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Wiki Markup
"Client-side locking entails guarding client code that uses some object X with the lock X uses to guard its own state. In order to use client-side locking, you must know what lock X uses." \[[Goetz 06|AA. Java References#Goetz 06]\].

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Wiki Markup
While client-side locking is acceptable if the thread-safe class commits to its locking strategy and clearly documents it, Goetz et al. \[[Goetz 06|AA. Java References#Goetz 06]\] caution against its misuse:

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If extending a class to add another atomic operation is fragile because it distributes the locking code for a class over multiple classes in an object hierarchy, client-side locking is even more fragile because it entails putting locking code for class C into classes that are totally unrelated to C. Exercise care when using client-side locking on classes that do not commit to their locking strategy.

Wiki Markup
The documentation of a class that does support client-side locking should explicitly state its applicability. 

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For instance, the class {{java.util.concurrent.ConcurrentHashMap<K,V>}}

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 should not be used for client-side locking, because documentation states \[[API 06|AA. Java References#API 06]\]:

...

...

...

even

...

though

...

all

...

operations

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are

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thread-safe,

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retrieval

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operations

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do

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not

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entail

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locking,

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and

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there

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is

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not

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any

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support

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for

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locking

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the

...

entire

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table

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in

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a

...

way

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that

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prevents

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all

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access.

...

This

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class

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is

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fully

...

interoperable

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with

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Hashtable

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in

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programs

...

that

...

rely

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on

...

its

...

thread

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safety

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but

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not

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on

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its

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synchronization

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details.

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Wiki Markup
In general, use client-side locking only when the documentation of the class recommends it. For example, the documentation of the wrapper method {{synchronizedList()}} of class {{java.util.Collections}} \[[API 06|AA. Java References#API 06]\] states:

In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list. It is imperative that the user manually synchronize on the returned list when iterating over it. Failure to follow this advice may result in non-deterministic behavior.

Note that this advice is compliant with CON40-J. Do not synchronize on a collection view if the backing collection is accessible when the backing list is inaccessible from a caller that can potentially inflict harm.

Noncompliant Code Example (intrinsic lock)

This noncompliant code example uses a thread-safe class Book that cannot be refactored. This could happen, for example, when the source code is not available for review or the class is part of a general library that cannot be extended.

Code Block


{quote}
In order to guarantee serial access, it is critical that all access to the backing list is accomplished through the returned list. It is imperative that the user manually synchronize on the returned list when iterating over it. Failure to follow this advice may result in non-deterministic behavior.
{quote} 

Note that the advice suggested above is compliant with [CON40-J. Do not synchronize on a collection view if the backing collection is accessible] when the backing list is inaccessible from a caller that can potentially inflict some harm.


h2. Noncompliant Code Example (intrinsic lock)

This noncompliant code example uses a thread-safe class {{Book}} that cannot be refactored. This could happen, for example, when the source code is not available for review or the class is part of a general library that cannot be extended.

{code}
final class Book {
  // May change its locking policy in the future to use private final locks
  private final String title;
  private Calendar dateIssued;
  private Calendar dateDue;

  Book(String title) {
    this.title = title; 
  }
  
  public synchronized void issue(int days) {
    dateIssued = Calendar.getInstance();
    dateDue = Calendar.getInstance();
    dateDue.add(dateIssued.DATE, days);	 
  }

  public synchronized Calendar getDueDate() {
    return dateDue;
  }
}
{code}

This

...

class

...

does

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not

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commit

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to

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its

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locking

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strategy.

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That

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is,

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it

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reserves

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the

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right

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to

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change

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its

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locking

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strategy

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without

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notice.

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Furthermore,

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it

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does

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not

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document

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that

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callers

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can

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safely

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use

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client-side

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locking.

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The

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client

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class

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BookWrapper

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uses

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client-side

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locking

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in

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the

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renew()

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method

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by

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synchronizing

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on

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a

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Book

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instance.

{:=
Code Block
bgColor
#FFCCCC
}
// Client
public class BookWrapper {
  private final Book book;

  BookWrapper(Book book) {
    this.book = book;
  }

  public void issue(int days) {
    book.issue();
  }

  public Calendar getDueDate() {
    return book.getDueDate();
  }

  public void renew() {
    synchronized(book) {
      if (book.getDueDate().after(Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14); // Issue book for 14 days
      }
    }
  }
}
{code}

If

...

class

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Book

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changes

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its

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synchronization

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policy

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in

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the

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future,

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the

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BookWrapper

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class's

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locking

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strategy

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might

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silently

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break.

...

For

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instance,

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the

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Bookwrapper

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class's

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locking

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strategy

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will

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definitely

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break

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if

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Book

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is

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modified

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to

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use

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a

...

private

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final

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lock

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object,

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as

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recommended

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by

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CON04-J.

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Use

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private

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final

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lock

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objects

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to

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synchronize

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classes

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that

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may

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interact

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with

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untrusted

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code

...

.

...

This

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is

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because

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threads

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that

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call

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BookWrapper.getDueDate()

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may

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perform

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operations

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on

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the

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thread-safe

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Book

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using

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its

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new

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locking

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policy

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. However,

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threads

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that

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call

...

method

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renew()

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will

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always

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synchronize

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on

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the

...

intrinsic

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lock

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of

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the

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Book

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instance.

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Consequently,

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the

...

implementation

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will

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use

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two

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different

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locks.

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Compliant Solution (private

...

final

...

lock

...

object)

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This

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compliant

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solution

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uses

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a

...

private

...

final

...

lock

...

object

...

and

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synchronizes

...

all

...

its

...

methods

...

using

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this

...

lock.

{:=
Code Block
bgColor
#ccccff
}
public final class BookWrapper {
  private final Book book;
  private final Object lock = new Object();

  BookWrapper(Book book) {
    this.book = book;
  }

  public void issue(int days) {
    synchronized(lock) {
      book.issue();
    }
  }

  public Calendar getDueDate() {
    synchronized(lock) {
      return book.getDueDate();
    }
  }

  public void renew() {
    synchronized(lock) {
      if (book.getDueDate().after(Calendar.getInstance())) {
        throw new IllegalStateException("Book overdue");
      } else {
        book.issue(14); // Issue book for 14 days
      }
    }
  }
}
{code}

Wiki Markup
Consequently, the {{BookWrapper}} class's locking strategy is independent of the locking policy of the {{Book}} instance. This solution incurs a very small performance penalty but the resulting code is much more robust \[[Goetz 06|AA. Java References#Goetz 06]\].

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Noncompliant Code Example (class

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extension

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and

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accessible

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member

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lock)

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Wiki Markup
Goetz et al. describe the 

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fragility of class extension for adding functionality to thread-safe classes \[[Goetz 06|AA. Java References#Goetz 06]\]:

Extension is more fragile than adding code directly to a class, because the implementation of the synchronization policy is now distributed over multiple, separately maintained source files. If the underlying class were to change its synchronization policy by choosing a different lock to guard its state variables, the subclass would subtly and silently break, because it no longer used the right lock to control concurrent access to the base class's state.

In this noncompliant code example, the class PrintableIPAddressList extends the thread-safe class IPAddressList. It locks on IpAddressList's member ips in the method addAndPrintIPAddresses(). This is another example of client-side locking, because a subclass uses an object owned and locked by its superclass.

Code Block
bgColor#FFCCCC


{quote}
Extension is more fragile than adding code directly to a class, because the implementation of the synchronization policy is now distributed over multiple, separately maintained source files. If the underlying class were to change its synchronization policy by choosing a different lock to guard its state variables, the subclass would subtly and silently break, because it no longer used the right lock to control concurrent access to the base class's state.
{quote}

In this noncompliant code example, the class {{PrintableIPAddressList}} extends the thread-safe class {{IPAddressList}}. It locks on {{IpAddressList}}'s member {{ips}} in the method {{addAndPrintIPAddresses()}}. This is another example of client-side locking, because a subclass uses an object owned and locked by its superclass.

{code:bgColor=#FFCCCC}
// This class may change its locking policy in the future, for example, 
// when new non-atomic methods are added
class IPAddressList {
  private final List<InetAddress> ips = Collections.synchronizedList(new ArrayList<InetAddress>());
  
  public List<InetAddress> getList() {
    return ips; // No defensive copies required as package-private visibility
  }

  public void addIPAddress(InetAddress address) {
    ips.add(address);
  }
}

class PrintableIPAddressList extends IPAddressList {
  public void addAndPrintIPAddresses(InetAddress address) {
    synchronized(getList()) {
      addIPAddress(address);
      InetAddress[] ia = (InetAddress[]) getList().toArray(new InetAddress[0]);      
      // ...
    }
  }
}
{code}

Wiki Markup
If the class {{IPAddressList}} is modified to use block synchronization on a private final lock object, as recommended by [CON04-J. Use private final lock objects to synchronize classes that may interact with untrusted code], the subclass {{PrintableIPAddressList}} will silently break. Moreover, when a wrapper such as {{Collections.synchronizedList()}} is used, it is unwieldy for a client to determine the type of the class ({{List}}) that is being wrapped to extend it \[[Goetz 06|AA. Java References#Goetz 06]\]. 

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Compliant Solution (Composition)

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Composition

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offers

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encapsulation

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benefits

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at

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the

...

cost

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of

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performance.

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The

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performance

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impact

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is

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usually

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minimal.

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Refer

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to

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OBJ07-J.

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Understand

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how

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a

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superclass

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can

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affect

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a

...

subclass

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for

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more

...

information

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on

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implementing

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composition.

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This

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compliant

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solution

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wraps

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an

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object

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of

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class

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IPAddressList

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and

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provides

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synchronized

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accessors

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that

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can

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be

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used

...

to

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manipulate

...

the

...

state

...

of

...

the

...

object.

{:=
Code Block
bgColor
#ccccff
}
// Class IPAddressList remains unchanged
class PrintableIPAddressList {
  private final IPAddressList ips;
 
  public PrintableIPAddressList(IPAddressList list) {
    this.ips = list;
  }
  
  public synchronized void addIPAddress(InetAddress address) {
    ips.addIPAddress(address);
  }

  public synchronized void addAndPrintIPAddresses(InetAddress address) {  
    addIPAddress(address); 
    InetAddress[] ia = (InetAddress[]) ips.getList().toArray(new InetAddress[0]);     
    // ...
  }
}
{code}

Wiki Markup
This approach allows the {{CompositeCollection}} class to use its own intrinsic lock in a way that is completely independent of the lock of the underlying list class.  This allows the underlying collection to be not thread-safe because the {{CompositeCollection}} wrapper prevents direct access to its methods by publishing its own synchronized equivalents. This approach provides consistent locking even if the underlying class changes its locking policy in the future. \[[Goetz 06|AA. Java References#Goetz 06]\]

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Risk Assessment

Using client-side

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locking

...

when

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the

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thread-safe

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class

...

does

...

not

...

commit

...

to

...

its

...

locking

...

strategy

...

can

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cause

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data

...

inconsistencies

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and

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deadlock.

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Rule

Severity

Likelihood

Remediation Cost

Priority

Level

CON31- J

low

probable

medium

P4

L3

Automated Detection

TODO

Related Vulnerabilities

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

References

Wiki Markup
\[[API 06|AA. Java References#API 06]\] Class Vector, Class WeakReference, Class ConcurrentHashMap<K,V>
\[[JavaThreads 04|AA. Java References#JavaThreads 04]\] 8.2 "Synchronization and Collection Classes"
\[[Goetz 06|AA. Java References#Goetz 06]\] 4.4.1. Client-side Locking, 4.4.2. Composition and 5.2.1. ConcurrentHashMap
\[[Lee 09|AA. Java References#Lee 09]\] "Map & Compound Operation"

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...

VOID CON06-J. Do not defer a thread that is holding a lock      11. Concurrency (CON)      Image Added