一、关于线程安全
1.是什么决定的线程安全问题？
线程安全问题基本是由全局变量及静态变量引起的。
若每个线程中对全局变量、静态变量只有读操作，而无写操作，一般来说，这个全局变量是线程安全的；若有多个线程同时执行写操作，一般都需要考虑线程同步，否则的话就可能影响线程安全。
2.可以解决多线程并发访问资源的方法有哪些？
主要有三种方式：分别是同步代码块 、同步方法和锁机制(Lock)
其中同步代码块和同步方法是通过关键字synchronized实现线程同步
本文主要是将synchronized关键字用法作为例子来去解释Java中的对象锁和类锁
二、synchronized关键字各种用法与实例
事实上，synchronized修饰非静态方法、同步代码块的synchronized (this)用法和synchronized (非this对象)的用法锁的是对象，线程想要执行对应同步代码，需要获得对象锁。
synchronized修饰静态方法以及同步代码块的synchronized (类.class)用法锁的是类，线程想要执行对应同步代码，需要获得类锁。
因此，事实上synchronized关键字可以细分为上面描述的五种用法。
1.同步块synchronized (this)
public class Ticket1 extends Thread{
private int nums = 0; //出票数
private int count =20; //剩余
@Override
public void run() {
while (true) {
synchronized (this) {
if(count <= 0) {
break;
}
nums++;
count--;
try {
Thread.sleep(550);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("显示出票信息："+Thread.currentThread().getName()+
"抢到第"+nums+"张票，剩余"+count+"张");
}
}
}
public static void main(String[] args) {
Ticket1 ticket1 = new Ticket1();
Thread anni = new Thread(ticket1,"安妮");
Thread jack = new Thread(ticket1,"jack");
anni.start();
jack.start();
}
}
这样是同步的，线程获取的是同步块synchronized (this)括号（）里面的对象实例的对象锁，这里就是Ticket1 实例对象的对象锁了。
需要注意的是synchronized (){}的{}前后的代码依旧是异步的
2.synchronized (非this对象)的用法锁的是对象
例1：
public class Ticket2 implements Runnable{
private int nums = 0; //出票数
private int count =25; //剩余
//实现线程安全三种方法
private final Object lock = new Object();//1.使用私有不变对象锁，使得攻击者无法获取到锁对象(推荐使用)
@Override
public void run() {
while (true) {
//2.this 使用对象自身的锁（隐式锁）--对象锁
//3.Ticket2.class 给Ticket2加锁 --类锁->使用说明：静态方法则一定会同步，非静态方
//法需在单例模式才生效(本例为单例)，但是也不能都用静态同步方法，总之用得不好可能会给性能带来极大的影响。
synchronized (lock) {
if(count <= 0) {
break;
}
nums++;
count--;
try {
Thread.sleep(750);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("显示出票信息："+Thread.currentThread().getName()+
"抢到第"+nums+"张票，剩余"+count+"张");
}
}
}
public static void main(String[] args) {
Ticket2 ticket2 = new Ticket2();
Thread zhangsan = new Thread(ticket2,"张三");
Thread zhaoyun = new Thread(ticket2,"赵云");
zhangsan.start();
zhaoyun.start();
}
}
例2：
public class Run2 {
public static void main(String[] args) {
Service service = new Service("xiaobaoge");
ThreadA2 a = new ThreadA2(service);
a.setName("A");
a.start();
ThreadB2 b = new ThreadB2(service);
b.setName("B");
b.start();
}
}
class Service {
String anyString = new String();
public Service(String anyString){
this.anyString = anyString;
}
public void setUsernamePassword(String username, String password) {
try {
synchronized (anyString) {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入同步块");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开同步块");
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class ThreadA2 extends Thread {
private Service service;
public ThreadA2(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.setUsernamePassword("a", "aa");
}
}
class ThreadB2 extends Thread {
private Service service;
public ThreadB2(Service service) {
super();
this.service = service;
}
@Override
public void run() {
service.setUsernamePassword("b", "bb");
}
}
3.synchronized (class)
public class Run {
public static void main(String[] args) {
ThreadA a = new ThreadA();
a.setName("A");
a.start();
ThreadB b = new ThreadB();
b.setName("B");
b.start();
}
}
class Service {
public static void printA() {
synchronized (Service.class) {
try {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void printB() {
synchronized (Service.class) {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printB");
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printB");
}
}
}
4.静态synchronized同步方法
public class Run {
public static void main(String[] args) {
ThreadA a = new ThreadA();
a.setName("A");
a.start();
ThreadB b = new ThreadB();
b.setName("B");
b.start();
}
}
class Service {
synchronized public static void printA() {
try {
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "进入printA");
Thread.sleep(3000);
System.out.println("线程名称为：" + Thread.currentThread().getName()
+ "在" + System.currentTimeMillis() + "离开printA");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
synchronized public static void printB() {
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "进入printB");
System.out.println("线程名称为：" + Thread.currentThread().getName() + "在"
+ System.currentTimeMillis() + "离开printB");
}
}
class ThreadA extends Thread {
@Override
public void run() {
Service.printA();
}
}
class ThreadB extends Thread {
@Override
public void run() {
Service.printB();
}
}
5.synchronized修饰非静态方法
public class Run {
public static void main(String[] args) {
HasSelfPrivateNum numRef = new HasSelfPrivateNum();
ThreadA athread = new ThreadA(numRef);
athread.start();
ThreadB bthread = new ThreadB(numRef);
bthread.start();
}
}
class HasSelfPrivateNum {
private int num = 0;
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!");
}
System.out.println(username + " num=" + num);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class ThreadA extends Thread {
private HasSelfPrivateNum numRef;
public ThreadA(HasSelfPrivateNum numRef) {
super();
this.numRef = numRef;
}
@Override
public void run() {
super.run();
numRef.addI("a");
}
}
class ThreadB extends Thread {
private HasSelfPrivateNum numRef;
public ThreadB(HasSelfPrivateNum numRef) {
super();
this.numRef = numRef;
}
@Override
public void run() {
super.run();
numRef.addI("b");
}
}
实验结论：两个线程访问同一个对象中的同步方法是一定是线程安全的。本实现由于是同步访问，所以先打印出a，然后打印出b
这里线程获取的是HasSelfPrivateNum的对象实例的锁——对象锁。