卓二妹原创，转载请注明来源。
@ThreadSafe
public class SafePoint {
@GuardedBy("this") private int x, y;
private SafePoint(int[] a) { this(a[0], a[1]); }
public SafePoint(SafePoint p) { this(p.get()); }
public SafePoint(int x, int y) {
this.x = x;
this.y = y;
}
public synchronized int[] get() {
return new int[] { x, y };
}
public synchronized void set(int x, int y) {
this.x = x;
this.y = y;
}
}
java并发编程中对这段代码有一段让人比较费解的描述：
写道
The private constructor exists to avoid the race condition that would occur if the copy constructor were implemented as this(p.x, p.y); this is an example of the private constructor capture idiom (Bloch and Gafter, 2005).
这里如果将private SafePoint(int[] a) { this(a[0], a[1]); }方法改为public就会有race condition问题。为什么呢？
因为如果我们要复制一个SafePoint对象时，我们会首先想到编写下面这样一个方法：
public SafePoint(SafePoint safePoint){
this(safePoint.x, safePoint.y);
}
但是当我们在通过复制originalSafePoint创建新的SafePoint对象时，可能有一个线程正在调用originalSafePoint.setXY方法，导致读取safePoint.x, safePoint.y这对值可能一个更新了而一个没更新，出现线程不安全。因此x，y必须同时读取出来。所以在复制的时候，不能像上面那样调用，而要用getXY()把x y一起取出来。
public SafePoint(SafePoint safePoint){
int [] xy = safePoint.getXY();
this(xy[0], xy[1]);
}
上面的代码会有编译错误，因为this()必须是body的第一行：
那么好吧，那我们这样写就行了是吗？
public SafePoint(SafePoint originalSafePoint){
this(originalSafePoint.getXY()[0],originalSafePoint.getXY()[1]);
}
显然这样也是有问题的，因为originalSafePoint.getXY()被调用了两次，这时候两次取出来的x y已经不是一对值了。
因此我们只有提供一个构造方法接受getXY()返回的数组作为参数，来复制一个对象：
public SafePoint(SafePoint originalSafePoint){
this(originalSafePoint.getXY());
}
private SafePoint(int [] xy){
this(xy[0], xy[1]);
}
这样就避免了上面的copy constructor的问题。现在再来读一下作者说的这句话就清楚很多了：
写道
The private constructor exists to avoid the race condition that would occur if the copy constructor were implemented as this(p.x, p.y);
私有的构造方法的存在，是为了避免在复制构造函数（即SafePoint(SafePoint originalSafePoint)）的中因调用this(p.x, p.y)而引起的race condition。
