Saturday, 7 July 2012

Item 71: Use lazy initialization judiciously

Lazy initialization is the act of delaying the initialization of a field until its value is needed. If the value is never needed, the field is never initialized. This technique is applicable to both static and instance fields. While lazy initialization is primarily an optimization, it can also be used to break harmful circularities in class and instance initialization.

As is the case for most optimizations, the best advice for lazy initialization is “don’t do it unless you need to” (Item 55). Lazy initialization is a double-edged sword. It decreases the cost of initializing a class or creating an instance, at the expense of increasing the cost of accessing the lazily initialized field. Depending on what fraction of lazily initialized fields eventually require initialization, how expensive it is to initialize them, and how often each field is accessed, lazy initialization can (like many “optimizations”) actually harm performance.

That said, lazy initialization has its uses. If a field is accessed only on a fraction of the instances of a class and it is costly to initialize the field, then lazy initialization may be worthwhile. The only way to know for sure is to measure the performance of the class with and without lazy initialization.

In the presence of multiple threads, lazy initialization is tricky. If two or more threads share a lazily initialized field, it is critical that some form of synchronization be employed, or severe bugs can result (Item 66). All of the initialization techniques discussed in this item are thread-safe.

Under most circumstances, normal initialization is preferable to lazy initialization. Here is a typical declaration for a normally initialized instance field. Note the use of the final modifier (Item 15):

// Normal initialization of an instance field
private final FieldType field = computeFieldValue();

If you use lazy initialization to break an initialization circularity, use a synchronized accessor, as it is the simplest, clearest alternative:

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

Both of these idioms (normal initialization and lazy initialization with a synchronized accessor) are unchanged when applied to static fields, except that you add the static modifier to the field and accessor declarations.

If you need to use lazy initialization for performance on a static field, use the lazy initialization holder class idiom. This idiom (also known as the initializeon- demand holder class idiom) exploits the guarantee that a class will not be initialized until it is used [JLS, 12.4.1]. Here’s how it looks:

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

When the getField method is invoked for the first time, it reads Field- Holder.field for the first time, causing the FieldHolder class to get initialized. The beauty of this idiom is that the getField method is not synchronized and performs only a field access, so lazy initialization adds practically nothing to the cost of access. A modern VM will synchronize field access only to initialize the class. Once the class is initialized, the VM will patch the code so that subsequent access to the field does not involve any testing or synchronization.

If you need to use lazy initialization for performance on an instance field, use the double-check idiom. This idiom avoids the cost of locking when accessing the field after it has been initialized (Item 67). The idea behind the idiom is to check the value of the field twice (hence the name double-check): once without locking, and then, if the field appears to be uninitialized, a second time with locking.  Only if the second check indicates that the field is uninitialized does the call initialize the field. Because there is no locking if the field is already initialized, it is critical that the field be declared volatile (Item 66). Here is the idiom:

// Double-check idiom for lazy initialization of instance fields
private 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;

Prior to release 1.5, the double-check idiom did not work reliably because the semantics of the volatile modifier were not strong enough to support it [Pugh01]. The memory model introduced in release 1.5 fixed this problem [JLS, 17, Goetz06 16]. Today, the double-check idiom is the technique of choice for lazily initializing an instance field. While you can apply the double-check idiom to static fields as well, there is no reason to do so: the lazy initialization holder class idiom is a better choice.

Two variants of the double-check idiom bear noting. Occasionally, you may need to lazily initialize an instance field that can tolerate repeated initialization. If you find yourself in this situation, you can use a variant of the double-check idiom that dispenses with the second check. It is, not surprisingly, known as the singlecheck idiom. Here is how it looks. Note that field is still declared volatile:

// 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;

If you don’t care whether every thread recalculates the value of a field, and the type of the field is a primitive other than long or double, then you may choose to remove the volatile modifier from the field declaration in the single-check idiom. This variant is known as the racy single-check idiom.

In summary, you should initialize most fields normally, not lazily. If you must initialize a field lazily in order to achieve your performance goals, or to break a harmful initialization circularity, then use the appropriate lazy initialization technique. For instance fields, it is the double-check idiom; for static fields, the lazy initialization holder class idiom. For instance fields that can tolerate repeated initialization, you may also consider the single-check idiom.

Reference: Effective Java 2nd Edition by Joshua Bloch