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Effective Java 66 Synchronize access to shared mutable data
synchronized - Only a single thread can execute a method or block at one time.
Not only does synchronization prevent a thread from observing an object in an inconsistent state, but it ensures that each thread entering a synchronized method or block sees the effects of all previous modifications that were guarded by the same lock.
Synchronization is required for reliable communication between threads as well as for mutual exclusion.
Principles
-
Do not use Thread.stop.
-
Synchronization has no effect unless both read and write operations are synchronized.
/**
* Demo for 66 Synchronize access to shared mutable data.
*/
package com.effectivejava.concurrency;
import java.util.concurrent.TimeUnit;
/**
* Properly synchronized cooperative thread termination
* @author Kaibo
*
*/
public class StopThread {
private static boolean stopRequested;
private static synchronized void requestStop() {
stopRequested = true;
System.out.println("request stop from another thread.");
}
private static synchronized boolean stopRequested() {
return stopRequested;
}
public static void main(String[] args) throws InterruptedException {
Thread backgroundThread = new Thread(new Runnable() {
public void run() {
int i = 0;
while (!stopRequested())
System.out.println(i++);
}
});
backgroundThread.start();
TimeUnit.SECONDS.sleep(1);
requestStop();
}
}
/**
* Cooperative thread termination with a volatile field
*
* @author Kaibo
*
*/
public class StopThreadWithVolatile {
private static volatile boolean stopRequested;
public static void main(String[] args) throws InterruptedException {
Thread backgroundThread = new Thread(new Runnable() {
public void run() {
int i = 0;
while (!stopRequested)
System.out.println(i++);
}
});
backgroundThread.start();
TimeUnit.SECONDS.sleep(1);
stopRequested = true;
}
}
NOTE
operator(++) is not atomic - If a second thread reads the field between the time a thread reads the old value and writes back a new one, the second thread will see the same value as the first and return the same serial number.
// Broken - requires synchronization!
private static volatile int nextSerialNumber = 0;
public static int generateSerialNumber() {
return nextSerialNumber++;
}
// Correct way
private static final Atomic Long nextSerialNum = new AtomicLong();
public static long generateSerialNumber() {
return nextSerialNum.getAndIncrement();
}
4. Confine mutable data to a single thread
Summary
When multiple threads share mutable data, each thread that reads or writes the data must perform synchronization. Without synchronization, there is no guarantee that one thread’s changes will be visible to another. The penalties for failing to synchronize shared mutable data are liveness and safety failures. If you need only inter-thread communication, and not mutual exclusion, the volatile modifier is an acceptable form of synchronization, but it can be tricky to use correctly.