CON26-J. Do not publish partially initialized objects

During initialization of a shared object, the object must only be accessible to the thread constructing it. However, the object can be safely published (that is, made visible to other threads) once it is initialized. The [Java Memory Model (JMM)] allows multiple threads to observe the object after its initialization has begun, but before it has concluded. Consequently, it is important to ensure that a partially initialized object is not published.

This guideline differs from other guidelines in that it prohibits publishing a reference to a partially initialized member object instance before initialization completes while [CON14-J. Do not let the "this" reference escape during object construction] refers to the this reference of the current object.

Noncompliant Code Example

This noncompliant code example constructs a Helper object in the initialize() method of class Foo. The helper field is initialized by Helper's constructor.

class Foo {
  private Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void initialize() {
    helper = new Helper(42);
  }
}

public class Helper {
  private int n;

  public Helper(int n) {
    this.n = n;
  }
  // ...
}

If a thread accesses helper using the getHelper() method before initialize() has been called, the thread will observe an uninitialized helper field. Later, if one thread calls initialize(), and another calls getHelper(), the second thread might observe the helper reference as null, or it might observe a fully-initialized Helper object with the n field set to 42, or it might observe a partially-initialized Helper object with an uninitialized n which contains the default value 0.

In particular, the [JMM] permits compilers to allocate memory for the new Helper object and assign it to the helper field before initializing it. In other words, the compiler can reorder the write to the helper instance field with the write that initializes the Helper object (that is, this.n = n) such that the former occurs first. This exposes a race window during which other threads may observe a partially-initialized Helper object instance.

There is a separate issue in that, if two threads call initialize(), then two Helper objects are created. This is a performance issue and not a correctness issue because n will be properly initialized and the unused Helper objects will be garbage-collected.

Compliant Solution (Synchronization)

Publishing partially-constructed object reference can be prevented by using method synchronization, as shown by this compliant solution.

class Foo {
  private Helper helper;

  public synchronized Helper getHelper() {
    return helper;
  }

  public synchronized void initialize() {
    helper = new Helper(42);
  }
}

Synchronizing both methods guarantees that they will not execute concurrently. If one thread calls initialize() just before another thread calls getHelper(), the synchronized initialize() method will always finish first. The synchronized keywords establish a [happens-before relationship] between the two threads. This guarantees that the thread calling getHelper() sees the fully initialized Helper object or none at all (that is, helper contains a null reference). This approach guarantees proper publication for both immutable and mutable members.

Compliant Solution (Final Field)

If the helper field is declared as final, it is guaranteed to be fully constructed before its reference is made visible.

class Foo {
  private final Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public Foo() {
    helper = new Helper(42);
  }
}

However, this solution requires that the helper field is assigned to a new object during construction. According to the Java Language Specification, Section 17.5.2, "Reading Final Fields During Construction" [[JLS 05]]:

A read of a final field of an object within the thread that constructs that object is ordered with respect to the initialization of that field within the constructor by the usual happens-before rules. If the read occurs after the field is set in the constructor, it sees the value the final field is assigned, otherwise it sees the default value.

Consequently, the reference to the helper field should not be published before class Foo's constructor has finished its initialization (see [CON14-J. Do not let the "this" reference escape during object construction]).

Compliant Solution (Final Field and Thread-safe Composition)

Some collection classes provide thread-safe access to contained elements. If the Helper object is inserted into such a collection, it is guaranteed to be fully initialized before its reference is made visible. This compliant solution encapsulates the helper field in a Vector<Helper>.

class Foo {
  private final Vector<Helper> helper;

  public Foo() {
    helper = new Vector<Helper>();  
  }

  public Helper getHelper() {
    if (helper.isEmpty()) {
      initialize();
    }
    return helper.elementAt(0);
  }

  public synchronized void initialize() {
    if (helper.isEmpty()) {
      helper.add(new Helper(42));
    }
  }
}

The helper field is declared as final to guarantee that the vector is created before any accesses take place. It can be suitably initialized using the initialize() method. The initialize() method is synchronized, and provides a check to ensure that exactly one Helper object is added to the vector. The getHelper() method does not need to be syncrhronized in the general case of returning the Helper, however, if it is invoked before initialize(), it ensures that the vector contains the Helper instance to avoid a null-pointer dereference. To do this it invokes initialize() method.

Compliant Solution (Static Initialization)

In this compliant solution, the helper field is initialized in a static block. When initialized statically, an object is guaranteed to be fully initialized before its reference is made visible.

// Immutable Foo
final class Foo {
  private static final Helper helper = new Helper(42);

  public static Helper getHelper() {
    return helper;
  } 
}

This requires the helper field to be declared static. Although not strictly necessary, it is recommended that the field be declared final to document the class's immutability.

According to JSR-133, Section 9.2.3, "Static Final Fields" [[JSR-133 04]]:

The rules for class initialization ensure that any thread that reads a static field will be synchronized with the static initialization of that class, which is the only place where static final fields can be set. Thus, no special rules in the JMM are needed for static final fields.

Compliant Solution (Immutable object - Final fields, Volatile Reference)

The Java memory model guarantees that any final fields of an object are fully initialized before a published object becomes visible [[Goetz 06]]. By declaring n as final, the Helper class is rendered [immutable]. Furthermore, if the helper field is declared volatile in compliance with [CON09-J. Ensure visibility of shared references to immutable objects], Helper's reference is guaranteed to be made visible to any thread that calls getHelper() after Helper has been fully initialized.

class Foo {
  private volatile Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void initialize() {
    helper = new Helper(42);
  }
}

// Immutable Helper
public final class Helper {
  private final int n;

  public Helper(int n) {
    this.n = n;
  }
  // ...
}

Similarly, a public static factory method that returns a new instance of Helper can be provided in class Helper. With this approach, the Helper instance can be created in a private constructor.

This compliant solution requires that helper be declared as volatile and class Helper be immutable. If it were not immutable, the code would violate [CON11-J. Do not assume that declaring an object reference volatile guarantees visibility of its members] and additional synchronization would be necessary (see the next compliant solution). And if the helper field were not volatile, it would violate [CON09-J. Ensure visibility of shared references to immutable objects]. See that rule for more information on safely publishing immutable objects.

Compliant Solution (Mutable Thread-safe Object, Volatile Reference)

If Helper is mutable, but thread-safe, it can be safely published by declaring the helper field in class Foo as volatile.

class Foo {
  private volatile Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void initialize() {
    helper = new Helper(42);
  }
}

// Mutable but thread-safe Helper
public class Helper {
  private volatile int n;
  private final Object lock = new Object();

  public Helper(int n) {
    this.n = n;
  }
  
  public void setN(int value) {
    synchronized (lock) {
      n = value;
    }
  }
}

Because the state of the Helper object can be changed after its construction, locking is necessary to ensure that all threads see the most recent value of n after the initial publication. This is typically the case when a mutable member object is expected to continually publish its most recent state when its reference is declared volatile. Consequently, in this compliant solution, the setN() method needs to be synchronized. (See [CON11-J. Do not assume that declaring an object reference volatile guarantees visibility of its members] for more information.)

In the absence of proper synchronization in class Helper, the use of volatile in class Foo guarantees the visibility of only the initial publication of Helper and not of subsequent state changes. Consequently, volatile is not useful for publishing objects that are not thread-safe.

When helper in class Foo is not declared as volatile, the field n should be declared as volatile so that a happens-before relationship is established between the initialization of n and the write of Helper to the field helper. This is in compliance with [CON11-J. Do not assume that declaring an object reference volatile guarantees visibility of its members]. This is only required when the caller (class Foo) cannot be trusted to declare helper as volatile.

Note that the Helper class uses a private lock to handle synchronization, because the class is public. (See [CON04-J. Use private final lock objects to synchronize classes that may interact with untrusted code] for more information.)

Exceptions

CON26-EX1: This exception uses a volatile initialized flag, as recommended by [CON28-J. Prevent partially initialized objects from being used]. The corresponding Foo class is the same as the noncompliant code example.

public class Helper {
  private int n;
  private volatile boolean initialized; // Defaults to false

  public Helper(int n) {
    this.n = n;
    this.initialized = true;
  }
  
  public void doSomething() {
    if(!initialized) {
      throw new SecurityException("Cannot use partially initialized instance");
    }
    // ... 
  }
  // ...
}

This ensures that even if the reference to the Helper object instance is published before its initialization is over, the instance is unusable. This is because every method within Helper must check the flag to determine whether the initialization has finished. This approach is more useful when the caller (Foo) is untrusted and consequently, may not declare the helper field as volatile.

Risk Assessment

Failing to synchronize access to shared mutable data can cause different threads to observe different states of the object or a partially initialized object.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]]
[[Bloch 01]] Item 48: "Synchronize access to shared mutable data"
[[Goetz 06]] Section 3.5.3 "Safe Publication Idioms"
[[Goetz 07]] Pattern #2: "one-time safe publication"
[[JPL 06]] 14.10.2. "Final Fields and Security"
[[Pugh 04]]

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