synchronized 加到 static 方法前面是給class 加鎖,即類鎖;而synchronized 加到非靜態方法前面是給對象上鎖。這兩者的區別我用代碼來演示下:
對象鎖和類鎖是不同的鎖,所以多個線程同時執行這2個不同鎖的方法時,是非同步的。
在Task2 中定義三個方法 doLongTimeTaskA和doLongTimeTaskB是類鎖,而doLongTimeTaskC是對象鎖。
public class Task2 {
public synchronized static void doLongTimeTaskA() {
System.out.println("name = " + Thread.currentThread().getName() + ", begain");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("name = " + Thread.currentThread().getName() + ", end");
}
public synchronized static void doLongTimeTaskB() {
System.out.println("name = " + Thread.currentThread().getName() + ", begain");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("name = " + Thread.currentThread().getName() + ", end");
}
public synchronized void doLongTimeTaskC() {
System.out.println("name = " + Thread.currentThread().getName() + ", begain");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("name = " + Thread.currentThread().getName() + ", end");
}
三個線程的代碼如下:
class ThreadA extends Thread{
private Task2 mTask2;
public ThreadA(Task2 tk){
mTask2 = tk;
}
public void run() {
mTask2.doLongTimeTaskA();
}
}
class ThreadB extends Thread{
private Task2 mTask2;
public ThreadB(Task2 tk){
mTask2 = tk;
}
public void run() {
mTask2.doLongTimeTaskB();
}
}
class ThreadC extends Thread{
private Task2 mTask2;
public ThreadC(Task2 tk){
mTask2 = tk;
}
public void run() {
mTask2.doLongTimeTaskC();
}
}
main函數中執行代碼如下:
Task2 mTask2 = new Task2();
ThreadA ta = new ThreadA(mTask2);
ThreadB tb = new ThreadB(mTask2);
ThreadC tc = new ThreadC(mTask2);
ta.setName("A");
tb.setName("B");
tc.setName("C");
ta.start();
tb.start();
tc.start();
執行的結果如下:
name = A, begain, time = 1487311199783
name = C, begain, time = 1487311199783
name = C, end, time = 1487311200784
name = A, end, time = 1487311200784
name = B, begain, time = 1487311200784
name = B, end, time = 1487311201784
可以看出由於 doLongTimeTaskA和doLongTimeTaskB都是類鎖,即同一個鎖,所以 A和B是按順序執行,即同步的。而C是對象鎖,和A/B不是同一種鎖,所以C和A、B是 非同步執行的。(A、B、C代指上面的3中方法)。
我們知道對象鎖要想保持同步執行,那麼鎖住的必須是同一個對象。下面就修改下上面的來證明:
Task2.java不變,修改ThreadA 和 ThreadB 如下:
class ThreadA extends Thread{
private Task2 mTask2;
public ThreadA(Task2 tk){
mTask2 = tk;
}
public void run() {
mTask2.doLongTimeTaskC();
}
}
class ThreadB extends Thread{
private Task2 mTask2;
public ThreadB(Task2 tk){
mTask2 = tk;
}
public void run() {
mTask2.doLongTimeTaskC();
}
}
main方法如下:
Task2 mTaska = new Task2();
Task2 mTaskb = new Task2();
ThreadA ta = new ThreadA(mTaska );
ThreadB tb = new ThreadB(mTaskb );
ta.setName("A");
tb.setName("B");
ta.start();
tb.start();
結果如下:
name = A, begain, time = 1487311905775
name = B, begain, time = 1487311905775
name = B, end, time = 1487311906775
name = A, end, time = 1487311906775
從結果看來,對象鎖鎖的對象不一樣,分別是mTaska , mTaskb,所以線程A和線程B調用 doLongTimeTaskC 是非同步執行的。
但是,類鎖可以對類的所有對象的實例起作用。只需修改ThradA和 ThreadB,main 方法不做改變,修改如下:
class ThreadA extends Thread{
private Task2 mTask2;
public ThreadA(Task2 tk){
mTask2 = tk;
}
public void run() {
//mTask2.doLongTimeTaskC();
mTask2.doLongTimeTaskA();
}
}
class ThreadB extends Thread{
private Task2 mTask2;
public ThreadB(Task2 tk){
mTask2 = tk;
}
public void run() {
//mTask2.doLongTimeTaskC();
mTask2.doLongTimeTaskA();
}
}
結果如下:
name = A, begain, time = 1487312239674
name = A, end, time = 1487312240674
name = B, begain, time = 1487312240674
name = B, end, time = 1487312241674
可以看出 在線程A執行完doLongTimeTaskA方法後,線程B才會獲得該類鎖接著去執行doLongTimeTaskA。也就是說,類鎖對所有的該類對象都能起作用。
總結:
1. 如果多線程同時訪問同一類的 類鎖(synchronized 修飾的靜態方法)以及對象鎖(synchronized 修飾的非靜態方法)這兩個方法執行是非同步的,原因:類鎖和對象鎖是2中不同的鎖。
2. 類鎖對該類的所有對象都能起作用,而對象鎖不能。