1.synchronized锁的是什么
synchronized可以锁定this、临界资源、Class类对象。
/***
* synchronized 关键字对某个对象加锁
*/
public class Test {
private int count = 10;
private Object o = new Object();
public void m(){
/**
* 任何线程要执行以下代码，必须拿到对象o的锁
*/
synchronized (o){
count --;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
}
}
/***
* synchronized关键字
* 对某个对象加锁
*/
public class Test {
private int count = 10;
public void m(){
/**
* 任何对象要想执行以下代码，必须拿到this对象的锁
*/
synchronized (this){
count--;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
}
/**
* 等同于在方法的代码执行时要synchronized(this)
*/
public synchronized void m1(){
count--;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
}
/**
* synchronized关键字
* 对某个对象加锁
*/
public class Test {
public static int count = 10;
/***
* 对静态增加synchronized关键字等同于synchronized (Test.class) 锁住当前类的Class对象
*/
public synchronized static void m(){
count--;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
public static void m1(){
synchronized (Test.class){
count--;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
}
}
2.synchronized 关键字的作用
首先先分析以下程序的输出
public class T implements Runnable {
private int count = 10;
public /*synchronized*/ void run() {
count--;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
public static void main(String[] args) {
T t = new T();
for(int i=0; i<5; i++) {
new Thread(t, "THREAD" + i).start();
}
}
}
输出如下：
THREAD4 count = 7
THREAD3 count = 7
THREAD2 count = 6
THREAD1 count = 5
五个线程访问t对象中的count属性，根据结果可以看出多线程情况下出现了数据不一致的情况。
加上synchronized 关键字之后再次运行
public class T implements Runnable {
private int count = 10;
public synchronized void run() {
count--;
System.out.println(Thread.currentThread().getName() + " count = " + count);
}
public static void main(String[] args) {
T t = new T();
for(int i=0; i<5; i++) {
new Thread(t, "THREAD" + i).start();
}
}
}
运行结果
THREAD0 count = 9
THREAD1 count = 8
THREAD4 count = 7
THREAD3 count = 6
THREAD2 count = 5
synchronized 的代码块是一个原子操作，不可分。
3.同步和非同步方法是否可以同时调用？
public class T {
public synchronized void m1() {
System.out.println(Thread.currentThread().getName() + " m1 start...");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " m1 end");
}
public void m2() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " m2 ");
}
public static void main(String[] args) {
T t = new T();
new Thread(t::m1, "t1").start();
new Thread(t::m2, "t2").start();
}
}
运行结果如下
t1 m1 start...
t2 m2
t1 m1 end
在一个synchronize的方法执行之中非同步方法是可以执行的，m2执行的过程总不需要申请this对象的锁。
