ThreadMXBean.java

/* ThreadMXBean.java - Interface for a thread bean
   Copyright (C) 2006 Free Software Foundation

This file is part of GNU Classpath.

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package java.lang.management;

/**
 * <p>
 * Provides access to information about the threads 
 * of the virtual machine.  An instance of this bean is
 * obtained by calling
 * {@link ManagementFactory#getThreadMXBean()}.
 * </p>
 * <p>
 * Each thread within the virtual machine is given an
 * identifier, which is guaranteed to be unique to a
 * particular thread over its lifetime (after which it
 * may be reused). The identifier for a thread may be
 * obtained by calling {@link java.lang.Thread#getId()}.
 * This identifier is used within implementations of this
 * interface to obtain information about a particular thread
 * (or series of threads, in the case of an array of identifiers).
 * </p>
 * <p>
 * This bean supports some optional behaviour, which all
 * virtual machines may not choose to implement.  Specifically,
 * this includes the monitoring of the CPU time used by a
 * thread, and the monitoring of thread contention.  The former
 * is further subdivided into the monitoring of either just
 * the current thread or all threads.  The methods
 * {@link #isThreadCpuTimeSupported()},
 * {@link #isCurrentThreadCpuTimeSupported()} and
 * {@link #isThreadContentionMonitoringSupported()} may be
 * used to determine whether or not this functionality is
 * supported.
 * </p>
 * <p>
 * Furthermore, both these facilities may be disabled.
 * In fact, thread contention monitoring is disabled by
 * default, and must be explictly turned on by calling
 * the {@link #setThreadContentionMonitoringEnabled(boolean)}
 * method.
 * </p>
 *
 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
 * @since 1.5
 */
public interface ThreadMXBean
{

  /**
   * <p>
   * This method obtains a list of threads which are deadlocked
   * waiting to obtain monitor ownership.  On entering a synchronized
   * method of an object, or re-entering it after returning from an
   * {@link java.lang.Object#wait()} call, a thread obtains ownership
   * of the object's monitor.  
   * </p>
   * <p>
   * Deadlocks can occur in this situation if one or more threads end up
   * waiting for a monitor, P, while also retaining ownership of a monitor,
   * Q, required by the thread that currently owns P.  To give a simple
   * example, imagine thread A calls a synchronized method, R, obtaining the
   * monitor, P.  It then sleeps within that method, allowing thread B
   * to run, but still retaining ownership of P.  B calls another
   * synchronized method, S, which causes it to obtain the monitor, Q,
   * of a different object.  While in that method, it then wants to
   * call the original synchronized method, R,  called by A.  Doing so
   * requires ownership of P, which is still held by A.  Hence, it
   * becomes blocked.  
   * </p>
   * <p>
   * A then finishes its sleep, becomes runnable, and is then allowed
   * to run, being the only eligible thread in this scenario.  A tries
   * to call the synchronized method, S.  It also gets blocked, because
   * B still holds the monitor, Q.  Hence, the two threads, A and B,
   * are deadlocked, as neither can give up its monitor without first
   * obtaining the monitor held by the other thread.
   * </p>
   * <p>
   * Calling this method in this scenario would return the thread IDs
   * of A and B.  Note that this method is not designed for controlling
   * synchronization, but for troubleshooting problems which cause such
   * deadlocks; it may be prohibitively expensive to use in normal
   * operation.
   * </p>
   * 
   * @return an array of thread identifiers, corresponding to threads
   *         which are currently in a deadlocked situation.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("monitor").
   */
  long[] findMonitorDeadlockedThreads();

  /**
   * Returns all live thread identifiers at the time of initial
   * execution.  Some thread identifiers in the returned array
   * may refer to terminated threads, if this occurs during the
   * lifetime of this method.
   *
   * @return an array of thread identifiers, corresponding to
   *         current live threads.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("monitor").
   */
  long[] getAllThreadIds();

  /**
   * <p>
   * Returns the total number of nanoseconds of CPU time
   * the current thread has used.  This is equivalent to calling
   * <code>{@link #getThreadCpuTime()}(Thread.currentThread.getId())</code>.
   * </p> 
   * <p>
   * Note that the value is only nanosecond-precise, and not accurate; there
   * is no guarantee that the difference between two values is really a
   * nanosecond.  Also, the value is prone to overflow if the offset
   * exceeds 2^63.  The use of this method depends on virtual machine
   * support for measurement of the CPU time of the current thread,
   * and on this functionality being enabled.
   * </p>
   *
   * @return the total number of nanoseconds of CPU time the current
   *         thread has used, or -1 if CPU time monitoring is disabled.
   * @throws UnsupportedOperationException if CPU time monitoring is not
   *                                       supported.
   * @see #getCurrentThreadUserTime()
   * @see #isCurrentThreadCpuTimeSupported()
   * @see #isThreadCpuTimeEnabled()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  long getCurrentThreadCpuTime();

  /**
   * <p>
   * Returns the total number of nanoseconds of CPU time
   * the current thread has executed in user mode.  This is
   * equivalent to calling
   * <code>{@link #getThreadUserTime()}(Thread.currentThread.getId())</code>.
   * </p> 
   * <p>
   * Note that the value is only nanosecond-precise, and not accurate; there
   * is no guarantee that the difference between two values is really a
   * nanosecond.  Also, the value is prone to overflow if the offset
   * exceeds 2^63.  The use of this method depends on virtual machine
   * support for measurement of the CPU time of the current thread,
   * and on this functionality being enabled.
   * </p>
   *
   * @return the total number of nanoseconds of CPU time the current
   *         thread has executed in user mode, or -1 if CPU time
   *         monitoring is disabled.
   * @throws UnsupportedOperationException if CPU time monitoring is not
   *                                       supported.
   * @see #getCurrentThreadCpuTime()
   * @see #isCurrentThreadCpuTimeSupported()
   * @see #isThreadCpuTimeEnabled()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  long getCurrentThreadUserTime();

  /**
   * Returns the number of live daemon threads.
   *
   * @return the number of live daemon threads.
   */
  int getDaemonThreadCount();

  /**
   * Returns the peak number of live threads since
   * the virtual machine was started or the count
   * reset using {@link #resetPeakThreadCount()}.
   *
   * @return the peak live thread count.
   * @see #resetPeakThreadCount()
   */
  int getPeakThreadCount();

  /**
   * Returns the number of live threads, including
   * both daemon threads and non-daemon threads.
   *
   * @return the current number of live threads.
   */
  int getThreadCount();

  /**
   * <p>
   * Returns the total number of nanoseconds of CPU time
   * the specified thread has used.  
   * </p> 
   * <p>
   * Note that the value is only nanosecond-precise, and not accurate; there
   * is no guarantee that the difference between two values is really a
   * nanosecond.  Also, the value is prone to overflow if the offset
   * exceeds 2^63.  The use of this method depends on virtual machine
   * support for measurement of the CPU time of the current thread,
   * and on this functionality being enabled.
   * </p>
   *
   * @param id the thread identifier of the thread whose CPU time is being
   *           monitored.
   * @return the total number of nanoseconds of CPU time the specified
   *         thread has used, or -1 if CPU time monitoring is disabled.
   * @throws IllegalArgumentException if <code>id</code> <= 0.
   * @throws UnsupportedOperationException if CPU time monitoring is not
   *                                       supported.
   * @see #getThreadUserTime(long)
   * @see #isThreadCpuTimeSupported()
   * @see #isThreadCpuTimeEnabled()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  long getThreadCpuTime(long id);

  /**
   * Returns information on the specified thread without any
   * stack trace information.  This is equivalent to
   * <code>{@link #getThreadInfo}(id, 0)</code>.  If the
   * identifier specifies a thread which is either non-existant
   * or not alive, then the method returns <code>null</code>.
   * 
   * @param id the identifier of the thread to return information
   *           on.
   * @return a {@link ThreadInfo} object pertaining to the specified
   *         thread, or <code>null</code> if the identifier specifies
   *         a thread that doesn't exist or is not alive.
   * @throws IllegalArgumentException if <code>id</code> <= 0.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("monitor").
   */
  ThreadInfo getThreadInfo(long id);

  /**
   * Returns information on the specified threads without any
   * stack trace information.  This is equivalent to
   * <code>{@link #getThreadInfo}(ids, 0)</code>.  If an
   * identifier specifies a thread which is either non-existant
   * or not alive, then the corresponding element in the returned
   * array is <code>null</code>.
   * 
   * @param ids an array of thread identifiers to return information
   *           on.
   * @return an array of {@link ThreadInfo} objects matching the
   *         specified threads.  The corresponding element is 
   *         <code>null</code> if the identifier specifies
   *         a thread that doesn't exist or is not alive.
   * @throws IllegalArgumentException if an identifier in the array is
   *                                  <= 0.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("monitor").
   */
  ThreadInfo[] getThreadInfo(long[] ids);

  /**
   * Returns information on the specified thread with
   * stack trace information to the supplied depth.  If the
   * identifier specifies a thread which is either non-existant
   * or not alive, then the method returns <code>null</code>.
   * A maximum depth of 0 corresponds to an empty stack trace
   * (an empty array is returned by the appropriate
   * {@link ThreadInfo} method).  A maximum depth of
   * <code>Integer.MAX_VALUE</code> returns the full stack trace.
   *
   * @param id the identifier of the thread to return information
   *           on.
   * @param maxDepth the maximum depth of the stack trace.
   *                 Values of 0 or <code>Integer.MAX_VALUE</code>
   *                 correspond to an empty and full stack trace
   *                 respectively.
   * @return a {@link ThreadInfo} object pertaining to the specified
   *         thread, or <code>null</code> if the identifier specifies
   *         a thread that doesn't exist or is not alive.
   * @throws IllegalArgumentException if <code>id</code> <= 0.
   * @throws IllegalArgumentException if <code>maxDepth</code> < 0.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("monitor").
   */
  ThreadInfo getThreadInfo(long id, int maxDepth);

  /**
   * Returns information on the specified threads with
   * stack trace information to the supplied depth.  If an
   * identifier specifies a thread which is either non-existant
   * or not alive, then the corresponding element in the returned
   * array is <code>null</code>.  A maximum depth of 0 corresponds
   * to an empty stack trace (an empty array is returned by the
   * appropriate {@link ThreadInfo} method).  A maximum depth of
   * <code>Integer.MAX_VALUE</code> returns the full stack trace.
   * 
   * @param ids an array of thread identifiers to return information
   *           on.
   * @param maxDepth the maximum depth of the stack trace.
   *                 Values of 0 or <code>Integer.MAX_VALUE</code>
   *                 correspond to an empty and full stack trace
   *                 respectively.
   * @return an array of {@link ThreadInfo} objects matching the
   *         specified threads.  The corresponding element is 
   *         <code>null</code> if the identifier specifies
   *         a thread that doesn't exist or is not alive.
   * @throws IllegalArgumentException if an identifier in the array is
   *                                  <= 0.
   * @throws IllegalArgumentException if <code>maxDepth</code> < 0.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("monitor").
   */
  ThreadInfo[] getThreadInfo(long[] ids, int maxDepth);

  /**
   * <p>
   * Returns the total number of nanoseconds of CPU time
   * the specified thread has executed in user mode.  
   * </p> 
   * <p>
   * Note that the value is only nanosecond-precise, and not accurate; there
   * is no guarantee that the difference between two values is really a
   * nanosecond.  Also, the value is prone to overflow if the offset
   * exceeds 2^63.  The use of this method depends on virtual machine
   * support for measurement of the CPU time of the current thread,
   * and on this functionality being enabled.
   * </p>
   *
   * @param id the thread identifier of the thread whose CPU time is being
   *           monitored.
   * @return the total number of nanoseconds of CPU time the specified
   *         thread has executed in user mode, or -1 if CPU time monitoring
   *         is disabled.
   * @throws IllegalArgumentException if <code>id</code> <= 0.
   * @throws UnsupportedOperationException if CPU time monitoring is not
   *                                       supported.
   * @see #getThreadCpuTime(long)
   * @see #isThreadCpuTimeSupported()
   * @see #isThreadCpuTimeEnabled()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  long getThreadUserTime(long id);

  /**
   * Returns the total number of threads that have been
   * created and started during the lifetime of the virtual
   * machine.
   *
   * @return the total number of started threads.
   */
  long getTotalStartedThreadCount();

  /**
   * Returns true if the virtual machine supports the monitoring
   * of the CPU time used by the current thread.  This is implied
   * by {@link isThreadCpuTimeSupported()} returning true.
   *
   * @return true if monitoring of the CPU time used by the current
   *         thread is supported by the virtual machine.
   * @see #isThreadCpuTimeEnabled()
   * @see #isThreadCpuTimeSupported()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  boolean isCurrentThreadCpuTimeSupported();

  /**
   * Returns true if thread contention monitoring is currently
   * enabled.
   *
   * @return true if thread contention monitoring is enabled.
   * @throws UnsupportedOperationException if the virtual
   *                                       machine does not
   *                                       support contention
   *                                       monitoring.
   * @see #isThreadContentionMonitoringSupported()
   * @see #setThreadContentionMonitoringEnabled(boolean)
   */
  boolean isThreadContentionMonitoringEnabled();

  /**
   * Returns true if thread contention monitoring is supported
   * by the virtual machine.
   *
   * @return true if thread contention monitoring is supported
   *         by the virtual machine.
   * @see #isThreadContentionMonitoringEnabled()
   * @see #setThreadContentionMonitoringEnabled(boolean)
   */
  boolean isThreadContentionMonitoringSupported();

  /**
   * Returns true if monitoring of the CPU time used by a thread
   * is currently enabled.
   *
   * @return true if thread CPU time monitoring is enabled.
   * @throws UnsupportedOperationException if the virtual
   *                                       machine does not
   *                                       support CPU time
   *                                       monitoring.
   * @see #isCurrentThreadCpuTimeSupported()
   * @see #isThreadCpuTimeSupported()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  boolean isThreadCpuTimeEnabled();

  /**
   * Returns true if the virtual machine supports the monitoring
   * of the CPU time used by all threads.  This implies
   * that {@link isCurrentThreadCpuTimeSupported()} returns true.
   *
   * @return true if monitoring of the CPU time used by the current
   *         thread is supported by the virtual machine.
   * @see #isCurrentThreadCpuTimeSupported()
   * @see #isThreadCpuTimeEnabled()
   * @see #setThreadCpuTimeEnabled(boolean)
   */
  boolean isThreadCpuTimeSupported();

  /**
   * Resets the peak live thread count to the
   * current number of live threads, as returned
   * by {@link #getThreadCount()}.
   *
   * @see #getPeakThreadCount()
   * @see #getThreadCount()
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("control").
   */
  void resetPeakThreadCount();

  /**
   * Toggles the monitoring of thread contention.  Thread
   * contention monitoring is disabled by default.  Each
   * time contention monitoring is re-enabled, the times
   * it maintains are reset.
   *
   * @param enable true if monitoring should be enabled,
   *               false if it should be disabled.
   * @throws UnsupportedOperationException if the virtual
   *                                       machine does not
   *                                       support contention
   *                                       monitoring.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("control").
   * @see #isThreadContentionMonitoringEnabled()
   * @see #isThreadContentionMonitoringSupported()
   */
  void setThreadContentionMonitoringEnabled(boolean enable);

  /**
   * Toggles the monitoring of CPU time used by threads. The
   * initial setting is dependent on the underlying virtual
   * machine.  On enabling CPU time monitoring, the virtual
   * machine may take any value up to and including the current
   * time as the start time for monitoring.
   *
   * @param enable true if monitoring should be enabled,
   *               false if it should be disabled.
   * @throws UnsupportedOperationException if the virtual
   *                                       machine does not
   *                                       support CPU time
   *                                       monitoring.
   * @throws SecurityException if a security manager exists and
   *                           denies ManagementPermission("control").
   * @see #isCurrentThreadCpuTimeSupported()
   * @see #isThreadCpuTimeEnabled()
   * @see #isThreadCpuTimeSupported()
   */
  void setThreadCpuTimeEnabled(boolean enable);

}