Double-checked locking

Double-checked locking
http://en.wikipedia.org/wiki/Double-checked_locking




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	// Single-threaded version
	public class DoubleCheckedLocking {
	private Helper helper;
	public Helper getHelper() {
	if (helper == null) {
	helper = new Helper();
	}
	return helper;
	}
	// other functions and members...
	}
	// Correct but possibly expensive multithreaded version
	class DoubleCheckedLocking1 {
	private Helper helper;
	public synchronized Helper getHelper() {
	if (helper == null) {
	helper = new Helper();
	}
	return helper;
	}
	// other functions and members...
	}
	// Broken multithreaded version
	// "Double-Checked Locking" idiom
	class DoubleCheckedLocking2 {
	private Helper helper;
	public Helper getHelper() {
	if (helper == null) {
	synchronized(this) {
	if (helper == null) {
	helper = new Helper();
	}
	}
	}
	return helper;
	}
	// other functions and members...
	}
	// Works with acquire/release semantics for volatile
	// Broken under Java 1.4 and earlier semantics for volatile
	class DoubleCheckedLocking3 {
	private volatile Helper helper;
	public Helper getHelper() {
	Helper result = helper;
	if (result == null) {
	synchronized(this) {
	result = helper;
	if (result == null) {
	helper = result = new Helper();
	}
	}
	}
	return result;
	}
	// other functions and members...
	}
	// Semantics of final field in Java 5 can be employed to safely publish the helper object without using volatile
	public class FinalWrapper<T> {
	public final T value;
	public FinalWrapper(T value) {
	this.value = value;
	}
	}
	public class DoubleCheckedLocking4 {
	private FinalWrapper<Helper> helperWrapper;
	public Helper getHelper() {
	FinalWrapper<Helper> wrapper = helperWrapper;
	if (wrapper == null) {
	synchronized(this) {
	if (helperWrapper == null) {
	helperWrapper = new FinalWrapper<Helper>(new Helper());
	}
	wrapper = helperWrapper;
	}
	}
	return wrapper.value;
	}
	}

view raw DoubleCheckedLocking.java hosted with ❤ by GitHub

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