VirtualMachine (Java SE 26 & JDK 26)
Description: declaration: module: jdk.attach, package: com.sun.tools.attach, class: VirtualMachine
Keywords: com.sun.tools.attach.VirtualMachine class

# Class VirtualMachine

java.lang.Object
com.sun.tools.attach.VirtualMachine

public abstract class VirtualMachine extends Object
A Java virtual machine.

A `VirtualMachine` represents a Java virtual machine to which this Java virtual machine has attached. The Java virtual machine to which it is attached is sometimes called the target virtual machine, or target VM. An application (typically a tool such as a managemet console or profiler) uses a VirtualMachine to load an agent into the target VM. For example, a profiler tool written in the Java Language might attach to a running application and load its profiler agent to profile the running application.

A VirtualMachine is obtained by invoking the `attach` method with an identifier that identifies the target virtual machine. The identifier is implementation-dependent but is typically the process identifier (or pid) in environments where each Java virtual machine runs in its own operating system process. Alternatively, a `VirtualMachine` instance is obtained by invoking the `attach` method with a `VirtualMachineDescriptor` obtained from the list of virtual machine descriptors returned by the `list` method. Once a reference to a virtual machine is obtained, the `loadAgent`, `loadAgentLibrary`, and `loadAgentPath` methods are used to load agents into target virtual machine. The `loadAgent` method is used to load agents that are written in the Java Language and deployed in a `JAR file`. (See `java.lang.instrument` for a detailed description on how these agents are loaded and started). The `loadAgentLibrary` and `loadAgentPath` methods are used to load agents that are deployed either in a dynamic library or statically linked into the VM and make use of the JVM Tools Interface.

In addition to loading agents a VirtualMachine provides read access to the `system properties` in the target VM. This can be useful in some environments where properties such as `java.home`, `os.name`, or `os.arch` are used to construct the path to agent that will be loaded into the target VM.

The following example demonstrates how VirtualMachine may be used:

         // attach to target VM
         VirtualMachine vm = VirtualMachine.attach("2177");

         // start management agent
         Properties props = new Properties();
         props.put("com.sun.management.jmxremote.port", "5000");
         vm.startManagementAgent(props);

         // detach
         vm.detach();

In this example we attach to a Java virtual machine that is identified by the process identifier `2177`. Then the JMX management agent is started in the target process using the supplied arguments. Finally, the client detaches from the target VM.

A VirtualMachine is safe for use by multiple concurrent threads.

Since:
  1.6

- ## Constructor Summary

  Constructors
  Modifier
  Constructor
  Description
  `protected`
  `VirtualMachine(AttachProvider provider, String id)`
  Initializes a new instance of this class.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `static VirtualMachine`
  `attach(VirtualMachineDescriptor vmd)`
  Attaches to a Java virtual machine.
  `static VirtualMachine`
  `attach(String id)`
  Attaches to a Java virtual machine.
  `abstract void`
  `detach()`
  Detach from the virtual machine.
  `boolean`
  `equals(Object ob)`
  Tests this VirtualMachine for equality with another object.
  `abstract Properties`
  `getAgentProperties()`
  Returns the current agent properties in the target virtual machine.
  `abstract Properties`
  `getSystemProperties()`
  Returns the current system properties in the target virtual machine.
  `int`
  `hashCode()`
  Returns a hash-code value for this VirtualMachine.
  `final String`
  `id()`
  Returns the identifier for this Java virtual machine.
  `static List<VirtualMachineDescriptor>`
  `list()`
  Return a list of Java virtual machines.
  `void`
  `loadAgent(String agent)`
  Loads an agent.
  `abstract void`
  `loadAgent(String agent, String options)`
  Loads an agent.
  `void`
  `loadAgentLibrary(String agentLibrary)`
  Loads an agent library.
  `abstract void`
  `loadAgentLibrary(String agentLibrary, String options)`
  Loads an agent library.
  `void`
  `loadAgentPath(String agentPath)`
  Load a native agent library by full pathname.
  `abstract void`
  `loadAgentPath(String agentPath, String options)`
  Load a native agent library by full pathname.
  `final AttachProvider`
  `provider()`
  Returns the provider that created this virtual machine.
  `abstract String`
  `startLocalManagementAgent()`
  Starts the local JMX management agent in the target virtual machine.
  `abstract void`
  `startManagementAgent(Properties agentProperties)`
  Starts the JMX management agent in the target virtual machine.
  `String`
  `toString()`
  Returns the string representation of the `VirtualMachine`.

  ### Methods declared in class Object

  `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected Object`
  `clone()`
  Creates and returns a copy of this object.
  `protected void`
  `finalize()`
  Deprecated, for removal: This API element is subject to removal in a future version.
  Finalization is deprecated and subject to removal in a future release.
  `final Class<?>`
  `getClass()`
  Returns the runtime class of this `Object`.
  `final void`
  `notify()`
  Wakes up a single thread that is waiting on this object's monitor.
  `final void`
  `notifyAll()`
  Wakes up all threads that are waiting on this object's monitor.
  `final void`
  `wait()`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
  `final void`
  `wait(long timeoutMillis)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
  `final void`
  `wait(long timeoutMillis, int nanos)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

- ## Constructor Details

  - ### VirtualMachine

    protected VirtualMachine(AttachProvider provider, String id)
    Initializes a new instance of this class.
    Parameters:
      `provider` - The attach provider creating this class.
      `id` - The abstract identifier that identifies the Java virtual machine.
    Throws:
      `NullPointerException` - If `provider` or `id` is `null`.

- ## Method Details

  - ### list

    public static List<VirtualMachineDescriptor> list()
    Return a list of Java virtual machines.

    This method returns a list of Java `VirtualMachineDescriptor` elements. The list is an aggregation of the virtual machine descriptor lists obtained by invoking the `listVirtualMachines` method of all installed `attach providers`. If there are no Java virtual machines known to any provider then an empty list is returned.

    Returns:
      The list of virtual machine descriptors.

  - ### attach

    public static VirtualMachine attach(String id) throws AttachNotSupportedException, IOException
    Attaches to a Java virtual machine.

    This method obtains the list of attach providers by invoking the `AttachProvider.providers()` method. It then iterates overs the list and invokes each provider's `attachVirtualMachine` method in turn. If a provider successfully attaches then the iteration terminates, and the VirtualMachine created by the provider that successfully attached is returned by this method. If the `attachVirtualMachine` method of all providers throws `AttachNotSupportedException` then this method also throws `AttachNotSupportedException`. This means that `AttachNotSupportedException` is thrown when the identifier provided to this method is invalid, or the identifier corresponds to a Java virtual machine that does not exist, or none of the providers can attach to it. This exception is also thrown if `AttachProvider.providers()` returns an empty list.

    Parameters:
      `id` - The abstract identifier that identifies the Java virtual machine.
    Returns:
      A VirtualMachine representing the target VM.
    Throws:
      `AttachNotSupportedException` - If the `attachVirtualmachine` method of all installed providers throws `AttachNotSupportedException`, or there aren't any providers installed.
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `id` is `null`.

  - ### attach

    public static VirtualMachine attach(VirtualMachineDescriptor vmd) throws AttachNotSupportedException, IOException
    Attaches to a Java virtual machine.

    This method first invokes the `provider()` method of the given virtual machine descriptor to obtain the attach provider. It then invokes the attach provider's `attachVirtualMachine` to attach to the target VM.

    Parameters:
      `vmd` - The virtual machine descriptor.
    Returns:
      A VirtualMachine representing the target VM.
    Throws:
      `AttachNotSupportedException` - If the attach provider's `attachVirtualmachine` throws `AttachNotSupportedException`.
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `vmd` is `null`.

  - ### detach

    public abstract void detach() throws IOException
    Detach from the virtual machine.

    After detaching from the virtual machine, any further attempt to invoke operations on that virtual machine will cause an `IOException` to be thrown. If an operation (such as `loadAgent` for example) is in progress when this method is invoked then the behaviour is implementation dependent. In other words, it is implementation specific if the operation completes or throws `IOException`.

    If already detached from the virtual machine then invoking this method has no effect.

    Throws:
      `IOException` - If an I/O error occurs

  - ### provider

    public final AttachProvider provider()
    Returns the provider that created this virtual machine.
    Returns:
      The provider that created this virtual machine.

  - ### id

    public final String id()
    Returns the identifier for this Java virtual machine.
    Returns:
      The identifier for this Java virtual machine.

  - ### loadAgentLibrary

    public abstract void loadAgentLibrary(String agentLibrary, String options) throws AgentLoadException, AgentInitializationException, IOException
    Loads an agent library.

    A JVM TI client is called an agent. It is developed in a native language. A JVM TI agent is deployed in a platform specific manner but it is typically the platform equivalent of a dynamic library. Alternatively, it may be statically linked into the VM. This method causes the given agent library to be loaded into the target VM (if not already loaded or if not statically linked into the VM). It then causes the target VM to invoke the `Agent_OnAttach` function or, for a statically linked agent named 'L', the `Agent_OnAttach_L` function as specified in the JVM Tools Interface specification. Note that the `Agent_OnAttach[_L]` function is invoked even if the agent library was loaded prior to invoking this method.

    The agent library provided is the name of the agent library. It is interpreted in the target virtual machine in an implementation-dependent manner. Typically an implementation will expand the library name into an operating system specific file name. For example, on UNIX systems, the name `L` might be expanded to `libL.so`, and located using the search path specified by the `LD_LIBRARY_PATH` environment variable. If the agent named 'L' is statically linked into the VM then the VM must export a function named `Agent_OnAttach_L`.

    If the `Agent_OnAttach[_L]` function in the agent library returns an error then an `AgentInitializationException` is thrown. The return value from the `Agent_OnAttach[_L]` can then be obtained by invoking the `returnValue` method on the exception.

    Parameters:
      `agentLibrary` - The name of the agent library.
      `options` - The options to provide to the `Agent_OnAttach[_L]` function (can be `null`).
    Throws:
      `AgentLoadException` - If the agent library does not exist, the agent library is not statically linked with the VM, or the agent library cannot be loaded for another reason.
      `AgentInitializationException` - If the `Agent_OnAttach[_L]` function returns an error.
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `agentLibrary` is `null`.
    See Also:
      - `AgentInitializationException.returnValue()`

  - ### loadAgentLibrary

    public void loadAgentLibrary(String agentLibrary) throws AgentLoadException, AgentInitializationException, IOException
    Loads an agent library.

    This convenience method works as if by invoking:

    > ``loadAgentLibrary`(agentLibrary, null);`

    Parameters:
      `agentLibrary` - The name of the agent library.
    Throws:
      `AgentLoadException` - If the agent library does not exist, the agent library is not statically linked with the VM, or the agent library cannot be loaded for another reason.
      `AgentInitializationException` - If the `Agent_OnAttach[_L]` function returns an error.
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `agentLibrary` is `null`.

  - ### loadAgentPath

    public abstract void loadAgentPath(String agentPath, String options) throws AgentLoadException, AgentInitializationException, IOException
    Load a native agent library by full pathname.

    A JVM TI client is called an agent. It is developed in a native language. A JVM TI agent is deployed in a platform specific manner but it is typically the platform equivalent of a dynamic library. Alternatively, the native library specified by the agentPath parameter may be statically linked with the VM. The parsing of the agentPath parameter into a statically linked library name is done in a platform specific manner in the VM. For example, in UNIX, an agentPath parameter of `/a/b/libL.so` would name a library 'L'. See the JVM TI Specification for more details. This method causes the given agent library to be loaded into the target VM (if not already loaded or if not statically linked into the VM). It then causes the target VM to invoke the `Agent_OnAttach` function or, for a statically linked agent named 'L', the `Agent_OnAttach_L` function as specified in the JVM Tools Interface specification. Note that the `Agent_OnAttach[_L]` function is invoked even if the agent library was loaded prior to invoking this method.

    The agent library provided is the absolute path from which to load the agent library. Unlike `loadAgentLibrary`, the library name is not expanded in the target virtual machine.

    If the `Agent_OnAttach[_L]` function in the agent library returns an error then an `AgentInitializationException` is thrown. The return value from the `Agent_OnAttach[_L]` can then be obtained by invoking the `returnValue` method on the exception.

    Parameters:
      `agentPath` - The full path of the agent library.
      `options` - The options to provide to the `Agent_OnAttach[_L]` function (can be `null`).
    Throws:
      `AgentLoadException` - If the agent library does not exist, the agent library is not statically linked with the VM, or the agent library cannot be loaded for another reason.
      `AgentInitializationException` - If the `Agent_OnAttach[_L]` function returns an error.
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `agentPath` is `null`.
    See Also:
      - `AgentInitializationException.returnValue()`

  - ### loadAgentPath

    public void loadAgentPath(String agentPath) throws AgentLoadException, AgentInitializationException, IOException
    Load a native agent library by full pathname.

    This convenience method works as if by invoking:

    > ``loadAgentPath`(agentLibrary, null);`

    Parameters:
      `agentPath` - The full path to the agent library.
    Throws:
      `AgentLoadException` - If the agent library does not exist, the agent library is not statically linked with the VM, or the agent library cannot be loaded for another reason.
      `AgentInitializationException` - If the `Agent_OnAttach[_L]` function returns an error.
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `agentPath` is `null`.

  - ### loadAgent

    public abstract void loadAgent(String agent, String options) throws AgentLoadException, AgentInitializationException, IOException
    Loads an agent.

    The agent provided to this method is a path name to a JAR file on the file system of the target virtual machine. This path is passed to the target virtual machine where it is interpreted. The target virtual machine attempts to start the agent as specified by the `java.lang.instrument` specification. That is, the specified JAR file is added to the system class path (of the target virtual machine), and the `agentmain` method of the agent class, specified by the `Agent-Class` attribute in the JAR manifest, is invoked. This method completes when the `agentmain` method completes.

    Parameters:
      `agent` - Path to the JAR file containing the agent.
      `options` - The options to provide to the agent's `agentmain` method (can be `null`).
    Throws:
      `AgentLoadException` - If the agent does not exist, or cannot be started in the manner specified in the `java.lang.instrument` specification.
      `AgentInitializationException` - If the `agentmain` throws an exception
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `agent` is `null`.

  - ### loadAgent

    public void loadAgent(String agent) throws AgentLoadException, AgentInitializationException, IOException
    Loads an agent.

    This convenience method works as if by invoking:

    > ``loadAgent`(agent, null);`

    Parameters:
      `agent` - Path to the JAR file containing the agent.
    Throws:
      `AgentLoadException` - If the agent does not exist, or cannot be started in the manner specified in the `java.lang.instrument` specification.
      `AgentInitializationException` - If the `agentmain` throws an exception
      `IOException` - If an I/O error occurs
      `NullPointerException` - If `agent` is `null`.

  - ### getSystemProperties

    public abstract Properties getSystemProperties() throws IOException
    Returns the current system properties in the target virtual machine.

    This method returns the system properties in the target virtual machine. Properties whose key or value is not a `String` are omitted. The method is approximately equivalent to the invocation of the method `System.getProperties` in the target virtual machine except that properties with a key or value that is not a `String` are not included.

    This method is typically used to decide which agent to load into the target virtual machine with `loadAgent`, or `loadAgentLibrary`. For example, the `java.home` or `user.dir` properties might be use to create the path to the agent library or JAR file.

    Returns:
      The system properties
    Throws:
      `AttachOperationFailedException` - If the target virtual machine is unable to complete the attach operation. A more specific error message will be given by `Throwable.getMessage()`.
      `IOException` - If an I/O error occurs, a communication error for example, that cannot be identified as an error to indicate that the operation failed in the target VM.
    See Also:
      - `System.getProperties()`
      - `loadAgentLibrary(String, String)`
      - `loadAgent(String, String)`

  - ### getAgentProperties

    public abstract Properties getAgentProperties() throws IOException
    Returns the current agent properties in the target virtual machine.

    The target virtual machine can maintain a list of properties on behalf of agents. The manner in which this is done, the names of the properties, and the types of values that are allowed, is implementation specific. Agent properties are typically used to store communication end-points and other agent configuration details. For example, a debugger agent might create an agent property for its transport address.

    This method returns the agent properties whose key and value is a `String`. Properties whose key or value is not a `String` are omitted. If there are no agent properties maintained in the target virtual machine then an empty property list is returned.

    Returns:
      The agent properties
    Throws:
      `AttachOperationFailedException` - If the target virtual machine is unable to complete the attach operation. A more specific error message will be given by `Throwable.getMessage()`.
      `IOException` - If an I/O error occurs, a communication error for example, that cannot be identified as an error to indicate that the operation failed in the target VM.

  - ### startManagementAgent

    public abstract void startManagementAgent(Properties agentProperties) throws IOException
    Starts the JMX management agent in the target virtual machine.

    The configuration properties are the same as those specified on the command line when starting the JMX management agent. In the same way as on the command line, you need to specify at least the `com.sun.management.jmxremote.port` property.

    See the online documentation for Monitoring and Management Using JMX Technology for further details.

    Parameters:
      `agentProperties` - A Properties object containing the configuration properties for the agent.
    Throws:
      `AttachOperationFailedException` - If the target virtual machine is unable to complete the attach operation. A more specific error message will be given by `Throwable.getMessage()`.
      `IOException` - If an I/O error occurs, a communication error for example, that cannot be identified as an error to indicate that the operation failed in the target VM.
      `IllegalArgumentException` - If keys or values in agentProperties are invalid.
      `NullPointerException` - If agentProperties is null.
    Since:
      1.8

  - ### startLocalManagementAgent

    public abstract String startLocalManagementAgent() throws IOException
    Starts the local JMX management agent in the target virtual machine.

    See the online documentation for Monitoring and Management Using JMX Technology for further details.

    Returns:
      The String representation of the local connector's service address. The value can be parsed by the `JMXServiceURL(String)` constructor.
    Throws:
      `AttachOperationFailedException` - If the target virtual machine is unable to complete the attach operation. A more specific error message will be given by `Throwable.getMessage()`.
      `IOException` - If an I/O error occurs, a communication error for example, that cannot be identified as an error to indicate that the operation failed in the target VM.
    Since:
      1.8

  - ### hashCode

    public int hashCode()
    Returns a hash-code value for this VirtualMachine. The hash code is based upon the VirtualMachine's components, and satisfies the general contract of the `Object.hashCode` method.
    Overrides:
      `hashCode` in class `Object`
    Returns:
      A hash-code value for this virtual machine
    See Also:
      - `Object.equals(java.lang.Object)`
      - `System.identityHashCode(Object)`

  - ### equals

    public boolean equals(Object ob)
    Tests this VirtualMachine for equality with another object.

    If the given object is not a VirtualMachine then this method returns `false`. For two VirtualMachines to be considered equal requires that they both reference the same provider, and their `identifiers` are equal.

    This method satisfies the general contract of the `Object.equals` method.

    Overrides:
      `equals` in class `Object`
    Parameters:
      `ob` - The object to which this object is to be compared
    Returns:
      `true` if, and only if, the given object is a VirtualMachine that is equal to this VirtualMachine.
    See Also:
      - `Object.hashCode()`
      - `HashMap`

  - ### toString

    public String toString()
    Returns the string representation of the `VirtualMachine`.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string representation of the object