Java中的多线程同步在Java示例中的思考
我现在读了《用Java思考》,关于同步的一章,有一个例子我看不懂
public abstract class IntGenerator {
private volatile boolean canceled = false;
public abstract int next();
public void cancel() {
canceled = true;
}
public boolean isCanceled() {
return canceled;
}
}
public class EvenGenerator extends IntGenerator {
private int currentEvenValue = 0;
final Object object = new Object();
@Override
public int next() {
++currentEvenValue;
++currentEvenValue;
return currentEvenValue;
}
public static void main(String[] args) {
EvenChecker.test(new EvenGenerator());
}
}
public class EvenChecker implements Runnable {
private IntGenerator generator;
private final int id;
public EvenChecker(IntGenerator generator, int id) {
this.generator = generator;
this.id = id;
}
@Override
public void run() {
while (!generator.isCanceled()) {
int val = generator.next();
if (val % 2 != 0) {
System.out.println(val + " odd");
generator.cancel();
}
}
}
public static void test(IntGenerator generator, int count) {
System.out.println("To finish press Ctrl + C");
final ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < count; i++) {
executorService.execute(new EvenChecker(generator, i));
}
}
public static void test(IntGenerator generator) {
test(generator, 10);
}
}
示例输出为:
1239 odd
1237 odd
1239 odd
我理解。这意味着3个线程在第一次增量后读取currentValue
这个问题的解决办法是:
public class SynchronizedEvenGenerator extends IntGenerator {
private int currentEvenValue = 0;
@Override
public synchronized int next() {
++currentEvenValue;
Thread.yield();
++currentEvenValue;
return currentEvenValue;
}
public static void main(String[] args) {
EvenChecker.test(new SynchronizedEvenGenerator());
}
}
现在程序正在无限期地工作,没有错误。 我尝试以这种方式仅同步增量:
public class SynchronizedEvenGenerator extends IntGenerator {
private int currentEvenValue = 0;
@Override
public int next() {
synchronized (this) {
++currentEvenValue;
Thread.yield();
++currentEvenValue;
}
return currentEvenValue;
}
public static void main(String[] args) {
EvenChecker.test(new SynchronizedEvenGenerator());
}
}
但现在的例子是:
345 odd
我无法理解,如果两个增量都是同步的,并且任何线程都不能在第一个增量和第二个增量之间读取currentValue,为什么可以读取currentValue的奇数值
为什么我得到这个输出。如何工作synchronized
# 1 楼答案
规则很简单:对共享状态(读或写)的所有访问都必须同步
# 2 楼答案
最后一个示例的
return currentEventValue;
语句不在synchronized
块内。因此,假设线程A和线程B都调用next()
:线程A:
currentEventValue
(现在的值是奇数)currentEventValue
(值再次为偶数)synchronized
块李>线程B:
currentEventValue
(现在的值是奇数)线程A:
currentEventValue
(奇数)线程B:
currentEventValue
(值再次为偶数)synchronized
块李>