第 83 条：慎用延迟初始化

慎用延迟初始化.jpeg

最简单同步访问方法

// Lazy initialization of instance field - synchronized accessor
private FieldType field;
private synchronized FieldType getField() {
    if (field == null)
        field = computeFieldValue();
    return field;
}

lazy initialization holder class 用法

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

双重检查模式（double-check idiom）

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

最单重检查模式（single-check idiom）

// Single-check idiom - can cause repeated initialization!
private volatile FieldType field;
private FieldType getField() {
    FieldType result = field;
    if (result == null)
        field = result = computeFieldValue();
    return result;
}