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| <H2> |
| <FONT SIZE="-1"> |
| org.apache.openjpa.lib.util.concurrent</FONT> |
| <BR> |
| Interface Lock</H2> |
| <DL> |
| <DT><B>All Known Implementing Classes:</B> <DD><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/ReentrantLock.html" title="class in org.apache.openjpa.lib.util.concurrent">ReentrantLock</A></DD> |
| </DL> |
| <HR> |
| <DL> |
| <DT><PRE>public interface <B>Lock</B></DL> |
| </PRE> |
| |
| <P> |
| <tt>Lock</tt> implementations provide more extensive locking |
| operations than can be obtained using <tt>synchronized</tt> methods |
| and statements. They allow more flexible structuring, may have |
| quite different properties, and may support multiple associated |
| <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent"><CODE>Condition</CODE></A> 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 <CODE>ReadWriteLock</CODE>. |
| The use of <tt>synchronized</tt> 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 <tt>synchronized</tt> 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 |
| <tt>Lock</tt> 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 <tt>synchronized</tt> |
| methods and statements. In most cases, the following idiom should be used: |
| |
| <pre><tt> Lock l = ...; l.lock(); try { |
| // access the resource protected by this lock } finally { l.unlock(); } |
| </tt></pre> |
| 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. |
| |
| <tt>Lock</tt> implementations provide additional functionality |
| over the use of <tt>synchronized</tt> methods and statements by |
| providing a non-blocking attempt to acquire a lock(<A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#tryLock()"><CODE>tryLock()</CODE></A>), an attempt to acquire the lock that can be |
| interrupted(<A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#lockInterruptibly()"><CODE>lockInterruptibly()</CODE></A>, and an attempt to acquire |
| the lock that can timeout(<A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#tryLock(long, org.apache.openjpa.lib.util.concurrent.TimeUnit)"><CODE>tryLock(long, TimeUnit)</CODE></A>). |
| A <tt>Lock</tt> 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 <tt>Lock</tt> instances are just normal objects and can |
| themselves be used as the target in a <tt>synchronized</tt> statement. |
| Acquiring the |
| monitor lock of a <tt>Lock</tt> instance has no specified relationship |
| with invoking any of the <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#lock()"><CODE>lock()</CODE></A> methods of that instance. |
| It is recommended that to avoid confusion you never use <tt>Lock</tt> |
| instances in this way, except within their own implementation. |
| Except where noted, passing a <tt>null</tt> value for any |
| parameter will result in a <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/NullPointerException.html" title="class or interface in java.lang"><CODE>NullPointerException</CODE></A> being thrown. |
| |
| <h3>Memory Synchronization</h3> |
| All <tt>Lock</tt> implementations <em>must</em> enforce the same |
| memory synchronization semantics as provided by the built-in monitor |
| lock, as described in <a href="http://java.sun.com/docs/books/jls/"> |
| The Java Language Specification, Third Edition(17.4 Memory Model)</a>: |
| <ul> |
| <li>A successful <tt>lock</tt> operation has the same memory |
| synchronization effects as a successful <em>Lock</em> action. |
| <li>A successful <tt>unlock</tt> operation has the same |
| memory synchronization effects as a successful <em>Unlock</em> action. |
| </ul> Unsuccessful locking and unlocking operations, and reentrant |
| locking/unlocking operations, do not require any memory |
| synchronization effects. |
| |
| <h3>Implementation Considerations</h3> |
| 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 <em>ongoing</em> |
| acquisition of a lock may not be available in a given <tt>Lock</tt> |
| 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. |
| <P> |
| |
| <P> |
| <DL> |
| <DT><B>Since:</B></DT> |
| <DD>1.5</DD> |
| <DT><B>Author:</B></DT> |
| <DD>Doug Lea</DD> |
| <DT><B>See Also:</B><DD><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/ReentrantLock.html" title="class in org.apache.openjpa.lib.util.concurrent"><CODE>ReentrantLock</CODE></A>, |
| <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent"><CODE>Condition</CODE></A>, |
| <CODE>ReadWriteLock</CODE></DL> |
| <HR> |
| |
| <P> |
| |
| <!-- ========== METHOD SUMMARY =========== --> |
| |
| <A NAME="method_summary"><!-- --></A> |
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| <TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"> |
| <B>Method Summary</B></FONT></TH> |
| </TR> |
| <TR BGCOLOR="white" CLASS="TableRowColor"> |
| <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"> |
| <CODE> void</CODE></FONT></TD> |
| <TD><CODE><B><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#lock()">lock</A></B>()</CODE> |
| |
| <BR> |
| Acquires the lock.</TD> |
| </TR> |
| <TR BGCOLOR="white" CLASS="TableRowColor"> |
| <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"> |
| <CODE> void</CODE></FONT></TD> |
| <TD><CODE><B><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#lockInterruptibly()">lockInterruptibly</A></B>()</CODE> |
| |
| <BR> |
| Acquires the lock unless the current thread is |
| <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupted</CODE></A>.</TD> |
| </TR> |
| <TR BGCOLOR="white" CLASS="TableRowColor"> |
| <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"> |
| <CODE> <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent">Condition</A></CODE></FONT></TD> |
| <TD><CODE><B><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#newCondition()">newCondition</A></B>()</CODE> |
| |
| <BR> |
| Returns a new <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent"><CODE>Condition</CODE></A> instance that is bound to this |
| <tt>Lock</tt> instance.</TD> |
| </TR> |
| <TR BGCOLOR="white" CLASS="TableRowColor"> |
| <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"> |
| <CODE> boolean</CODE></FONT></TD> |
| <TD><CODE><B><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#tryLock()">tryLock</A></B>()</CODE> |
| |
| <BR> |
| Acquires the lock only if it is free at the time of invocation.</TD> |
| </TR> |
| <TR BGCOLOR="white" CLASS="TableRowColor"> |
| <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"> |
| <CODE> boolean</CODE></FONT></TD> |
| <TD><CODE><B><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#tryLock(long, org.apache.openjpa.lib.util.concurrent.TimeUnit)">tryLock</A></B>(long time, |
| <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/TimeUnit.html" title="class in org.apache.openjpa.lib.util.concurrent">TimeUnit</A> unit)</CODE> |
| |
| <BR> |
| Acquires the lock if it is free within the given waiting time and the |
| current thread has not been <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupted</CODE></A>.</TD> |
| </TR> |
| <TR BGCOLOR="white" CLASS="TableRowColor"> |
| <TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"> |
| <CODE> void</CODE></FONT></TD> |
| <TD><CODE><B><A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Lock.html#unlock()">unlock</A></B>()</CODE> |
| |
| <BR> |
| Releases the lock.</TD> |
| </TR> |
| </TABLE> |
| |
| <P> |
| |
| <!-- ============ METHOD DETAIL ========== --> |
| |
| <A NAME="method_detail"><!-- --></A> |
| <TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""> |
| <TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"> |
| <TH ALIGN="left" COLSPAN="1"><FONT SIZE="+2"> |
| <B>Method Detail</B></FONT></TH> |
| </TR> |
| </TABLE> |
| |
| <A NAME="lock()"><!-- --></A><H3> |
| lock</H3> |
| <PRE> |
| void <B>lock</B>()</PRE> |
| <DL> |
| <DD>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. |
| <b>Implementation Considerations</b> |
| A <tt>Lock</tt> 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 |
| <tt>Lock</tt> implementation. |
| <P> |
| <DD><DL> |
| </DL> |
| </DD> |
| </DL> |
| <HR> |
| |
| <A NAME="lockInterruptibly()"><!-- --></A><H3> |
| lockInterruptibly</H3> |
| <PRE> |
| void <B>lockInterruptibly</B>() |
| throws <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/InterruptedException.html" title="class or interface in java.lang">InterruptedException</A></PRE> |
| <DL> |
| <DD>Acquires the lock unless the current thread is |
| <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupted</CODE></A>. |
| 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: |
| <ul> |
| <li>The lock is acquired by the current thread; or |
| <li>Some other thread <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupts</CODE></A> the current |
| thread, and interruption of lock acquisition is supported. |
| </ul> If the current thread: |
| <ul> |
| <li>has its interrupted status set on entry to this method; or |
| <li>is <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupted</CODE></A> while acquiring |
| the lock, and interruption of lock acquisition is supported, |
| </ul> |
| then <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/InterruptedException.html" title="class or interface in java.lang"><CODE>InterruptedException</CODE></A> is thrown and the current thread's |
| interrupted status is cleared. |
| |
| <b>Implementation Considerations</b> |
| 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 <tt>Lock</tt> 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 <tt>Lock</tt> implementation. |
| <P> |
| <DD><DL> |
| |
| <DT><B>Throws:</B> |
| <DD><CODE><A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/InterruptedException.html" title="class or interface in java.lang">InterruptedException</A></CODE> - if the current thread is interrupted |
| while acquiring the lock(and interruption of lock acquisition is |
| supported).<DT><B>See Also:</B><DD><A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>Thread.interrupt()</CODE></A></DL> |
| </DD> |
| </DL> |
| <HR> |
| |
| <A NAME="tryLock()"><!-- --></A><H3> |
| tryLock</H3> |
| <PRE> |
| boolean <B>tryLock</B>()</PRE> |
| <DL> |
| <DD>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 <tt>true</tt>. |
| If the lock is not available then this method will return |
| immediately with the value <tt>false</tt>. |
| A typical usage idiom for this method would be: |
| <pre> Lock lock = ...; if (lock.tryLock()) { try { |
| // manipulate protected state } finally { lock.unlock(); } } else { |
| // perform alternative actions } |
| </pre> |
| This usage ensures that the lock is unlocked if it was acquired, and |
| doesn't try to unlock if the lock was not acquired. |
| <P> |
| <DD><DL> |
| |
| <DT><B>Returns:</B><DD><tt>true</tt> if the lock was acquired and <tt>false</tt> |
| otherwise.</DL> |
| </DD> |
| </DL> |
| <HR> |
| |
| <A NAME="tryLock(long, org.apache.openjpa.lib.util.concurrent.TimeUnit)"><!-- --></A><H3> |
| tryLock</H3> |
| <PRE> |
| boolean <B>tryLock</B>(long time, |
| <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/TimeUnit.html" title="class in org.apache.openjpa.lib.util.concurrent">TimeUnit</A> unit) |
| throws <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/InterruptedException.html" title="class or interface in java.lang">InterruptedException</A></PRE> |
| <DL> |
| <DD>Acquires the lock if it is free within the given waiting time and the |
| current thread has not been <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupted</CODE></A>. |
| If the lock is available this method returns immediately |
| with the value <tt>true</tt>. 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: |
| <ul> |
| <li>The lock is acquired by the current thread; or |
| <li>Some other thread <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupts</CODE></A> the current |
| thread, and interruption of lock acquisition is supported; or |
| <li>The specified waiting time elapses |
| </ul> If the lock is acquired then the value <tt>true</tt> is returned. |
| If the current thread: |
| <ul> |
| <li>has its interrupted status set on entry to this method; or |
| <li>is <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>interrupted</CODE></A> while acquiring |
| the lock, and interruption of lock acquisition is supported, |
| </ul> |
| then <A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/InterruptedException.html" title="class or interface in java.lang"><CODE>InterruptedException</CODE></A> is thrown and the current thread's |
| interrupted status is cleared. |
| If the specified waiting time elapses then the value <tt>false</tt> |
| is returned. If the time is |
| less than or equal to zero, the method will not wait at all. |
| |
| <b>Implementation Considerations</b> |
| 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 <tt>Lock</tt> 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 |
| <tt>Lock</tt> implementation. |
| <P> |
| <DD><DL> |
| <DT><B>Parameters:</B><DD><CODE>time</CODE> - the maximum time to wait for the lock<DD><CODE>unit</CODE> - the time unit of the <tt>time</tt> argument. |
| <DT><B>Returns:</B><DD><tt>true</tt> if the lock was acquired and <tt>false</tt> |
| if the waiting time elapsed before the lock was acquired. |
| <DT><B>Throws:</B> |
| <DD><CODE><A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/InterruptedException.html" title="class or interface in java.lang">InterruptedException</A></CODE> - if the current thread is interrupted |
| while acquiring the lock(and interruption of lock acquisition is |
| supported).<DT><B>See Also:</B><DD><A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Thread.html#interrupt()" title="class or interface in java.lang"><CODE>Thread.interrupt()</CODE></A></DL> |
| </DD> |
| </DL> |
| <HR> |
| |
| <A NAME="unlock()"><!-- --></A><H3> |
| unlock</H3> |
| <PRE> |
| void <B>unlock</B>()</PRE> |
| <DL> |
| <DD>Releases the lock. |
| <b>Implementation Considerations</b> |
| A <tt>Lock</tt> 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 <tt>Lock</tt> implementation. |
| <P> |
| <DD><DL> |
| </DL> |
| </DD> |
| </DL> |
| <HR> |
| |
| <A NAME="newCondition()"><!-- --></A><H3> |
| newCondition</H3> |
| <PRE> |
| <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent">Condition</A> <B>newCondition</B>()</PRE> |
| <DL> |
| <DD>Returns a new <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent"><CODE>Condition</CODE></A> instance that is bound to this |
| <tt>Lock</tt> instance. |
| Before waiting on the condition the lock must be held by the |
| current thread. |
| A call to <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html#await()"><CODE>Condition.await()</CODE></A> will atomically release the lock |
| before waiting and re-acquire the lock before the wait returns. |
| <b>Implementation Considerations</b> |
| The exact operation of the <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent"><CODE>Condition</CODE></A> instance depends on the |
| <tt>Lock</tt> implementation and must be documented by that |
| implementation. |
| <P> |
| <DD><DL> |
| |
| <DT><B>Returns:</B><DD>A new <A HREF="../../../../../../org/apache/openjpa/lib/util/concurrent/Condition.html" title="interface in org.apache.openjpa.lib.util.concurrent"><CODE>Condition</CODE></A> instance for this <tt>Lock</tt> instance. |
| <DT><B>Throws:</B> |
| <DD><CODE><A HREF="http://java.sun.com/j2se/1.5.0/docs/api/java/lang/UnsupportedOperationException.html" title="class or interface in java.lang">UnsupportedOperationException</A></CODE> - if this <tt>Lock</tt> |
| implementation does not support conditions.</DL> |
| </DD> |
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