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An exceptional condition can circumvent the release of a lock, leading to deadlock. According to the Java API \[[API

...

2006|AA. Java References#API 06]\]:

A ReentrantLock is owned by the thread last successfully locking, but not yet unlocking it. A thread invoking lock will return, successfully acquiring the lock, when the lock is not owned by another thread.

Consequently, an unreleased lock in any thread will stop other threads from acquiring the same lock. Intrinsic locks of class objects used for method and block synchronization are automatically released on exceptional conditions (such as abnormal thread termination).

Noncompliant Code Example (Checked Exception)

This noncompliant code example protects a resource using a ReentrantLock but fails to release the lock if an exception occurs while performing operations on the open file. If an exception is thrown, control transfers to the catch block and the call to unlock() is not executed.

Code Block

bgColor	#FFcccc



{quote}
A {{ReentrantLock}} is owned by the thread last successfully locking, but not yet unlocking it. A thread invoking {{lock}} will return, successfully acquiring the lock, when the lock is not owned by another thread.
{quote}

Consequently, an unreleased lock in any thread will stop other threads from acquiring the same lock. Intrinsic locks of class objects used for method and block synchronization are automatically released on exceptional conditions (such as abnormal thread termination).

h2. Noncompliant Code Example (Checked Exception)

This noncompliant code example protects a resource using a {{ReentrantLock}} but fails to release the lock if an exception occurs while performing operations on the open file. If an exception is thrown, control transfers to the {{catch}} block and the call to {{unlock()}} is not executed.

{code:bgColor=#FFcccc}
public final class Client {
  public void doSomething(File file) {
    final Lock lock = new ReentrantLock();
    try {
      lock.lock();
      InputStream in = new FileInputStream(file);
      // Perform operations on the open file
      lock.unlock();
    } catch (FileNotFoundException fnf) {
      // Handle the exception
    }
  }
}
{code}

Note

...

that

...

the

...

lock

...

is

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not

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released,

...

even

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when

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the

...

doSomething()

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method

...

returns.

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This

...

noncompliant

...

code

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example

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does

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not

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close

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the

...

input

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stream

...

and

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, consequently,

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also

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violates

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guideline FIO06-J.

...

...

...

...

...

...

...

...

...

...

...

...

Compliant Solution (`finally` block)

This compliant solution encapsulates operations that may throw an exception in a try block immediately after acquiring the lock. The lock is acquired just before the try block, which guarantees that it is held when the finally block executes. Invoking Lock.unlock() in the finally block ensures that the lock is released, regardless of whether or not an exception occurs.

Code Block

bgColor	#ccccff

|FIO06-J. Ensure all resources are properly closed when they are no longer needed].


h2. Compliant Solution ({{finally}} block)

This compliant solution encapsulates operations that may throw an exception in a {{try}} block immediately after acquiring the lock.  The lock is acquired just before the try block, which guarantees that it is held when the finally block executes. The lock is released in the corresponding {{finally}} block, which ensures the lock is released regardless of whether or not an exception occurs.

{code:bgColor=#ccccff}
public final class Client {
  public void doSomething(File file) {
    final Lock lock = new ReentrantLock();
    InputStream in = null;
    lock.lock();
    try {
      in = new FileInputStream(file);
      // Perform operations on the open file
    } catch (FileNotFoundException fnf) {
      // Forward to handler
    } finally {
      lock.unlock();

      if (in != null) {
        try {
          in.close();
        } catch (IOException e) {
           // Forward to handler
        }
      }
    }
  }
}
{code}

h2. Compliant Solution

Compliant Solution (Execute-around

...

Idiom)

...

The

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execute-around

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idiom

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provides

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a

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generic

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mechanism

...

to

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perform

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resource

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allocation

...

and

...

clean-up

...

operations

...

so

...

that

...

the

...

client

...

can

...

focus

...

on

...

specifying

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only

...

the

...

required

...

functionality.

...

This

...

idiom

...

reduces

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clutter

...

in

...

client

...

code

...

and

...

provides

...

a

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secure

...

mechanism

...

for

...

resource

...

management.

...

In

...

this

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compliant

...

solution,

...

the

...

client's

...

doSomething()

...

method

...

provides

...

only

...

the

...

required

...

functionality

...

by

...

implementing

...

the

...

doSomethingWithFile()

...

method

...

of

...

the

...

LockAction

...

interface,

...

without

...

having

...

to

...

manage

...

the acquisition

...

and

...

release

...

of

...

locks

...

or

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the open

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and

...

close

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operations

...

of

...

files.

...

The

...

ReentrantLockAction

...

class

...

encapsulates

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all

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resource

...

management

...

actions.

{:=

Code Block

bgColor

	#ccccff

}
public interface LockAction {
  void doSomethingWithFile(InputStream in);
}

public final class ReentrantLockAction {
  public static void doSomething(File file, LockAction action)  {
    Lock lock = new ReentrantLock();
    InputStream in = null;
    lock.lock();
    try {
      in = new FileInputStream(file);
      action.doSomethingWithFile(in);
    } catch (FileNotFoundException fnf) {
      // Forward to handler
    } finally {
      lock.unlock();

      if (in != null) {
        try {
          in.close();
        } catch (IOException e) {
          // Forward to handler
        }
      }
    }
  }
}

public final class Client {
  public void doSomething(File file) {
    ReentrantLockAction.doSomething(file, new LockAction() {
      public void doSomethingWithFile(InputStream in) {
        // Perform operations on the open file
      }
    });
  }
}
{code}

h2. Noncompliant Code Example

Noncompliant Code Example (unchecked

...

exception)

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This

...

noncompliant

...

code

...

example

...

uses

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a

...

ReentrantLock

...

to

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protect

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a

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java.util.Date

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instance,

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which

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is

...

not

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thread-safe

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by

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design.

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The

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doSomethingSafely()

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method

...

must

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catch

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Throwable

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to

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comply

...

with

...

guideline EXC06-J.

...

...

...

...

...

...

...

...

Code Block

bgColor	#FFcccc

|EXC06-J. Do not allow exceptions to transmit sensitive information].

{mc} Do not declare lock as private as it need package-wide accessibility for illustrative purposes {mc}

{code:bgColor=#FFcccc}
final class DateHandler {
  private final Date date = new Date();
  final Lock lock = new ReentrantLock();

  public void doSomethingSafely(String str) {
    try {
      doSomething(str);
    } catch(Throwable t) {
      // Forward to handler
    }
  }

  public void doSomething(String str) {
    lock.lock();
    String dateString = date.toString();
    if (str.equals(dateString)) {
      // ...
    }
    lock.unlock();
  }
}
{code}

Because

...

the

...

doSomething()

...

method

...

fails

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to

...

check

...

if

...

str

...

is

...

null

...

,

...

a

...

runtime

...

exception

...

can

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occur,

...

preventing

...

the

...

lock

...

from

...

being

...

released.

Compliant Solution (`finally` block)

This compliant solution encapsulates all operations that can throw an exception in a try block and release the lock in the associated finally block.

Code Block

bgColor	#ccccff




h2. Compliant Solution ({{finally}} block)

This compliant solution encapsulates all operations that can throw an exception in a try block and release the lock in the associated {{finally}} block.

{code:bgColor=#ccccff}
final class DateHandler {
  private final Date date = new Date();
  final Lock lock = new ReentrantLock();

  public void doSomethingSafely(String str) {
    try {
      doSomething(str);
    } catch(Throwable t) {
      // Forward to handler
    }
  }

  public void doSomething(String str) {
    lock.lock();
    try {
      String dateString = date.toString();
      if (str != null && str.equals(dateString)) {
        // ...
      }
    } finally {
      lock.unlock();
    }
  }
}
{code}

Consequently,

...

the

...

lock

...

is

...

released

...

even

...