ThreadReference (Java SE 26 & JDK 26)
Description: declaration: module: jdk.jdi, package: com.sun.jdi, interface: ThreadReference
Keywords: com.sun.jdi.ThreadReference interface

# Interface ThreadReference

All Superinterfaces:
  `Mirror, ObjectReference, Value`

public interface ThreadReference extends ObjectReference
A thread object from the target VM. A ThreadReference is an `ObjectReference` with additional access to thread-specific information from the target VM.

Since:
  1.3

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final int`
  `THREAD_STATUS_MONITOR`
  Thread is waiting on a java monitor
  `static final int`
  `THREAD_STATUS_NOT_STARTED`
  Thread has not yet been started
  `static final int`
  `THREAD_STATUS_RUNNING`
  Thread is runnable
  `static final int`
  `THREAD_STATUS_SLEEPING`
  Thread is sleeping - `Thread.sleep(long)`.
  `static final int`
  `THREAD_STATUS_UNKNOWN`
  Thread status is unknown
  `static final int`
  `THREAD_STATUS_WAIT`
  Thread is waiting - `Object.wait()` or `LockSupport.park()`.
  `static final int`
  `THREAD_STATUS_ZOMBIE`
  Thread has completed execution

  ### Fields declared in interface ObjectReference

  `INVOKE_NONVIRTUAL, INVOKE_SINGLE_THREADED`
  Modifier and Type
  Field
  Description
  `static final int`
  `INVOKE_NONVIRTUAL`
  Perform non-virtual method invocation
  `static final int`
  `INVOKE_SINGLE_THREADED`
  Perform method invocation with only the invoking thread resumed

- ## Method Summary

  Modifier and Type
  Method
  Description
  `ObjectReference`
  `currentContendedMonitor()`
  Returns an `ObjectReference` for the monitor, if any, for which this thread is currently waiting.
  `void`
  `forceEarlyReturn(Value value)`
  Force a method to return before it reaches a return statement.
  `StackFrame`
  `frame(int index)`
  Returns the `StackFrame` at the given index in the thread's current call stack.
  `int`
  `frameCount()`
  Returns the number of stack frames in the thread's current call stack.
  `List<StackFrame>`
  `frames()`
  Returns a List containing each `StackFrame` in the thread's current call stack.
  `List<StackFrame>`
  `frames(int start, int length)`
  Returns a List containing a range of `StackFrame` mirrors from the thread's current call stack.
  `void`
  `interrupt()`
  Interrupts this thread unless the thread has been suspended by the debugger.
  `boolean`
  `isAtBreakpoint()`
  Determines whether the thread is suspended at a breakpoint.
  `boolean`
  `isSuspended()`
  Determines whether the thread has been suspended by the the debugger.
  `default boolean`
  `isVirtual()`
  Returns `true` if the thread is a virtual thread.
  `String`
  `name()`
  Returns the name of this thread.
  `List<ObjectReference>`
  `ownedMonitors()`
  Returns a List containing an `ObjectReference` for each monitor owned by the thread.
  `List<MonitorInfo>`
  `ownedMonitorsAndFrames()`
  Returns a List containing a `MonitorInfo` object for each monitor owned by the thread.
  `void`
  `popFrames(StackFrame frame)`
  Pop stack frames.
  `void`
  `resume()`
  Resumes this thread.
  `int`
  `status()`
  Returns the thread's status.
  `void`
  `stop(ObjectReference throwable)`
  Stops this thread with an asynchronous exception.
  `void`
  `suspend()`
  Suspends this thread.
  `int`
  `suspendCount()`
  Returns the number of pending suspends for this thread.
  `ThreadGroupReference`
  `threadGroup()`
  Returns this thread's thread group.

  ### Methods declared in interface Mirror

  `toString, virtualMachine`
  Modifier and Type
  Method
  Description
  `String`
  `toString()`
  Returns a String describing this mirror
  `VirtualMachine`
  `virtualMachine()`
  Gets the VirtualMachine to which this Mirror belongs.

  ### Methods declared in interface ObjectReference

  `disableCollection, enableCollection, entryCount, equals, getValue, getValues, hashCode, invokeMethod, isCollected, owningThread, referenceType, referringObjects, setValue, uniqueID, waitingThreads`
  Modifier and Type
  Method
  Description
  `void`
  `disableCollection()`
  Prevents garbage collection for this object.
  `void`
  `enableCollection()`
  Permits garbage collection for this object.
  `int`
  `entryCount()`
  Returns the number of times this object's monitor has been entered by the current owning thread if the owning thread is platform thread; Returns 0 if not owned by a platform thread.
  `boolean`
  `equals(Object obj)`
  Compares the specified Object with this ObjectReference for equality.
  `Value`
  `getValue(Field sig)`
  Gets the value of a given instance or static field in this object.
  `Map<Field,Value>`
  `getValues(List<? extends Field> fields)`
  Gets the value of multiple instance and/or static fields in this object.
  `int`
  `hashCode()`
  Returns the hash code value for this ObjectReference.
  `Value`
  `invokeMethod(ThreadReference thread, Method method, List<? extends Value> arguments, int options)`
  Invokes the specified `Method` on this object in the target VM.
  `boolean`
  `isCollected()`
  Determines if this object has been garbage collected in the target VM.
  `ThreadReference`
  `owningThread()`
  Returns a `ThreadReference` for the platform thread, if any, which currently owns this object's monitor.
  `ReferenceType`
  `referenceType()`
  Gets the `ReferenceType` that mirrors the type of this object.
  `List<ObjectReference>`
  `referringObjects(long maxReferrers)`
  Returns objects that directly reference this object.
  `void`
  `setValue(Field field, Value value)`
  Sets the value of a given instance or static field in this object.
  `long`
  `uniqueID()`
  Returns a unique identifier for this ObjectReference.
  `List<ThreadReference>`
  `waitingThreads()`
  Returns a List containing a `ThreadReference` for each platform thread currently waiting for this object's monitor.

  ### Methods declared in interface Value

  `type`
  Modifier and Type
  Method
  Description
  `Type`
  `type()`
  Returns the run-time type of this value.

- ## Field Details

  - ### THREAD_STATUS_UNKNOWN

    static final int THREAD_STATUS_UNKNOWN
    Thread status is unknown
    See Also:
      - Constant Field Values

  - ### THREAD_STATUS_ZOMBIE

    static final int THREAD_STATUS_ZOMBIE
    Thread has completed execution
    See Also:
      - Constant Field Values

  - ### THREAD_STATUS_RUNNING

    static final int THREAD_STATUS_RUNNING
    Thread is runnable
    See Also:
      - Constant Field Values

  - ### THREAD_STATUS_SLEEPING

    static final int THREAD_STATUS_SLEEPING
    Thread is sleeping - `Thread.sleep(long)`.
    See Also:
      - Constant Field Values

  - ### THREAD_STATUS_MONITOR

    static final int THREAD_STATUS_MONITOR
    Thread is waiting on a java monitor
    See Also:
      - Constant Field Values

  - ### THREAD_STATUS_WAIT

    static final int THREAD_STATUS_WAIT
    Thread is waiting - `Object.wait()` or `LockSupport.park()`. A virtual thread that is sleeping in `Thread.sleep(long)` may have this thread status instead of `THREAD_STATUS_SLEEPING`.
    See Also:
      - Constant Field Values

  - ### THREAD_STATUS_NOT_STARTED

    static final int THREAD_STATUS_NOT_STARTED
    Thread has not yet been started
    See Also:
      - Constant Field Values

- ## Method Details

  - ### name

    String name()
    Returns the name of this thread.
    Returns:
      the string containing the thread name.

  - ### suspend

    void suspend()
    Suspends this thread. The thread can be resumed through `resume()` or resumed with other threads through `VirtualMachine.resume()`.

    Suspends of both the virtual machine and individual threads are counted. Before a thread will run again, it must be resumed (through `resume()` or `VirtualMachine.resume()`) the same number of times it has been suspended.

    Suspending single threads with this method is inherently deadlock-prone. If the suspended thread holds a monitor needed by another running thread, deadlock is possible in the target VM (at least until the suspended thread is resumed again).

    The suspended thread is guaranteed to remain suspended until resumed through one of the JDI resume methods mentioned above.

    Throws:
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only
    See Also:
      - `VirtualMachine.canBeModified()`

  - ### resume

    void resume()
    Resumes this thread. If this thread was not previously suspended through `suspend()` or through `VirtualMachine.suspend()`, or because of a SUSPEND_ALL or SUSPEND_EVENT_THREAD event, then invoking this method has no effect. Otherwise, the count of pending suspends on this thread is decremented. If it is decremented to 0, the thread will continue to execute. Note: the normal way to resume from an event related suspension is via `EventSet.resume()`.
    Throws:
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only
    See Also:
      - `VirtualMachine.canBeModified()`

  - ### suspendCount

    int suspendCount()
    Returns the number of pending suspends for this thread. See `suspend()` for an explanation of counted suspends.
    Returns:
      pending suspend count as an integer

  - ### stop

    void stop(ObjectReference throwable) throws InvalidTypeException
    Stops this thread with an asynchronous exception. A debugger thread in the target VM will stop this thread with the given `Throwable` object.

    This method may be used to send an asynchronous exception to a virtual thread when it is suspended at an event. An implementation may support sending an asynchronous exception to a suspended virtual thread in other cases.

    Parameters:
      `throwable` - the asynchronous exception to throw
    Throws:
      `InvalidTypeException` - if `throwable` is not an instance of java.lang.Throwable in the target VM
      `IllegalThreadStateException` - if the thread has terminated, or if the thread is a virtual thread and was not suspended
      `OpaqueFrameException` - if the thread is a suspended virtual thread and the implementation was unable to throw an asynchronous exception from the thread's current frame
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only
    See Also:
      - `VirtualMachine.canBeModified()`

  - ### interrupt

    void interrupt()
    Interrupts this thread unless the thread has been suspended by the debugger.
    Throws:
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only
    See Also:
      - `Thread.interrupt()`
      - `VirtualMachine.canBeModified()`

  - ### status

    int status()
    Returns the thread's status. If the thread is not suspended the thread's current status is returned. If the thread is suspended, the thread's status before the suspension is returned (or `THREAD_STATUS_UNKNOWN` if this information is not available. `isSuspended()` can be used to determine if the thread has been suspended.
    Returns:
      one of `THREAD_STATUS_UNKNOWN`, `THREAD_STATUS_ZOMBIE`, `THREAD_STATUS_RUNNING`, `THREAD_STATUS_SLEEPING`, `THREAD_STATUS_MONITOR`, `THREAD_STATUS_WAIT`, `THREAD_STATUS_NOT_STARTED`,

  - ### isSuspended

    boolean isSuspended()
    Determines whether the thread has been suspended by the the debugger.
    Returns:
      `true` if the thread is currently suspended; `false` otherwise.

  - ### isAtBreakpoint

    boolean isAtBreakpoint()
    Determines whether the thread is suspended at a breakpoint.
    Returns:
      `true` if the thread is currently stopped at a breakpoint; `false` otherwise.

  - ### threadGroup

    ThreadGroupReference threadGroup()
    Returns this thread's thread group.
    Returns:
      a `ThreadGroupReference` that mirrors this thread's thread group in the target VM.

  - ### frameCount

    int frameCount() throws IncompatibleThreadStateException
    Returns the number of stack frames in the thread's current call stack. The thread must be suspended (normally through an interruption to the VM) to get this information, and it is only valid until the thread is resumed again.
    Returns:
      an integer frame count
    Throws:
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM

  - ### frames

    List<StackFrame> frames() throws IncompatibleThreadStateException
    Returns a List containing each `StackFrame` in the thread's current call stack. The thread must be suspended (normally through an interruption to the VM) to get this information, and it is only valid until the thread is resumed again.
    Returns:
      a List of `StackFrame` with the current frame first followed by each caller's frame.
    Throws:
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM

  - ### frame

    StackFrame frame(int index) throws IncompatibleThreadStateException
    Returns the `StackFrame` at the given index in the thread's current call stack. Index 0 retrieves the current frame; higher indices retrieve caller frames. The thread must be suspended (normally through an interruption to the VM) to get this information, and it is only valid until the thread is resumed again.
    Parameters:
      `index` - the desired frame
    Returns:
      the requested `StackFrame`
    Throws:
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM
      `IndexOutOfBoundsException` - if the index is greater than or equal to `frameCount()` or is negative.

  - ### frames

    List<StackFrame> frames(int start, int length) throws IncompatibleThreadStateException
    Returns a List containing a range of `StackFrame` mirrors from the thread's current call stack. The thread must be suspended (normally through an interruption to the VM) to get this information, and it is only valid until the thread is resumed again.
    Parameters:
      `start` - the index of the first frame to retrieve. Index 0 represents the current frame.
      `length` - the number of frames to retrieve
    Returns:
      a List of `StackFrame` with the current frame first followed by each caller's frame.
    Throws:
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM
      `IndexOutOfBoundsException` - if the specified range is not within the range of stack frame indices. That is, the exception is thrown if any of the following are true:

              start < 0
             start >= frameCount()
             length < 0
             (start+length) > frameCount()

  - ### ownedMonitors

    List<ObjectReference> ownedMonitors() throws IncompatibleThreadStateException
    Returns a List containing an `ObjectReference` for each monitor owned by the thread. A monitor is owned by a thread if it has been entered (via the synchronized statement or entry into a synchronized method) and has not been relinquished through `Object.wait()`.

    Not all target virtual machines support this operation. Use `VirtualMachine.canGetOwnedMonitorInfo()` to determine if the operation is supported.

    Returns:
      a List of `ObjectReference` objects. The list has zero length if no monitors are owned by this thread.
    Throws:
      `UnsupportedOperationException` - if the target virtual machine does not support this operation.
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM

  - ### ownedMonitorsAndFrames

    List<MonitorInfo> ownedMonitorsAndFrames() throws IncompatibleThreadStateException
    Returns a List containing a `MonitorInfo` object for each monitor owned by the thread. A monitor is owned by a thread if it has been entered (via the synchronized statement or entry into a synchronized method) and has not been relinquished through `Object.wait()`.

    Not all target virtual machines support this operation. Use `VirtualMachine.canGetMonitorFrameInfo()` to determine if the operation is supported.

    Returns:
      a List of `MonitorInfo` objects. The list has zero length if no monitors are owned by this thread.
    Throws:
      `UnsupportedOperationException` - if the target virtual machine does not support this operation.
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM
    Since:
      1.6

  - ### currentContendedMonitor

    ObjectReference currentContendedMonitor() throws IncompatibleThreadStateException
    Returns an `ObjectReference` for the monitor, if any, for which this thread is currently waiting. The thread can be waiting for a monitor through entry into a synchronized method, the synchronized statement, or `Object.wait()` waiting to re-enter the monitor after being notified, interrupted, or timed-out.

    Not all target virtual machines support this operation. Use `VirtualMachine.canGetCurrentContendedMonitor()` to determine if the operation is supported.

    Returns:
      the `ObjectReference` corresponding to the contended monitor, or null if it is not waiting for a monitor.
    Throws:
      `UnsupportedOperationException` - if the target virtual machine does not support this operation.
      `IncompatibleThreadStateException` - if the thread is not suspended in the target VM

  - ### popFrames

    void popFrames(StackFrame frame) throws IncompatibleThreadStateException
    Pop stack frames.

    All frames up to and including the `frame` are popped off the stack. The frame previous to the parameter `frame` will become the current frame.

    After this operation, this thread will be suspended at the invoke instruction of the target method that created `frame`. The `frame`'s method can be reentered with a step into the instruction.

    The operand stack is restored, however, any changes to the arguments that occurred in the called method, remain. For example, if the method `foo`:

           void foo(int x) {
               System.out.println("Foo: " + x);
               x = 4;
               System.out.println("pop here");
           }

    was called with `foo(7)` and `foo` is popped at the second `println` and resumed, it will print: `Foo: 4`.

    Locks acquired by a popped frame are released when it is popped. This applies to synchronized methods that are popped, and to any synchronized blocks within them.

    Finally blocks are not executed.

    No aspect of state, other than this thread's execution point and locks, is affected by this call. Specifically, the values of fields are unchanged, as are external resources such as I/O streams. Additionally, the target program might be placed in a state that is impossible with normal program flow; for example, order of lock acquisition might be perturbed. Thus the target program may proceed differently than the user would expect.

    This thread must be suspended.

    All `StackFrame` objects for this thread are invalidated.

    This method may be used to pop frames of a virtual thread when it is suspended at an event. An implementation may support popping the frames of a suspended virtual thread in other cases.

    No events are generated by this method.

    None of the frames through and including the frame for the caller of frame may be native.

    Not all target virtual machines support this operation. Use `VirtualMachine.canPopFrames()` to determine if the operation is supported.

    Parameters:
      `frame` - Stack frame to pop. `frame` is on this thread's call stack.
    Throws:
      `UnsupportedOperationException` - if the target virtual machine does not support this operation - see `VirtualMachine.canPopFrames()`.
      `IncompatibleThreadStateException` - if this thread is not suspended.
      `IllegalArgumentException` - if `frame` is not on this thread's call stack.
      `OpaqueFrameException` - if the target VM is unable to pop this frame (e.g. a virtual thread is suspended, but not at an event).
      `NativeMethodException` - if one of the frames that would be popped is that of a native method or if the frame previous to frame is native.
      `InvalidStackFrameException` - if `frame` has become invalid. Once this thread is resumed, the stack frame is no longer valid. This exception is also thrown if there are no more frames.
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only.
    Since:
      1.4
    See Also:
      - `VirtualMachine.canBeModified()`

  - ### forceEarlyReturn

    void forceEarlyReturn(Value value) throws InvalidTypeException, ClassNotLoadedException, IncompatibleThreadStateException
    Force a method to return before it reaches a return statement.

    The method which will return early is referred to as the called method. The called method is the current method (as defined by the Frames section in the Java Virtual Machine Specification) for the specified thread at the time this method is called.

    The thread must be suspended. The return occurs when execution of Java programming language code is resumed on this thread. Between the call to this method and resumption of thread execution, the state of the stack is undefined.

    This method may be used to force a return from the current frame of a virtual thread when it is suspended at an event. An implementation may support forcing a return from the current frame of a suspended virtual thread in other cases.

    No further instructions are executed in the called method. Specifically, finally blocks are not executed. Note: this can cause inconsistent states in the application.

    A lock acquired by calling the called method (if it is a synchronized method) and locks acquired by entering synchronized blocks within the called method are released. Note: this does not apply to native locks or java.util.concurrent.locks locks.

    Events, such as MethodExit, are generated as they would be in a normal return.

    The called method must be a non-native Java programming language method. Forcing return on a thread with only one frame on the stack causes the thread to exit when resumed.

    The `value` argument is the value that the method is to return. If the return type of the method is void, then value must be a `VoidValue`. Object values must be assignment compatible with the method return type (This implies that the method return type must be loaded through the enclosing class's class loader). Primitive values must be either assignment compatible with the method return type or must be convertible to the variable type without loss of information. See JLS section 5.2 for more information on assignment compatibility.

    Not all target virtual machines support this operation. Use `VirtualMachine.canForceEarlyReturn()` to determine if the operation is supported.

    Parameters:
      `value` - the value the method is to return.
    Throws:
      `UnsupportedOperationException` - if the target virtual machine does not support this operation - see `canForceEarlyReturn()`
      `IncompatibleThreadStateException` - if this thread is not suspended.
      `OpaqueFrameException` - if the target VM is unable to force the method to return (e.g. a virtual thread is suspended, but not at an event).
      `NativeMethodException` - if the frame to be returned from is that of a native method.
      `InvalidStackFrameException` - if there are no frames.
      `InvalidTypeException` - if the value's type does not match the method's return type.
      `ClassNotLoadedException` - if the method's return type has not yet been loaded through the appropriate class loader.
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only.
    Since:
      1.6
    See Also:
      - `VirtualMachine.canBeModified()`

  - ### isVirtual

    default boolean isVirtual()
    Returns `true` if the thread is a virtual thread.
    Implementation Requirements:
      The default implementation throws `UnsupportedOperationException`.
    Returns:
      true if the thread is a virtual thread
    Since:
      21