# Interface ObjectReference

All Superinterfaces:
  `Mirror, Value`

All Known Subinterfaces:
  `ArrayReference, ClassLoaderReference, ClassObjectReference, ModuleReference, StringReference, ThreadGroupReference, ThreadReference`

public interface ObjectReference extends Value
An object that currently exists in the target VM. An ObjectReference mirrors only the object itself and is not specific to any `Field` or `LocalVariable` to which it is currently assigned. An ObjectReference can have 0 or more references from field(s) and/or variable(s).

Any method on `ObjectReference` which directly or indirectly takes `ObjectReference` as a parameter may throw `VMDisconnectedException` if the target VM is disconnected and the `VMDisconnectEvent` has been or is available to be read from the `EventQueue`.

Any method on `ObjectReference` which directly or indirectly takes `ObjectReference` as a parameter may throw `VMOutOfMemoryException` if the target VM has run out of memory.

Any method on `ObjectReference` or which directly or indirectly takes `ObjectReference` as parameter may throw `ObjectCollectedException` if the mirrored object has been garbage collected.

Since:
  1.3

- ## Field Summary

  Fields
  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
  `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 Mirror

  `toString, virtualMachine`

  ### Methods declared in interface Value

  `type`

- ## Field Details

  - ### INVOKE_SINGLE_THREADED

    static final int INVOKE_SINGLE_THREADED
    Perform method invocation with only the invoking thread resumed
    See Also:
      - Constant Field Values

  - ### INVOKE_NONVIRTUAL

    static final int INVOKE_NONVIRTUAL
    Perform non-virtual method invocation
    See Also:
      - Constant Field Values

- ## Method Details

  - ### referenceType

    ReferenceType referenceType()
    Gets the `ReferenceType` that mirrors the type of this object. The type may be a subclass or implementor of the declared type of any field or variable which currently holds it. For example, right after the following statement.

    `Object obj = new String("Hello, world!");`

    The ReferenceType of obj will mirror java.lang.String and not java.lang.Object.

    The type of an object never changes, so this method will always return the same ReferenceType over the lifetime of the mirrored object.

    The returned ReferenceType will be a `ClassType` or `ArrayType` and never an `InterfaceType`.

    Returns:
      the `ReferenceType` for this object.

  - ### getValue

    Value getValue(Field sig)
    Gets the value of a given instance or static field in this object. The Field must be valid for this ObjectReference; that is, it must be from the mirrored object's class or a superclass of that class.
    Parameters:
      `sig` - the field containing the requested value
    Returns:
      the `Value` of the instance field.
    Throws:
      `IllegalArgumentException` - if the field is not valid for this object's class.

  - ### getValues

    Map<Field,Value> getValues(List<? extends Field> fields)
    Gets the value of multiple instance and/or static fields in this object. The Fields must be valid for this ObjectReference; that is, they must be from the mirrored object's class or a superclass of that class.
    Parameters:
      `fields` - a list of `Field` objects containing the requested values.
    Returns:
      a Map of the requested `Field` objects with their `Value`.
    Throws:
      `IllegalArgumentException` - if any field is not valid for this object's class.

  - ### setValue

    void setValue(Field field, Value value) throws InvalidTypeException, ClassNotLoadedException
    Sets the value of a given instance or static field in this object. The `Field` must be valid for this ObjectReference; that is, it must be from the mirrored object's class or a superclass of that class. The field must not be final.

    Object values must be assignment compatible with the field type (This implies that the field type must be loaded through the enclosing class's class loader). Primitive values must be either assignment compatible with the field type or must be convertible to the field type without loss of information. See section 5.2 of The Java Language Specification for more information on assignment compatibility.

    Parameters:
      `field` - the field containing the requested value
      `value` - the new value to assign
    Throws:
      `IllegalArgumentException` - if the field is not valid for this object's class or the field is final.
      `InvalidTypeException` - if the value's type does not match the field's type.
      `ClassNotLoadedException` - if 'value' is not null, and the field type has not yet been loaded through the appropriate class loader.
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only - see `VirtualMachine.canBeModified()`.

  - ### invokeMethod

    Value invokeMethod(ThreadReference thread, Method method, List<? extends Value> arguments, int options) throws InvalidTypeException, ClassNotLoadedException, IncompatibleThreadStateException, InvocationException
    Invokes the specified `Method` on this object in the target VM. The specified method can be defined in this object's class, in a superclass of this object's class, or in an interface implemented by this object. The method may be a static method or an instance method, but not a static initializer or constructor. Use `ClassType.newInstance(ThreadReference, Method, List, int)` to create a new object and run its constructor.

    The method invocation will occur in the specified thread. Method invocation can occur only if the specified thread has been suspended by an event which occurred in that thread. Method invocation is not supported when the target VM has been suspended through `VirtualMachine.suspend()` or when the specified thread is suspended through `ThreadReference.suspend()`.

    The specified method is invoked with the arguments in the specified argument list. The method invocation is synchronous; this method does not return until the invoked method returns in the target VM. If the invoked method throws an exception, this method will throw an `InvocationException` which contains a mirror to the exception object thrown.

    Object arguments must be assignment compatible with the argument type (This implies that the argument type must be loaded through the enclosing class's class loader). Primitive arguments must be either assignment compatible with the argument type or must be convertible to the argument type without loss of information. If the method being called accepts a variable number of arguments, then the last argument type is an array of some component type. The argument in the matching position can be omitted, or can be null, an array of the same component type, or an argument of the component type followed by any number of other arguments of the same type. If the argument is omitted, then a 0 length array of the component type is passed. The component type can be a primitive type. Autoboxing is not supported. See section 5.2 of The Java Language Specification for more information on assignment compatibility.

    By default, the method is invoked using dynamic lookup as documented in section 15.12.4.4 of The Java Language Specification in particular, overriding based on the runtime type of the object mirrored by this `ObjectReference` will occur. This behavior can be changed by specifying the `INVOKE_NONVIRTUAL` bit flag in the `options` argument. If this flag is set, the specified method is invoked whether or not it is overridden for this object's runtime type. The method, in this case, must have an implementation, either in a class or an interface. This option is useful for performing method invocations like those done with the `super` keyword in the Java programming language.

    By default, all threads in the target VM are resumed while the method is being invoked if they were previously suspended by an event or by `VirtualMachine.suspend()` or `ThreadReference.suspend()`. This is done to prevent the deadlocks that will occur if any of the threads own resources, such as monitors, that will be needed by the invoked method. Note, however, that this implicit resume acts exactly like `ThreadReference.resume()`, so if the thread's suspend count is greater than 1, it will remain in a suspended state during the invocation and thus a deadlock could still occur. By default, when the invocation completes, all threads in the target VM are suspended, regardless their state before the invocation. It is possible that breakpoints or other events might occur during the invocation. This can cause deadlocks as described above. It can also cause a deadlock if invokeMethod is called from the client's event handler thread. In this case, this thread will be waiting for the invokeMethod to complete and won't read the EventSet that comes in for the new event. If this new EventSet is SUSPEND_ALL, then a deadlock will occur because no one will resume the EventSet. To avoid this, all EventRequests should be disabled before doing the invokeMethod, or the invokeMethod should not be done from the client's event handler thread.

    The resumption of other threads during the invocation can be prevented by specifying the `INVOKE_SINGLE_THREADED` bit flag in the `options` argument; however, there is no protection against or recovery from the deadlocks described above, so this option should be used with great caution. Only the specified thread will be resumed (as described for all threads above). Upon completion of a single threaded invoke, the invoking thread will be suspended once again. Note that any threads started during the single threaded invocation will not be suspended when the invocation completes.

    If the target VM is disconnected during the invoke (for example, through `VirtualMachine.dispose()`) the method invocation continues.

    API Note:
      Virtual threads are typically user-mode threads scheduled by the Java runtime rather than the operating system. As such, there may be more cases with virtual threads where the `INVOKE_SINGLE_THREADED` option may cause a deadlock. For example, suspending all threads in the target VM may suspend threads that support the timer mechanism for virtual threads, and thus methods such as `Thread.sleep(long)` may deadlock.
    Parameters:
      `thread` - the thread in which to invoke.
      `method` - the `Method` to invoke.
      `arguments` - the list of `Value` arguments bound to the invoked method. Values from the list are assigned to arguments in the order they appear in the method signature.
      `options` - the integer bit flag options.
    Returns:
      a `Value` mirror of the invoked method's return value.
    Throws:
      `IllegalArgumentException` - if the method is not a member of this object's class, if the size of the argument list does not match the number of declared arguments for the method, if the method is a constructor or static initializer, or if `INVOKE_NONVIRTUAL` is specified and the method is abstract.
      `ClassNotLoadedException` - if any argument type has not yet been loaded through the appropriate class loader.
      `IncompatibleThreadStateException` - if the specified thread has not been suspended by an event.
      `InvocationException` - if the method invocation resulted in an exception in the target VM.
      `InvalidTypeException` - If the arguments do not meet this requirement -- Object arguments must be assignment compatible with the argument type. This implies that the argument type must be loaded through the enclosing class's class loader. Primitive arguments must be either assignment compatible with the argument type or must be convertible to the argument type without loss of information. See JLS section 5.2 for more information on assignment compatibility.
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only - see `VirtualMachine.canBeModified()`.

  - ### disableCollection

    void disableCollection()
    Prevents garbage collection for this object. By default all `ObjectReference` values returned by JDI may be collected at any time the target VM is running. A call to this method guarantees that the object will not be collected. `enableCollection()` can be used to allow collection once again.

    Calls to this method are counted. Every call to this method requires a corresponding call to `enableCollection()` before garbage collection is re-enabled.

    Note that while the target VM is suspended, no garbage collection will occur because all threads are suspended. The typical examination of variables, fields, and arrays during the suspension is safe without explicitly disabling garbage collection.

    This method should be used sparingly, as it alters the pattern of garbage collection in the target VM and, consequently, may result in application behavior under the debugger that differs from its non-debugged behavior.

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

  - ### enableCollection

    void enableCollection()
    Permits garbage collection for this object. By default all `ObjectReference` values returned by JDI may be collected at any time the target VM is running. A call to this method is necessary only if garbage collection was previously disabled with `disableCollection()`.
    Throws:
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only - see `VirtualMachine.canBeModified()`.

  - ### isCollected

    boolean isCollected()
    Determines if this object has been garbage collected in the target VM.
    Returns:
      `true` if this `ObjectReference` has been collected; `false` otherwise.
    Throws:
      `VMCannotBeModifiedException` - if the VirtualMachine is read-only - see `VirtualMachine.canBeModified()`.

  - ### uniqueID

    long uniqueID()
    Returns a unique identifier for this ObjectReference. It is guaranteed to be unique among all ObjectReferences from the same VM that have not yet been disposed. The guarantee applies as long as this ObjectReference has not yet been disposed.
    Returns:
      a long unique ID

  - ### waitingThreads

    List<ThreadReference> waitingThreads() throws IncompatibleThreadStateException
    Returns a List containing a `ThreadReference` for each platform thread currently waiting for this object's monitor. See `ThreadReference.currentContendedMonitor()` for information about when a thread is considered to be waiting for a monitor.

    Not all target VMs support this operation. See `VirtualMachine.canGetMonitorInfo()` to determine if the operation is supported.

    Returns:
      a List of `ThreadReference` objects. The list has zero length if no threads are waiting for the monitor, or only virtual threads are waiting for the monitor.
    Throws:
      `UnsupportedOperationException` - if the target VM does not support this operation.
      `IncompatibleThreadStateException` - if any waiting thread is not suspended in the target VM

  - ### owningThread

    ThreadReference owningThread() throws IncompatibleThreadStateException
    Returns a `ThreadReference` for the platform thread, if any, which currently owns this object's monitor. See `ThreadReference.ownedMonitors()` for a definition of ownership.

    Not all target VMs support this operation. See `VirtualMachine.canGetMonitorInfo()` to determine if the operation is supported.

    Returns:
      the `ThreadReference` of the platform thread which currently owns the monitor, or null if the monitor is owned by a virtual thread or not owned.
    Throws:
      `UnsupportedOperationException` - if the target VM does not support this operation.
      `IncompatibleThreadStateException` - if the owning thread is not suspended in the target VM

  - ### entryCount

    int entryCount() throws IncompatibleThreadStateException
    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. See `ThreadReference.ownedMonitors()` for a definition of ownership.

    Not all target VMs support this operation. See `VirtualMachine.canGetMonitorInfo()` to determine if the operation is supported.

    Returns:
      the integer count of the number of entries.
    Throws:
      `UnsupportedOperationException` - if the target VM does not support this operation.
      `IncompatibleThreadStateException` - if the owning thread is not suspended in the target VM
    See Also:
      - `owningThread()`

  - ### referringObjects

    List<ObjectReference> referringObjects(long maxReferrers)
    Returns objects that directly reference this object. Only objects that are reachable for the purposes of garbage collection are returned. Note that an object can also be referenced in other ways, such as from a local variable in a stack frame, or from a JNI global reference. Such non-object referrers are not returned by this method.

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

    Parameters:
      `maxReferrers` - The maximum number of referring objects to return. Must be non-negative. If zero, all referring objects are returned.
    Returns:
      a List of `ObjectReference` objects. If there are no objects that reference this object, a zero-length list is returned..
    Throws:
      `UnsupportedOperationException` - if the target virtual machine does not support this operation - see `VirtualMachine.canGetInstanceInfo()`
      `IllegalArgumentException` - if maxReferrers is less than zero.
    Since:
      1.6
    See Also:
      - `VirtualMachine.instanceCounts(List)`
      - `ReferenceType.instances(long)`

  - ### equals

    boolean equals(Object obj)
    Compares the specified Object with this ObjectReference for equality.
    Overrides:
      `equals` in class `Object`
    Parameters:
      `obj` - the reference object with which to compare.
    Returns:
      true if the Object is an ObjectReference, if the ObjectReferences belong to the same VM, and if applying the "==" operator on the mirrored objects in that VM evaluates to true.
    See Also:
      - `Object.hashCode()`
      - `HashMap`

  - ### hashCode

    int hashCode()
    Returns the hash code value for this ObjectReference.
    Overrides:
      `hashCode` in class `Object`
    Returns:
      the integer hash code
    See Also:
      - `Object.equals(java.lang.Object)`
      - `System.identityHashCode(Object)`