并发案例(二)
1.非线程安全
main线程更新ready址 ， ReaderThread 线程持续检测ready状态 ，直到ready为true.
正常来讲，程序会输出42 ；但也有可能输出0 , 甚至可能程序复发终止。
输出0:
@NotSafeThread
public class NoVisibility {
private static boolean ready;
private static int number;
private static class ReaderThread extends Thread {
public void run() {
while (!ready)
Thread.yield();
System.out.println(number);
}
}
public static void main(String[] args) {
new ReaderThread().start();
number = 42;
ready = true;
}
}
@NotThreadSafe
public class MutableInteger {
private int value;
public int get() { return value; }
public void set(int value) { this.value = value; }
}
@ThreadSafe
public class SynchronizedInteger {
@GuardedBy("this") private int value;
public synchronized int get() { return value; }
public synchronized void set(int value) { this.value = value; }
}
@NotThreadSafe
class UnsafeStates {
private String[] states = new String[] {
"AK", "AL" ...
};
public String[] getStates() { return states; }
}
@NotSafeThread
public class ThisEscape {
public ThisEscape(EventSource source) {
source.registerListener(
new EventListener() {
public void onEvent(Event e) {
doSomething(e);
} });
} }
临时阻塞
这个技术尽量少用，比较脆弱。应当使用更加健壮的 栈阻塞 或者是 ThreadLocal
@SafeThread
public class SafeListener {
private final EventListener listener;
private SafeListener() {
listener = new EventListener() {
public void onEvent(Event e) {
doSomething(e);
} };
}
public static SafeListener newInstance(EventSource source) {
SafeListener safe = new SafeListener();
source.registerListener(safe.listener);
return safe;
} }
栈阻塞
通过 局部变量（local variables) —— 本例中是局部的原始变量, 运行在线程的栈上 ，不会和别的线程共享，来保证线程安全。
public int loadTheArk(Collection<Animal> candidates) {
SortedSet<Animal> animals;
int numPairs = 0;
Animal candidate = null;
// animals confined to method, don't let them escape!
animals = new TreeSet<Animal>(new SpeciesGenderComparator());
animals.addAll(candidates);
for (Animal a : animals) {
if (candidate == null || !candidate.isPotentialMate(a))
candidate = a;
else {
ark.load(new AnimalPair(candidate, a));
++numPairs;
candidate = null;
} }
return numPairs;
}
利用ThreadLocal保证线程安全
以jdbc connection 为例，每个线程一个connection 。
private static ThreadLocal<Connection> connectionHolder
= new ThreadLocal<Connection>() {
public Connection initialValue() {
return DriverManager.getConnection(DB_URL);
} };
public static Connection getConnection() {
return connectionHolder.get();
}
@Immutable
public final class ThreeStooges {
private final Set<String> stooges = new HashSet<String>();
public ThreeStooges() {
stooges.add("Moe");
stooges.add("Larry");
stooges.add("Curly");
}
public boolean isStooge(String name) {
return stooges.contains(name);
} }
