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 must remain exclusive to the thread constructing it. However, once the object is initialized, it can be safely published. That is, it can be made visible to other threads. The [Java Memory Model (JMM)|BB. Definitions#Java Memory Model] 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 is similar to [CON14-J. Do not let the "this" reference escape during object construction]. In this guideline, a reference to a partially initialized member object instance is published before initialization is over, instead of the {{this}} reference of the current object.

h2. Noncompliant Code Example

This noncompliant code example uses an {{initialize()}} method to construct a {{Helper}} object inside class {{Foo}}, and initializes the field {{helper}} accordingly.

{code:bgColor=#FFCCCC}
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;
  }
  // ...
}
{code}

Suppose a thread accesses the field {{helper}} using the {{getHelper()}} method, and {{initialize()}} has not been called yet. The thread will see the {{helper}} field as uninitialized. Later, if one thread calls {{initialize()}}, and another calls {{getHelper()}}, the second thread might see 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 {{n}} that has not been initialized yet, and which contains the default value {{0}}.

In particular, the [JMM|BB. Definitions#memory model] 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 instance field {{helper}} with the write that initializes the object {{Helper}} (that is, {{this.n = n}}) such that the former occurs first. This introduces a race window during which other threads may see a partially-initialized {{Helper}} object instance.

There is another consideration in that if two threads both call {{initialize()}}, then two {{Helper}} objects will be created, with one eventually being garbage-collected. This is a performance issue rather than a correctness issue, because {{n}} always contains the value 42.


h2. Compliant Solution (synchronization)

The reference of a partially-constructed object can be prevented from being made visible by using method synchronization.

{code:bgColor=#CCCCFF}
class Foo {
  private Helper helper;

  public synchronized Helper getHelper() {
    return helper;
  }

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

Synchronizing both methods guarantees that they will never run simultaneously in different threads. 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|BB. Definitions#happens-before order] between the two threads. This guarantees that the thread calling {{getHelper()}} sees the fully initialized {{Helper}} object or none at all ({{helper}} may contain a {{null}} reference). This approach guarantees proper publication for both immutable and mutable members.


h2. 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. According to the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\], section 17.5.2 "Reading Final Fields During Construction":

{code:bgColor=#CCCCFF}
class Foo {
  private final Helper helper;

  public Helper getHelper() {
    return helper;
  }

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

However, now the compiler disallows setting the {{helper}} field to a new object using the {{initialize()}} method. Instead, a constructor is required to initialize {{helper}}.

{quote}
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.
{quote}

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]). {mc} this sentence must occur with the quote because it stems from the last line of the quote {mc}


h2. Compliant Solution ({{final}} field and thread-safe composition)

Some collection classes provide thread-safety of accesses 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 encases the {{helper}} field in a {{Vector<Helper>}}. 

{code:bgColor=#CCCCFF}

class Foo {
  private Helper helper;

  public synchronized Helper getHelper() {
    return helper;
  }

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

class Foo {
  private final Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public Foo() {
    // Point 1
    helper = new Helper(42);
    // Point 2
  }
}

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));
    }
  }
}
{code}

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 needs no synchronization in the general case of returning the {{Helper}}, however, if it is invoked before {{initialize()}}, it may ensure that the vector contains the {{Helper}} instance to avoid a null-pointer dereference. Consequently, it invokes {{initialize()}} if the vector is still empty. 


h2. 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. {mc} cite JLS section here {mc}

{code:bgColor=#CCCCFF}
class Foo {
  private static final Helper helper = new Helper(42);

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

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

According to JSR-133 \[[JSR-133 04|AA. Java References#JSR-133 04]\], 9.2.3 Static Final Fields:

{quote}
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.
{quote}

h2. Compliant Solution (immutable object - final fields, {{volatile}} reference)

The Java memory model guarantees that any {{final}} fields of the object will be fully initialized before a published object becomes visible \[[Goetz 06|AA. Java References#Goetz 06]\]. By making {{n}} {{final}}, we can render the {{Helper}} class [immutable|BB. Definitions#immutable]. Furthermore, if the {{helper}} field is declared {{volatile}}, the {{Helper}}'s reference is guaranteed to be made visible to any thread that calls {{getHelper()}} after {{Helper}} has been fully initialized.

{code:

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

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

bgColor=#CCCCFF}
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;
  }
  // ...
}
{code

class Foo {
  private volatile Helper helper;

  public Helper getHelper(}

h2. Compliant Solution (immutable object - factory method, volatile reference)

An alternative is to use a {{public static}} factory method in class {{Helper}} to create a new instance of {{Helper}}. The {{Helper}} instance is created using a {{private}} constructor.

{code:bgColor=#CCCCFF}
// Immutable Helper
class Helper { // Class is not required to be public or final in this case
  private final int n;

  private Helper(int n) {
    this.n = nreturn helper;
  }
  
  public staticvoid Helper getHelperinitialize() {
    Helper helper = new Helper(42);
    return helper;
  }
  // ...
}
{code}


This compliant solution, as well as the previous one, require 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 required to fix it (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 how to safely publish immutable objects.

h2. 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}}. 

{code:bgColor=#CCCCFF}
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;
    }
  }
}
{code}

Because the state of the {{Helper}} object can be changed after its construction, additional locking is necessary to ensure that all threads see the most recent value of {{n}} after the initial publication. See [CON11-J. Do not assume that declaring an object reference volatile guarantees visibility of its members] for more information. 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.

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 thread-unsafe objects. 

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}}. {mc} please double check this para {mc}

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.


h2. Exceptions

*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. 

{mc} not a good idea to call this class thread-safe, bad for maintainability because someone might forget to use the flag in a new method if you call it thread-safe and even other wise, the flag only guarantees proper initializatin, not thread-safety {mc}

{code:bgColor=#CCCCFF}
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");
    }
    // ... 
  }
  // ...
}
{code}

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}}.

h2. 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.

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON26-J | medium | probable | medium | {color:#cc9900}{*}P8{*}{color} | {color:#cc9900}{*}L2{*}{color} |

h3. Automated Detection

TODO

h3. Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON26-J].

h2. References

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

----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|FIO36-J. Do not create multiple buffered wrappers on an InputStream]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|09. Input Output (FIO)]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|09. Input Output (FIO)]
}
}

// 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;
    }
  }
}

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");
    }
    // ...
  }
  // ...
}