Effective Java 71 Use lazy initialization judiciously

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Lazy initialization - 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.  

The only way to know for sure is to measure the performance of the class with and without lazy initialization.  

Principle

• Normal initialization of an instance field - Under most circumstances, normal initialization is preferable to lazy initialization.

  // Normal initialization of an instance field

  private final FieldType field = computeFieldValue();  

• synchronized accessor - 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.  

• lazy initialization holder class idiom - If you need to use lazy initialization for performance on a static field, use the lazy initialization holder class idiom .

  // Lazy initialization holder class idiom for static fields

  private static class FieldHolder {

  static final FieldType field = computeFieldValue();

  }

  static FieldType getField() { return FieldHolder.field; }  

• Double-check idiom - If you need to use lazy initialization for performance on an instance field, use the double-check 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;

  }  

• Single-check idiom - Occasionally, you may need to lazily initialize an instance field that can tolerate repeated initialization.

  // 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;

  }  

  Note

  When the double- check or single-check idiom is applied to a numerical primitive field, the field‘s value is checked against 0 (the default value for numerical primitive variables) rather than null.  

• Racy single-check idiom - 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(e.g. String instances to cache their hash codes).

  // racy single-check idiom - can cause repeated initialization!

  private FieldType field;

  private FieldType getField() {

  FieldType result = field;

  if (result == null)

  field = result = computeFieldValue();

  return result;

  }  

  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.

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Effective Java 71 Use lazy initialization judiciously

标签：des   style   class   code   java   color   

原文地址：http://www.cnblogs.com/haokaibo/p/use-lazy-initialization-judiciously.html