + i);
if(i == 2){
//Integer.parseInt("a");
throw new RuntimeException();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
final SyncException se = new SyncException();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
se.operation();
}
},"t1");
t1.start();
// try {
// Thread.sleep(1000);
// } catch (Exception e) {
// // TODO: handle exception
// }
se.operation();
}
}
打印
main , i = 1 main , i = 2 Exception in thread "main" java.lang.RuntimeException at com.bjsxt.base.sync005.SyncException.operation(SyncException.java:18) at com.bjsxt.base.sync005.SyncException.main(SyncException.java:41) t1 , i = 3 t1 , i = 4 t1 , i = 5
5.锁的可重入性
（1）同一个对象的不同方法
当一个线程再次请求自己持有的对象锁的另一个临界资源时，这个情况属于重入锁，就是一个线程得到一个对象锁之后再次请求该对象锁，这种是允许的。
package com.bjsxt.base.sync005;
/**
* synchronized的重入
*
*/
public class SyncDubbo1 {
public synchronized void method1(){
System.out.println(Thread.currentThread().getName()+" "+"method1..");
method2();
}
public synchronized void method2(){
System.out.println(Thread.currentThread().getName()+" "+"method2..");
method3();
}
public synchronized void method3(){
System.out.println(Thread.currentThread().getName()+" "+"method3..");
}
public static void main(String[] args) {
final SyncDubbo1 sd = new SyncDubbo1();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
sd.method1();
}
});
t1.start();
}
}
（2）子类与父类
子类继承父类时，也是可以通过重入锁调用父类的同步方法
package com.bjsxt.base.sync005;
/**
* synchronized的重入
*
*/
public class SyncDubbo2 {
static class Main {
public int i = 5;
public synchronized void operationSup(){
try {
i--;
System.out.println("Main print i = " + i);
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class Sub extends Main {
public synchronized void operationSub(){
try {
while(i > 0) {
i--;
System.out.println("Sub print i = " + i);
Thread.sleep(100);
this.operationSup();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
Sub sub = new Sub();
sub.operationSub();
}
});
t1.start();
}
}
6.锁对象改变的情况
（1）synchronized String、Integer等类型，当变量值发生改变时，会生成一个新对象
- String值改变，会new Stirng()，获得的是不同的对象锁，可以用intern()方法
这篇文章有介绍：https://www.cnblogs.com/xrq730/p/6662232.html
- Integer类型自动装箱和解箱会生成一个新的对象
package com.bjsxt.base.sync006;
/**
* 锁对象的改变问题
*synchronized代码块对字符串的锁，注意String常量池的缓存功能
*/
public class ChangeLock {
private String lock = "lock";
private void method(){
//new String("字符串常量")
//synchronized ("字符串常量") {
synchronized (lock) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + "开始");
lock = "change lock";
Thread.sleep(2000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + "结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
final ChangeLock changeLock = new ChangeLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
changeLock.method();
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
changeLock.method();
}
},"t2");
t1.start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
打印
当前线程 : t1开始
当前线程 : t2开始
当前线程 : t1结束
当前线程 : t2结束
（2）同一对象的属性值修改时，不会影响锁
package com.bjsxt.base.sync006;
/**
* 同一对象属性的修改不会影响锁的情况
*
*/
public class ModifyLock {
private String name ;
private int age ;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public synchronized void changeAttributte(String name, int age) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 开始");
this.setName(name);
this.setAge(age);
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 修改对象内容为： "
+ this.getName() + ", " + this.getAge());
Thread.sleep(2000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
final ModifyLock modifyLock = new ModifyLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
modifyLock.changeAttributte("张三", 20);
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
modifyLock.changeAttributte("李四", 21);
}
},"t2");
t1.start();
// try {
// Thread.sleep(100);
// } catch (InterruptedException e) {
// e.printStackTrace();
// }
t2.start();
}
}
7.synchronized 锁对象的不同方式
package com.bjsxt.base.sync006;
/**
* 使用synchronized代码块加锁,比较灵活
*/
public class ObjectLock {
public void method1(){
synchronized (this) { //对象锁
try {
System.out.println("do method1..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public void method2(){ //类锁
synchronized (ObjectLock.class) {
try {
System.out.println("do method2..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
private Object lock = new Object();
public void method3(){ //任何对象锁
synchronized (lock) {
try {
System.out.println("do method3..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
final ObjectLock objLock = new ObjectLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method1();
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method2();
}
});
Thread t3 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method3();
}
});
t1.start();
t2.start();
t3.start();
}
}
8.死锁
package com.bjsxt.base.sync006;
/**
* 死锁问题，在设计程序时就应该避免双方相互持有对方的锁的情况
*/
public class DeadLock implements Runnable{
private String tag;
private static Object lock1 = new Object();
private static Object lock2 = new Object();
public void setTag(String tag){
this.tag = tag;
}
@Override
public void run() {
if(tag.equals("a")){
synchronized (lock1) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock1执行");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock2) {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock2执行");
}
}
}
if(tag.equals("b")){
synchronized (lock2) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock2执行");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock1) {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 进入lock1执行");
}
}
}
}
public static void main(String[] args) {
DeadLock d1 = new DeadLock();
d1.setTag("a");
DeadLock d2 = new DeadLock();
d2.setTag("b");
Thread t1 = new Thread(d1, "t1");
Thread t2 = new Thread(d2, "t2");
t1.start();
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
}
}
9.尽量减小锁的粒度
package com.bjsxt.base.sync006;
/**
* 使用synchronized代码块减小锁的粒度，提高性能
*
*/
public class Optimize {
public void doLongTimeTask(){
try {
System.out.println("当前线程开始：" + Thread.currentThread().getName() +
", 正在执行一个较长时间的业务操作，其内容不需要同步");
Thread.sleep(2000);
synchronized(this){
System.out.println("当前线程：" + Thread.currentThread().getName() +
", 执行同步代码块，对其同步变量进行操作");
Thread.sleep(1000);
}
System.out.println("当前线程结束：" + Thread.currentThread().getName() +
", 执行完毕");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
final Optimize otz = new Optimize();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
otz.doLongTimeTask();
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
otz.doLongTimeTask();
}
},"t2");
t1.start();
t2.start();
}
}