Java Doc for Lock.java in  » Library » Apache-beehive-1.0.2-src » org » apache » beehive » netui » util » internal » concurrent » Java Source Code / Java DocumentationJava Source Code and Java Documentation

Java Source Code / Java Documentation
1. 6.0 JDK Core
2. 6.0 JDK Modules
3. 6.0 JDK Modules com.sun
4. 6.0 JDK Modules com.sun.java
5. 6.0 JDK Modules sun
6. 6.0 JDK Platform
7. Ajax
8. Apache Harmony Java SE
9. Aspect oriented
10. Authentication Authorization
11. Blogger System
12. Build
13. Byte Code
14. Cache
15. Chart
16. Chat
17. Code Analyzer
18. Collaboration
19. Content Management System
20. Database Client
21. Database DBMS
22. Database JDBC Connection Pool
23. Database ORM
24. Development
25. EJB Server geronimo
26. EJB Server GlassFish
27. EJB Server JBoss 4.2.1
28. EJB Server resin 3.1.5
29. ERP CRM Financial
30. ESB
31. Forum
32. GIS
33. Graphic Library
34. Groupware
35. HTML Parser
36. IDE
37. IDE Eclipse
38. IDE Netbeans
39. Installer
40. Internationalization Localization
41. Inversion of Control
42. Issue Tracking
43. J2EE
44. JBoss
45. JMS
46. JMX
47. Library
48. Mail Clients
49. Net
50. Parser
51. PDF
52. Portal
53. Profiler
54. Project Management
55. Report
56. RSS RDF
57. Rule Engine
58. Science
59. Scripting
60. Search Engine
61. Security
62. Sevlet Container
63. Source Control
64. Swing Library
65. Template Engine
66. Test Coverage
67. Testing
68. UML
69. Web Crawler
70. Web Framework
71. Web Mail
72. Web Server
73. Web Services
74. Web Services apache cxf 2.0.1
75. Web Services AXIS2
76. Wiki Engine
77. Workflow Engines
78. XML
79. XML UI
Java
Java Tutorial
Java Open Source
Jar File Download
Java Articles
Java Products
Java by API
Photoshop Tutorials
Maya Tutorials
Flash Tutorials
3ds-Max Tutorials
Illustrator Tutorials
GIMP Tutorials
C# / C Sharp
C# / CSharp Tutorial
C# / CSharp Open Source
ASP.Net
ASP.NET Tutorial
JavaScript DHTML
JavaScript Tutorial
JavaScript Reference
HTML / CSS
HTML CSS Reference
C / ANSI-C
C Tutorial
C++
C++ Tutorial
Ruby
PHP
Python
Python Tutorial
Python Open Source
SQL Server / T-SQL
SQL Server / T-SQL Tutorial
Oracle PL / SQL
Oracle PL/SQL Tutorial
PostgreSQL
SQL / MySQL
MySQL Tutorial
VB.Net
VB.Net Tutorial
Flash / Flex / ActionScript
VBA / Excel / Access / Word
XML
XML Tutorial
Microsoft Office PowerPoint 2007 Tutorial
Microsoft Office Excel 2007 Tutorial
Microsoft Office Word 2007 Tutorial
Java Source Code / Java Documentation » Library » Apache beehive 1.0.2 src » org.apache.beehive.netui.util.internal.concurrent 
Source Cross Reference  Class Diagram Java Document (Java Doc) 


org.apache.beehive.netui.util.internal.concurrent.Lock

All known Subclasses:   org.apache.beehive.netui.util.internal.concurrent.ReentrantLock,
Lock
interface Lock (Code)
Lock implementations provide more extensive locking operations than can be obtained using synchronized methods and statements. They allow more flexible structuring, may have quite different properties, and may support multiple associated Condition objects.

A lock is a tool for controlling access to a shared resource by multiple threads. Commonly, a lock provides exclusive access to a shared resource: only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first. However, some locks may allow concurrent access to a shared resource, such as the read lock of a ReadWriteLock .

The use of synchronized methods or statements provides access to the implicit monitor lock associated with every object, but forces all lock acquisition and release to occur in a block-structured way: when multiple locks are acquired they must be released in the opposite order, and all locks must be released in the same lexical scope in which they were acquired.

While the scoping mechanism for synchronized methods and statements makes it much easier to program with monitor locks, and helps avoid many common programming errors involving locks, there are occasions where you need to work with locks in a more flexible way. For example, some algorithms for traversing concurrently accessed data structures require the use of "hand-over-hand" or "chain locking": you acquire the lock of node A, then node B, then release A and acquire C, then release B and acquire D and so on. Implementations of the Lock interface enable the use of such techniques by allowing a lock to be acquired and released in different scopes, and allowing multiple locks to be acquired and released in any order.

With this increased flexibility comes additional responsibility. The absence of block-structured locking removes the automatic release of locks that occurs with synchronized methods and statements. In most cases, the following idiom should be used:

     Lock l = ...;
 l.lock();
 try {
 // access the resource protected by this lock
 } finally {
 l.unlock();
 }
 
When locking and unlocking occur in different scopes, care must be taken to ensure that all code that is executed while the lock is held is protected by try-finally or try-catch to ensure that the lock is released when necessary.

Lock implementations provide additional functionality over the use of synchronized methods and statements by providing a non-blocking attempt to acquire a lock ( Lock.tryLock() ), an attempt to acquire the lock that can be interrupted ( Lock.lockInterruptibly , and an attempt to acquire the lock that can timeout ( Lock.tryLock(long,TimeUnit) ).

A Lock class can also provide behavior and semantics that is quite different from that of the implicit monitor lock, such as guaranteed ordering, non-reentrant usage, or deadlock detection. If an implementation provides such specialized semantics then the implementation must document those semantics.

Note that Lock instances are just normal objects and can themselves be used as the target in a synchronized statement. Acquiring the monitor lock of a Lock instance has no specified relationship with invoking any of the Lock.lock methods of that instance. It is recommended that to avoid confusion you never use Lock instances in this way, except within their own implementation.

Except where noted, passing a null value for any parameter will result in a NullPointerException being thrown.

Memory Synchronization

All Lock implementations must enforce the same memory synchronization semantics as provided by the built-in monitor lock:

  • A successful lock operation acts like a successful monitorEnter action
  • A successful unlock operation acts like a successful monitorExit action
Unsuccessful locking and unlocking operations, and reentrant locking/unlocking operations, do not require any memory synchronization effects.

Implementation Considerations

The three forms of lock acquisition (interruptible, non-interruptible, and timed) may differ in their performance characteristics, ordering guarantees, or other implementation qualities. Further, the ability to interrupt the ongoing acquisition of a lock may not be available in a given Lock class. Consequently, an implementation is not required to define exactly the same guarantees or semantics for all three forms of lock acquisition, nor is it required to support interruption of an ongoing lock acquisition. An implementation is required to clearly document the semantics and guarantees provided by each of the locking methods. It must also obey the interruption semantics as defined in this interface, to the extent that interruption of lock acquisition is supported: which is either totally, or only on method entry.

As interruption generally implies cancellation, and checks for interruption are often infrequent, an implementation can favor responding to an interrupt over normal method return. This is true even if it can be shown that the interrupt occurred after another action may have unblocked the thread. An implementation should document this behavior.
See Also:   ReentrantLock
See Also:   Condition
See Also:   ReadWriteLock
since:
   1.5
author:
   Doug Lea





Method Summary
 voidlock()
     Acquires the lock.
 voidlockInterruptibly()
     Acquires the lock unless the current thread is Thread.interrupt interrupted .

Acquires the lock if it is available and returns immediately.

If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:

  • The lock is acquired by the current thread; or
  • Some other thread Thread.interrupt interrupts the current thread, and interruption of lock acquisition is supported.

If the current thread:

  • has its interrupted status set on entry to this method; or
  • is Thread.interrupt interrupted while acquiring the lock, and interruption of lock acquisition is supported,
then InterruptedException is thrown and the current thread's interrupted status is cleared.

Implementation Considerations

The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation.

 ConditionnewCondition()
     Returns a new Condition instance that is bound to this Lock instance.
 booleantryLock()
     Acquires the lock only if it is free at the time of invocation.
 booleantryLock(long time, TimeUnit unit)
     Acquires the lock if it is free within the given waiting time and the current thread has not been Thread.interrupt interrupted .

If the lock is available this method returns immediately with the value true. If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

  • The lock is acquired by the current thread; or
  • Some other thread Thread.interrupt interrupts the current thread, and interruption of lock acquisition is supported; or
  • The specified waiting time elapses

If the lock is acquired then the value true is returned.

If the current thread:

  • has its interrupted status set on entry to this method; or
  • is Thread.interrupt interrupted while acquiring the lock, and interruption of lock acquisition is supported,
then InterruptedException is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all.

Implementation Considerations

The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case.

 voidunlock()
     Releases the lock.



Method Detail
lock
void lock()(Code)
Acquires the lock.

If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until the lock has been acquired.

Implementation Considerations

A Lock implementation may be able to detect erroneous use of the lock, such as an invocation that would cause deadlock, and may throw an (unchecked) exception in such circumstances. The circumstances and the exception type must be documented by that Lock implementation.




lockInterruptibly
void lockInterruptibly() throws InterruptedException(Code)
Acquires the lock unless the current thread is Thread.interrupt interrupted .

Acquires the lock if it is available and returns immediately.

If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:

  • The lock is acquired by the current thread; or
  • Some other thread Thread.interrupt interrupts the current thread, and interruption of lock acquisition is supported.

If the current thread:

  • has its interrupted status set on entry to this method; or
  • is Thread.interrupt interrupted while acquiring the lock, and interruption of lock acquisition is supported,
then InterruptedException is thrown and the current thread's interrupted status is cleared.

Implementation Considerations

The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case. An implementation should document when this is the case.

An implementation can favor responding to an interrupt over normal method return.

A Lock implementation may be able to detect erroneous use of the lock, such as an invocation that would cause deadlock, and may throw an (unchecked) exception in such circumstances. The circumstances and the exception type must be documented by that Lock implementation.
throws:
  InterruptedException - if the current thread is interruptedwhile acquiring the lock (and interruption of lock acquisition issupported).
See Also:   Thread.interrupt




newCondition
Condition newCondition()(Code)
Returns a new Condition instance that is bound to this Lock instance.

Before waiting on the condition the lock must be held by the current thread. A call to Condition.await will atomically release the lock before waiting and re-acquire the lock before the wait returns.

Implementation Considerations

The exact operation of the Condition instance depends on the Lock implementation and must be documented by that implementation. A new Condition instance for this Lockinstance.
throws:
  UnsupportedOperationException - if this Lockimplementation does not support conditions.




tryLock
boolean tryLock()(Code)
Acquires the lock only if it is free at the time of invocation.

Acquires the lock if it is available and returns immediately with the value true. If the lock is not available then this method will return immediately with the value false.

A typical usage idiom for this method would be:

 Lock lock = ...;
 if (lock.tryLock()) {
 try {
 // manipulate protected state
 } finally {
 lock.unlock();
 }
 } else {
 // perform alternative actions
 }
 
This usage ensures that the lock is unlocked if it was acquired, and doesn't try to unlock if the lock was not acquired. true if the lock was acquired and falseotherwise.



tryLock
boolean tryLock(long time, TimeUnit unit) throws InterruptedException(Code)
Acquires the lock if it is free within the given waiting time and the current thread has not been Thread.interrupt interrupted .

If the lock is available this method returns immediately with the value true. If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

  • The lock is acquired by the current thread; or
  • Some other thread Thread.interrupt interrupts the current thread, and interruption of lock acquisition is supported; or
  • The specified waiting time elapses

If the lock is acquired then the value true is returned.

If the current thread:

  • has its interrupted status set on entry to this method; or
  • is Thread.interrupt interrupted while acquiring the lock, and interruption of lock acquisition is supported,
then InterruptedException is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all.

Implementation Considerations

The ability to interrupt a lock acquisition in some implementations may not be possible, and if possible may be an expensive operation. The programmer should be aware that this may be the case. An implementation should document when this is the case.

An implementation can favor responding to an interrupt over normal method return, or reporting a timeout.

A Lock implementation may be able to detect erroneous use of the lock, such as an invocation that would cause deadlock, and may throw an (unchecked) exception in such circumstances. The circumstances and the exception type must be documented by that Lock implementation.
Parameters:
  time - the maximum time to wait for the lock
Parameters:
  unit - the time unit of the time argument. true if the lock was acquired and falseif the waiting time elapsed before the lock was acquired.
throws:
  InterruptedException - if the current thread is interruptedwhile acquiring the lock (and interruption of lock acquisition issupported).
See Also:   Thread.interrupt




unlock
void unlock()(Code)
Releases the lock.

Implementation Considerations

A Lock implementation will usually impose restrictions on which thread can release a lock (typically only the holder of the lock can release it) and may throw an (unchecked) exception if the restriction is violated. Any restrictions and the exception type must be documented by that Lock implementation.




www.java2java.com | Contact Us
Copyright 2009 - 12 Demo Source and Support. All rights reserved.
All other trademarks are property of their respective owners.