Package java.lang

## Class Thread

- java.lang.Object
- - java.lang.Thread

- All Implemented Interfaces:
    `Runnable`

      public class Thread
      extends Object
      implements Runnable

  A thread is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently.

  Every thread has a priority. Threads with higher priority are executed in preference to threads with lower priority. Each thread may or may not also be marked as a daemon. When code running in some thread creates a new `Thread` object, the new thread has its priority initially set equal to the priority of the creating thread, and is a daemon thread if and only if the creating thread is a daemon.

  When a Java Virtual Machine starts up, there is usually a single non-daemon thread (which typically calls the method named `main` of some designated class). The Java Virtual Machine continues to execute threads until either of the following occurs:

  - The `exit` method of class `Runtime` has been called and the security manager has permitted the exit operation to take place.
  - All threads that are not daemon threads have died, either by returning from the call to the `run` method or by throwing an exception that propagates beyond the `run` method.

  There are two ways to create a new thread of execution. One is to declare a class to be a subclass of `Thread`. This subclass should override the `run` method of class `Thread`. An instance of the subclass can then be allocated and started. For example, a thread that computes primes larger than a stated value could be written as follows:

  >      class PrimeThread extends Thread {
  >             long minPrime;

  >             PrimeThread(long minPrime) {
  >                     this.minPrime = minPrime;
  >             }

  >             public void run() {
  >                  // compute primes larger than minPrime
  >                   . . .
  >              }
  >      }

  The following code would then create a thread and start it running:

  >      PrimeThread p = new PrimeThread(143);
  >      p.start();

  The other way to create a thread is to declare a class that implements the `Runnable` interface. That class then implements the `run` method. An instance of the class can then be allocated, passed as an argument when creating `Thread`, and started. The same example in this other style looks like the following:

  >      class PrimeRun implements Runnable {
  >             long minPrime;

  >             PrimeRun(long minPrime) {
  >                     this.minPrime = minPrime;
  >             }

  >             public void run() {
  >                  // compute primes larger than minPrime
  >                   . . .
  >              }
  >      }

  The following code would then create a thread and start it running:

  >      PrimeRun p = new PrimeRun(143);
  >      new Thread(p).start();

  Every thread has a name for identification purposes. More than one thread may have the same name. If a name is not specified when a thread is created, a new name is generated for it.

  Unless otherwise noted, passing a `null` argument to a constructor or method in this class will cause a `NullPointerException` to be thrown.

  See Also:
    `Runnable`, `Runtime.exit(int)`, `run()`

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class | Description
    --- | --- | ---
    `static class` | `Thread.State` | A thread state.
    `static interface` | `Thread.UncaughtExceptionHandler` | Interface for handlers invoked when a `Thread` abruptly terminates due to an uncaught exception.

  - ### Field Summary

    Fields
    Modifier and Type | Field | Description
    --- | --- | ---
    `static int` | `MAX_PRIORITY` | The maximum priority that a thread can have.
    `static int` | `MIN_PRIORITY` | The minimum priority that a thread can have.
    `static int` | `NORM_PRIORITY` | The default priority that is assigned to a thread.

  - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `Thread()` | Allocates a new `Thread` object.
    `Thread​(Runnable target)` | Allocates a new Thread object with a specific target object whose run method is called.
    `Thread​(Runnable target, String name)` | Allocates a new `Thread` object.
    `Thread​(String name)` | Allocates a new `Thread` object.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `static int` | `activeCount()` | Returns the current number of active threads in the virtual machine.
    `void` | `checkAccess()` | Determines if the currently running thread has permission to modify this thread.
    `static Thread` | `currentThread()` | Returns a reference to the currently executing thread object.
    `static void` | `dumpStack()` | Prints a stack trace of the current thread to the standard error stream.
    `static int` | `enumerate​(Thread[] tarray)` | Copies into the specified array every active thread.
    `static Map<Thread,​StackTraceElement[]>` | `getAllStackTraces()` | Returns a map of stack traces for all live threads.
    `static Thread.UncaughtExceptionHandler` | `getDefaultUncaughtExceptionHandler()` | Returns the default handler invoked when a thread abruptly terminates due to an uncaught exception.
    `long` | `getId()` | Returns the identifier of this Thread.
    `String` | `getName()` | Returns this thread's name.
    `int` | `getPriority()` | Returns this thread's priority.
    `StackTraceElement[]` | `getStackTrace()` | Returns an array of stack trace elements representing the stack dump of this thread.
    `Thread.State` | `getState()` | Returns the state of this thread.
    `Thread.UncaughtExceptionHandler` | `getUncaughtExceptionHandler()` | Returns the handler invoked when this thread abruptly terminates due to an uncaught exception.
    `static boolean` | `holdsLock​(Object obj)` | Returns `true` if and only if the current thread holds the monitor lock on the specified object.
    `void` | `interrupt()` | Interrupts this thread.
    `static boolean` | `interrupted()` | Tests whether the current thread has been interrupted.
    `boolean` | `isAlive()` | Tests if this thread is alive.
    `boolean` | `isDaemon()` | Tests if this thread is a daemon thread.
    `boolean` | `isInterrupted()` | Tests whether this thread has been interrupted.
    `void` | `join()` | Waits for this thread to die.
    `void` | `join​(long millis)` | Waits at most `millis` milliseconds for this thread to die.
    `void` | `join​(long millis, int nanos)` | Waits at most `millis` milliseconds plus `nanos` nanoseconds for this thread to die.
    `void` | `run()` | If this thread was constructed using a separate Runnable run object, then that Runnable object's run method is called; otherwise, this method does nothing and returns.
    `void` | `setDaemon​(boolean on)` | Marks this thread as either a daemon thread or a user thread.
    `static void` | `setDefaultUncaughtExceptionHandler​(Thread.UncaughtExceptionHandler eh)` | Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.
    `void` | `setName​(String name)` | Changes the name of this thread to be equal to the argument `name`.
    `void` | `setPriority​(int newPriority)` | Changes the priority of this thread.
    `void` | `setUncaughtExceptionHandler​(Thread.UncaughtExceptionHandler eh)` | Set the handler invoked when this thread abruptly terminates due to an uncaught exception.
    `static void` | `sleep​(long millis)` | Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers.
    `static void` | `sleep​(long millis, int nanos)` | Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds plus the specified number of nanoseconds, subject to the precision and accuracy of system timers and schedulers.
    `void` | `start()` | Causes this thread to begin execution; the Java Virtual Machine calls the `run` method of this thread.
    `String` | `toString()` | Returns a string representation of this thread, including the thread's name, priority, and thread group.
    `static void` | `yield()` | A hint to the scheduler that the current thread is willing to yield its current use of a processor.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### MAX_PRIORITY

          public static final int MAX_PRIORITY

      The maximum priority that a thread can have.
      See Also:
        Constant Field Values

    - #### MIN_PRIORITY

          public static final int MIN_PRIORITY

      The minimum priority that a thread can have.
      See Also:
        Constant Field Values

    - #### NORM_PRIORITY

          public static final int NORM_PRIORITY

      The default priority that is assigned to a thread.
      See Also:
        Constant Field Values

  - ### Constructor Detail

    - #### Thread

          public Thread()

      Allocates a new `Thread` object.

    - #### Thread

          public Thread​(@Nullable
                        Runnable target)

      Allocates a new Thread object with a specific target object whose run method is called.
      Parameters:
        `target` - the object whose `run` method is invoked when this thread is started. If `null`, this classes `run` method does nothing.

    - #### Thread

          public Thread​(@Nullable
                        Runnable target,
                        String name)

      Allocates a new `Thread` object.
      Parameters:
        `target` - the object whose `run` method is invoked when this thread is started. If `null`, this classes `run` method does nothing.
        `name` - the name of the new thread

    - #### Thread

          public Thread​(String name)

      Allocates a new `Thread` object.
      Parameters:
        `name` - the name of the new thread

  - ### Method Detail

    - #### activeCount

          public static int activeCount()

      Returns the current number of active threads in the virtual machine.
      Returns:
        the number of active threads

    - #### currentThread

          public static Thread currentThread()

      Returns a reference to the currently executing thread object.
      Returns:
        the currently executing thread.

    - #### dumpStack

          public static void dumpStack()

      Prints a stack trace of the current thread to the standard error stream. This method is used only for debugging.
      See Also:
        `Throwable.printStackTrace()`

    - #### enumerate

          public static int enumerate​(Thread[] tarray)

      Copies into the specified array every active thread.

      An application might use the activeCount method to get an estimate of how big the array should be, however if the array is too short to hold all the threads, the extra threads are silently ignored. If it is critical to obtain every active thread in the current thread's thread group and its subgroups, the invoker should verify that the returned int value is strictly less than the length of `tarray`.

      Due to the inherent race condition in this method, it is recommended that the method only be used for debugging and monitoring purposes.

      Parameters:
        `tarray` - an array into which to put the list of threads
      Returns:
        the number of threads put into the array

    - #### getAllStackTraces

          public static Map<Thread,​StackTraceElement[]> getAllStackTraces()

      Returns a map of stack traces for all live threads. The map keys are threads and each map value is an array of `StackTraceElement` that represents the stack dump of the corresponding `Thread`. The returned stack traces are in the format specified for the `getStackTrace` method.

      The threads may be executing while this method is called. The stack trace of each thread only represents a snapshot and each stack trace may be obtained at different time. A zero-length array will be returned in the map value if the virtual machine has no stack trace information about a thread.

      If there is a security manager, then the security manager's `checkPermission` method is called with a `RuntimePermission("getStackTrace")` permission to see if it is ok to get the stack trace of all threads.

      Returns:
        a `Map` from `Thread` to an array of `StackTraceElement` that represents the stack trace of the corresponding thread.
      Throws:
        `SecurityException` - if a security manager exists and its `checkPermission` method doesn't allow getting the stack trace of thread.
      See Also:
        `getStackTrace()`, `SecurityManager.checkPermission(java.security.Permission)`, `RuntimePermission`, `Throwable.getStackTrace()`

    - #### getDefaultUncaughtExceptionHandler

          @Nullable
          public static Thread.UncaughtExceptionHandler getDefaultUncaughtExceptionHandler()

      Returns the default handler invoked when a thread abruptly terminates due to an uncaught exception. If the returned value is `null`, there is no default.
      Returns:
        the default uncaught exception handler for all threads
      See Also:
        `setDefaultUncaughtExceptionHandler(java.lang.Thread.UncaughtExceptionHandler)`

    - #### holdsLock

          public static boolean holdsLock​(Object obj)

      Returns `true` if and only if the current thread holds the monitor lock on the specified object.

      This method is designed to allow a program to assert that the current thread already holds a specified lock:

           assert Thread.holdsLock(obj);

      Parameters:
        `obj` - the object on which to test lock ownership
      Returns:
        `true` if the current thread holds the monitor lock on the specified object.
      Throws:
        `NullPointerException` - if obj is `null`

    - #### interrupted

          public static boolean interrupted()

      Tests whether the current thread has been interrupted. The interrupted status of the thread is cleared by this method. In other words, if this method were to be called twice in succession, the second call would return false (unless the current thread were interrupted again, after the first call had cleared its interrupted status and before the second call had examined it).

      A thread interruption ignored because a thread was not alive at the time of the interrupt will be reflected by this method returning false.

      Returns:
        `true` if the current thread has been interrupted; `false` otherwise.
      See Also:
        `isInterrupted()`

    - #### setDefaultUncaughtExceptionHandler

          public static void setDefaultUncaughtExceptionHandler​(@Nullable
                                                                Thread.UncaughtExceptionHandler eh)

      Set the default handler invoked when a thread abruptly terminates due to an uncaught exception, and no other handler has been defined for that thread.

      Uncaught exception handling is controlled first by the thread and by the default uncaught exception handler. If the thread does not have an explicit uncaught exception handler set, and the thread's thread group (including parent thread groups) does not specialize its `uncaughtException` method, then the default handler's `uncaughtException` method will be invoked.

      By setting the default uncaught exception handler, an application can change the way in which uncaught exceptions are handled (such as logging to a specific device, or file) for those threads that would already accept whatever "default" behavior the system provided.

      Parameters:
        `eh` - the object to use as the default uncaught exception handler. If `null` then there is no default handler.
      Throws:
        `SecurityException` - if a security manager is present and it denies ``RuntimePermission` ("setDefaultUncaughtExceptionHandler")`
      See Also:
        `setUncaughtExceptionHandler(java.lang.Thread.UncaughtExceptionHandler)`, `getUncaughtExceptionHandler()`

    - #### sleep

          public static void sleep​(long millis)
                            throws InterruptedException

      Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors.
      Parameters:
        `millis` - the length of time to sleep in milliseconds
      Throws:
        `IllegalArgumentException` - if the value of `millis` is negative
        `InterruptedException` - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

    - #### sleep

          public static void sleep​(long millis,
                                   int nanos)
                            throws InterruptedException

      Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds plus the specified number of nanoseconds, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors.
      Parameters:
        `millis` - the length of time to sleep in milliseconds
        `nanos` - `0-999999` additional nanoseconds to sleep
      Throws:
        `IllegalArgumentException` - if the value of `millis` is negative, or the value of `nanos` is not in the range `0-999999`
        `InterruptedException` - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

    - #### yield

          public static void yield()

      A hint to the scheduler that the current thread is willing to yield its current use of a processor. The scheduler is free to ignore this hint.

      Yield is a heuristic attempt to improve relative progression between threads that would otherwise over-utilise a CPU. Its use should be combined with detailed profiling and benchmarking to ensure that it actually has the desired effect.

    - #### checkAccess

          public final void checkAccess()

      Determines if the currently running thread has permission to modify this thread.

      If there is a security manager, its `checkAccess` method is called with this thread as its argument. This may result in throwing a `SecurityException`.

      Throws:
        `SecurityException` - if the current thread is not allowed to access this thread.
      See Also:
        `SecurityManager.checkAccess(Thread)`

    - #### getId

          public long getId()

      Returns the identifier of this Thread. The thread ID is a positive `long` number generated when this thread was created. The thread ID is unique and remains unchanged during its lifetime. When a thread is terminated, this thread ID may be reused.
      Returns:
        this thread's ID.

    - #### getName

          public final String getName()

      Returns this thread's name.
      Returns:
        this thread's name.
      See Also:
        `setName(String)`

    - #### getPriority

          public final int getPriority()

      Returns this thread's priority.
      Returns:
        this thread's priority.
      See Also:
        `setPriority(int)`

    - #### getStackTrace

          public StackTraceElement[] getStackTrace()

      Returns an array of stack trace elements representing the stack dump of this thread. This method will return a zero-length array if this thread has not started, has started but has not yet been scheduled to run by the system, or has terminated. If the returned array is of non-zero length then the first element of the array represents the top of the stack, which is the most recent method invocation in the sequence. The last element of the array represents the bottom of the stack, which is the least recent method invocation in the sequence.

      If there is a security manager, and this thread is not the current thread, then the security manager's `checkPermission` method is called with a `RuntimePermission("getStackTrace")` permission to see if it's ok to get the stack trace.

      Some virtual machines may, under some circumstances, omit one or more stack frames from the stack trace. In the extreme case, a virtual machine that has no stack trace information concerning this thread is permitted to return a zero-length array from this method.

      Returns:
        an array of `StackTraceElement`, each represents one stack frame.
      Throws:
        `SecurityException` - if a security manager exists and its `checkPermission` method doesn't allow getting the stack trace of thread.
      See Also:
        `SecurityManager.checkPermission(java.security.Permission)`, `RuntimePermission`, `Throwable.getStackTrace()`

    - #### getState

          public Thread.State getState()

      Returns the state of this thread. This method is designed for use in monitoring of the system state, not for synchronization control.
      Returns:
        this thread's state.

    - #### getUncaughtExceptionHandler

          @Nullable
          public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler()

      Returns the handler invoked when this thread abruptly terminates due to an uncaught exception. If this thread has not had an uncaught exception handler explicitly set then `null` is returned.
      Returns:
        the uncaught exception handler for this thread

    - #### interrupt

          public void interrupt()

      Interrupts this thread.

      Unless the current thread is interrupting itself, which is always permitted, the `checkAccess` method of this thread is invoked, which may cause a `SecurityException` to be thrown.

      If this thread is blocked in an invocation of the `wait()`, `wait(long)`, or `wait(long, int)` methods of the `Object` class, or of the `join()`, `join(long)`, `join(long, int)`, `sleep(long)`, or `sleep(long, int)`, methods of this class, then its interrupt status will be cleared and it will receive an `InterruptedException`.

      If none of the previous conditions hold then this thread's interrupt status will be set.

      Interrupting a thread that is not alive need not have any effect.

      Throws:
        `SecurityException` - if the current thread cannot modify this thread

    - #### isAlive

          public final boolean isAlive()

      Tests if this thread is alive. A thread is alive if it has been started and has not yet died.
      Returns:
        `true` if this thread is alive; `false` otherwise.

    - #### isInterrupted

          public boolean isInterrupted()

      Tests whether this thread has been interrupted. The interrupted status of the thread is unaffected by this method.

      A thread interruption ignored because a thread was not alive at the time of the interrupt will be reflected by this method returning false.

      Returns:
        `true` if this thread has been interrupted; `false` otherwise.
      See Also:
        `interrupted()`

    - #### join

          public final void join()
                          throws InterruptedException

      Waits for this thread to die.

      An invocation of this method behaves in exactly the same way as the invocation

      > join`(0)`

      Throws:
        `InterruptedException` - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

    - #### join

          public final void join​(long millis)
                          throws InterruptedException

      Waits at most `millis` milliseconds for this thread to die. A timeout of `0` means to wait forever.

      This implementation uses a loop of `this.wait` calls conditioned on `this.isAlive`. As a thread terminates the `this.notifyAll` method is invoked. It is recommended that applications not use `wait`, `notify`, or `notifyAll` on `Thread` instances.

      Parameters:
        `millis` - the time to wait in milliseconds
      Throws:
        `IllegalArgumentException` - if the value of `millis` is negative
        `InterruptedException` - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

    - #### join

          public final void join​(long millis,
                                 int nanos)
                          throws InterruptedException

      Waits at most `millis` milliseconds plus `nanos` nanoseconds for this thread to die.

      This implementation uses a loop of `this.wait` calls conditioned on `this.isAlive`. As a thread terminates the `this.notifyAll` method is invoked. It is recommended that applications not use `wait`, `notify`, or `notifyAll` on `Thread` instances.

      Parameters:
        `millis` - the time to wait in milliseconds
        `nanos` - `0-999999` additional nanoseconds to wait
      Throws:
        `IllegalArgumentException` - if the value of `millis` is negative, or the value of `nanos` is not in the range `0-999999`
        `InterruptedException` - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.

    - #### run

          public void run()

      If this thread was constructed using a separate Runnable run object, then that Runnable object's run method is called; otherwise, this method does nothing and returns. Subclasses of Thread should override this method.
      Specified by:
        `run` in interface `Runnable`
      See Also:
        `run()`

    - #### setName

          public final void setName​(String name)

      Changes the name of this thread to be equal to the argument `name`.

      First the `checkAccess` method of this thread is called with no arguments. This may result in throwing a `SecurityException`.

      Parameters:
        `name` - the new name for this thread.
      Throws:
        `SecurityException` - if the current thread cannot modify this thread.
      See Also:
        `getName()`

    - #### setDaemon

          public final void setDaemon​(boolean on)

      Marks this thread as either a daemon thread or a user thread. The Java Virtual Machine exits when the only threads running are all daemon threads.

      This method must be invoked before the thread is started.

      Parameters:
        `on` - if `true`, marks this thread as a daemon thread
      Throws:
        `IllegalThreadStateException` - if this thread is alive
        `SecurityException` - if `checkAccess()` determines that the current thread cannot modify this thread

    - #### isDaemon

          public final boolean isDaemon()

      Tests if this thread is a daemon thread.
      Returns:
        `true` if this thread is a daemon thread; `false` otherwise.
      See Also:
        `setDaemon(boolean)`

    - #### setPriority

          public final void setPriority​(int newPriority)

      Changes the priority of this thread.

      Parameters:
        `newPriority` - priority to set this thread to
      Throws:
        `IllegalArgumentException` - If the priority is not in the range `MIN_PRIORITY` to `MAX_PRIORITY`.
        `SecurityException` - if the current thread cannot modify this thread.
      See Also:
        `getPriority()`, `MAX_PRIORITY`, `MIN_PRIORITY`

    - #### setUncaughtExceptionHandler

          public void setUncaughtExceptionHandler​(@Nullable
                                                  Thread.UncaughtExceptionHandler eh)

      Set the handler invoked when this thread abruptly terminates due to an uncaught exception.

      A thread can take full control of how it responds to uncaught exceptions by having its uncaught exception handler explicitly set.

      Parameters:
        `eh` - the object to use as this thread's uncaught exception handler. If `null` then this thread has no explicit handler.
      Throws:
        `SecurityException` - if the current thread is not allowed to modify this thread.
      See Also:
        `setDefaultUncaughtExceptionHandler(java.lang.Thread.UncaughtExceptionHandler)`

    - #### start

          public void start()

      Causes this thread to begin execution; the Java Virtual Machine calls the `run` method of this thread.

      The result is that two threads are running concurrently: the current thread (which returns from the call to the `start` method) and the other thread (which executes its `run` method).

      It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

      Throws:
        `IllegalThreadStateException` - if the thread was already started.
      See Also:
        `run()`

    - #### toString

          public String toString()

      Returns a string representation of this thread, including the thread's name, priority, and thread group.
      Overrides:
        `toString` in class `Object`
      Returns:
        a string representation of this thread.