[英]synchronized(obj) does not work ,why the result can be -1 or 0?
synchronized(obj)不起作用,為什么結果可以是-1或0? 當我使用synchronized(this)時它會好的,為什么會發生這種情況
public class RunnableSales {
public static void main(String[] args) {
Ticket1 t = new Ticket1();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
Thread t4 = new Thread(t);
t1.start();
t2.start();
t3.start();
t4.start();
}
}
class Ticket1 implements Runnable {
private int tick = 10;
public void run() {
Object obj = new Object();
while (true) {
synchronized (obj) {
if (tick > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName() + "sale:" + tick--);
} else {
break;
}
}
}
}
}
控制台打印是:
Thread-1sale:10
Thread-2sale:10
Thread-3sale:10
Thread-0sale:9
Thread-2sale:8
Thread-3sale:7
Thread-1sale:8
Thread-0sale:8
Thread-1sale:6
Thread-0sale:3
Thread-2sale:5
Thread-3sale:4
Thread-1sale:2
Thread-0sale:1
Thread-2sale:0
Thread-3sale:-1
synchronized(obj)不起作用,為什么結果可以是-1或0? 當我使用synchronized(this)時它會好的,為什么會發生這種情況
查看代碼,可以發現每次調用run()
生成一個新的Object obj
,它僅用於此調用的同步。 因此,實際上沒有同步進行。 修復非常簡單:使Object obj
成為實例字段。 您應該將其設置為final
以禁止重新分配(這可能會導致同步混亂)。
固定版本看起來像這樣:
public class Test {
public static void main(String[] args) {
Ticket1 t = new Ticket1(4);
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
Thread t4 = new Thread(t);
t1.start();
t2.start();
t3.start();
t4.start();
try{
t1.join();
t2.join();
t3.join();
t4.join();
} catch (InterruptedException e){
}
}
}
class Ticket1 implements Runnable {
private int tick = 10;
private final Object obj = new Object();
public void run() {
while (true) {
synchronized (obj) {
if (tick > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName() + "sale:" + tick--);
} else {
break;
}
}
}
}
}
我添加了代碼來顯式等待線程終止。 您應始終包含此代碼,以確保在主線程退出之前完成所有生成的線程。 輸出如下:
Thread-3sale:10
Thread-3sale:9
Thread-3sale:8
Thread-3sale:7
Thread-3sale:6
Thread-3sale:5
Thread-3sale:4
Thread-3sale:3
Thread-3sale:2
Thread-3sale:1
請注意,最終輸出顯示值1
。 關於你的程序,這是正確的,因為在tick
減少到0
之后沒有輸出。
如評論中所述,您需要使您的鎖對象成為類級變量,而不是在run()方法中實例化一個新的。 這樣,每個線程訪問此類將使用鎖對象的相同實例,而不是run()方法中新創建的實例。
碼
static class Ticket1 implements Runnable{
private final int tick = 10;
private final Object obj = new Object();
public void run() {
while(true)
{
synchronized(obj)
{
if(tick>0)
{
try {
Thread.sleep(10);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName()+"sale:"+tick--);
}
else {
break;
}
}
}
}
}
Ouptut
Thread-0sale:10
Thread-0sale:9
Thread-0sale:8
Thread-0sale:7
Thread-0sale:6
Thread-0sale:5
Thread-3sale:4
Thread-3sale:3
Thread-3sale:2
Thread-3sale:1
你以為obj就像鎖一樣。
但是當你啟動4個線程時,它們將調用run方法,並且每個obj都在每個線程中創建,因此上面的代碼沒有任何意義作為鎖。
只有4個線程共享tick。
如果要正常工作,則需要將obj聲明為類成員變量。
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