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 _Lazy initialization_ defers the construction of a member field or an object referred to by a member field until an instance is actually required, rather than computing the field value or constructing the referenced object in the class's constructor. Lazy initialization helps to break harmful circularities in class and instance initialization . andIt inalso performingenables other optimizations \[[Bloch 2005|AA. Bibliography#Bloch 05]\].

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Lazy initialization must be synchronized in multithreaded applications , to prevent multiple threads from creating extraneous instances of the member object:

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The double-checked locking idiom improves performance by limiting synchronization to the rare case of computing the field's value or constructing a new instance for the field to reference , and by foregoing synchronization during the common case of retrieving an already-created instance or value.

Incorrect forms of the double-checked locking idiom include those that allow publication of an uninitialized or partially initialized object. Consequently, only those forms of the double-checked locking idiom that correctly establish a happens-before relationship both for the helper reference and also for the complete construction of the Helper instance are permitted.

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The double-checked locking pattern uses block synchronization rather than method synchronization , and installs an additional null reference check before attempting synchronization. This noncompliant code example uses the incorrect form of the double-checked locking idiom.

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According to Pugh \[[Pugh 2004|AA. Bibliography#Pugh 04]\]

... writes Writes that initialize the Helper object and the write to the helper field can be done or perceived out of order. As a result, a thread which invokes getHelper() could see a non-null reference to a helper object, but see the default values for fields of the helper object, rather than the values set in the constructor.

Even if the compiler does not reorder those writes, on a multiprocessor, the processor or the memory system may reorder those writes, as perceived by a thread running on another processor.

This code also violates rule TSM03-J. Do not publish partially initialized objects.

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Variables that are declared static and initialized at declaration , or from a static initializer , are guaranteed to be fully constructed before being made visible to other threads. However, this solution forgoes the benefits of lazy initialization.

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Initialization of the static {{helper}} field is deferred until the {{getInstance()}} method is called. The necessary happens-before relationships are created by the combination of the class loader's actions loading and initializing the {{Holder}} instance, and the guarantees provided by the Java memory model. This idiom is a better choice than the double-checked locking idiom for lazily initializing static fields \[[Bloch 2008|AA. Bibliography#Bloch 08]\].  However, this idiom cannot be used to lazily initialize instance fields \[[Bloch 2001|AA. Bibliography#Bloch 01]\].

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final class Foo {
  private final ThreadLocal<Foo> perThreadInstance = 
      new ThreadLocal<Foo>();
  private Helper helper = null;

  public Helper getHelper() {
    if (perThreadInstance.get() == null) {
      createHelper();
    }
    return helper;
  }

  private synchronized void createHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    // Any non-null value can be used as an argument to set()
    perThreadInstance.set(this);
  }
}

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In this compliant solution, suppose that the Helper class is immutable. The Java Memory Model (JMM) guarantees that immutable objects are fully constructed before they become visible to any other thread. Additionally, the block synchronization in the getHelper() method suffices to ensure that all methods that can see a non-null value of the helper field have a proper happens-before relationship for the update to the helper reference. This synchronization and the aforementioned JMM guarantee combine to ensure that only fully - initialized Helper objects are visible to threads that see non-null values. Consequently, this compliant solution correctly creates both of the needed happens-before relationships.

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*LCK10-EX0:* Use of the noncompliant form of the double-checked locking idiom is permitted for 32-bit primitive values (for example, {{int}} or {{float}}) \[[Pugh 2004|AA. Bibliography#Pugh 04]\], although this usage is discouraged. The noncompliant form establishes the necessary happens-before relationship between threads that see an initialized version of the primitive value. The second happens-before relationship (that for the initialization of the fields of the referent) is of no practical value, because unsynchronized reads and writes of primitive values up to 32-bits are guaranteed to be atomic. Consequently, the noncompliant form establishes the only needed happens-before relationship in this case. Note, however, that the noncompliant form fails for {{long}} or {{double}} because unsynchronized reads or writes of 64-bit primitives lack a guarantee of atomicity, and consequently require a second happens-before relationship to guarantee that all threads see only fully assigned 64-bit values (See rule [VNA05-J. Ensure atomicity when reading and writing 64-bit values] for more information.)

Risk Assessment

Using incorrect forms of the double-checked locking idiom can lead to synchronization problems and can expose partially - initialized objects.

Related Guidelines

Bibliography

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