Java延迟初始化的2种经典模式

静态域的延迟初始化：

// Lazy initialization holder class idiom for static fieldsprivate static class FieldHolder {static final FieldType field = computeFieldValue();}static FieldType getField() {return FieldHolder.field;}

实例域的延迟初始化：

// Double-check idiom for lazy initialization of instance fieldsprivate volatile FieldType field;FieldType getField() {FieldType result = field;if (result == null) { // First check (no locking)synchronized(this) {result = field;if (result == null) { // Second check (with locking)field = result = computeFieldValue();}}}return result;}

参见《Effective Java第二版》第71条：慎用延迟初始化。