本文转载自
IT江湖,原文链接:Click me
如果println函数的参数为常量则不会出现线程并发问题,但是如果参数为表达式形式,则JVM在执行println函数的时候会分为几步来执行,从而造成并发问题。
如下例子所示:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
|
public
class Test
{
public
static void
main(String[]
args)
{
ExecutorService
pool =
Executors.newFixedThreadPool(2);
Runnable t1
= new
MyRunnable("张三",
2000);
Runnable
t2 =
new MyRunnable("李四",
3600);
Runnable t3
= new
MyRunnable("王五",
2700);
Runnable
t4 =
new MyRunnable("老张",
600);
Runnable t5
= new
MyRunnable("老牛",
1300);
Runnable
t6 =
new MyRunnable("老朱",
800);
//执行各个线程
pool.execute(t1);
pool.execute(t2);
pool.execute(t3);
pool.execute(t4);
pool.execute(t5);
pool.execute(t6);
//关闭线程池
pool.shutdown();
}
}
class MyRunnable
implements Runnable
{
private
static AtomicLong
aLong =
new AtomicLong(10000);
//原子量,每个线程都可以自由操作
private
String name;
//操作人
private
int data;
//操作数
MyRunnable(String
name,
int data)
{
this.name
= name;
this.data
= data;
}
public
void run()
{
System.out.println(name
+ "执行了"
+ data
+ ",当前余额:"
+ aLong.addAndGet(data));
}
}
|
执行的结果为:
张三执行了2000,当前余额:12000
李四执行了3600,当前余额:15600
王五执行了2700,当前余额:18300
老牛执行了1300,当前余额:20200
老朱执行了800,当前余额:21000
老张执行了600,当前余额:18900
通过反编译,上面println函数的主要处理过程分为下面三步:
|
long
l =
aLong.addAndGet(data);
String sb
=new
StringBuilder(String.valueOf(name))).append("执行了").append(data).append(",当前余额:").append(l);
synchronized
(this)
{
print(sb.toString);
newLine();
}
|
所以可以看出上面程序的执行顺序为:、
张三执行完->李四执行完->王五执行完->老张只执行到加操作,被老牛打断->老牛执行完->老朱执行完->老张执行剩下的输出操作
要想有一个正常的输出顺序,应该修改代码为:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
|
import
java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import
java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.Lock;
import
java.util.concurrent.locks.ReentrantLock;
public
class Test
{
public
static void
main(String[]
args)
{
ExecutorService
pool =
Executors.newFixedThreadPool(2);
Lock lock
= new
ReentrantLock(false);
Runnable
t1 =
new MyRunnable("张三",
2000,
lock);
Runnable t2
= new
MyRunnable("李四",
3600,
lock);
Runnable
t3 =
new MyRunnable("王五",
2700,
lock);
Runnable t4
= new
MyRunnable("老张",
600,
lock);
Runnable
t5 =
new MyRunnable("老牛",
1300,
lock);
Runnable t6
= new
MyRunnable("老朱",
800,
lock);
//执行各个线程
pool.execute(t1);
pool.execute(t2);
pool.execute(t3);
pool.execute(t4);
pool.execute(t5);
pool.execute(t6);
//关闭线程池
pool.shutdown();
}
}
class
MyRunnable implements
Runnable
{
private
static AtomicLong
aLong =
new AtomicLong(10000);
//原子量,每个线程都可以自由操作
private
String name;
//操作人
private
int data;
//操作数
private
Lock lock;
MyRunnable(String
name,
int data,
Lock lock)
{
this.name
= name;
this.data
= data;
this.lock
= lock;
}
public
void run()
{
lock.lock();
System.out.println(name
+ "执行了"
+ data
+ ",当前余额:"
+ aLong.addAndGet(data));
lock.unlock();
}
}
|
结果为:
张三执行了2000,当前余额:12000
王五执行了2700,当前余额:14700
老张执行了600,当前余额:15300
老牛执行了1300,当前余额:16600
老朱执行了800,当前余额:17400
李四执行了3600,当前余额:21000
【注意】这里使用了一个对象锁,来控制对并发代码的访问。不管运行多少次,执行次序如何,最终余额均为21000,这个结果是正确的。有关原子量的用法很简单,关键是对原子量的认识,原子仅仅是保证变量操作的原子性,但整个程序还需要考虑线程安全的。