public abstract class

VirtualMachine

extends Object
java.lang.Object
   ↳ com.sun.tools.attach.VirtualMachine
Known Direct Subclasses

Class Overview

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 in a dynamic library 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");

      // get system properties in target VM
      Properties props = vm.getSystemProperties();

      // construct path to management agent
      String home = props.getProperty("java.home");
      String agent = home + File.separator + "lib" + File.separator
          + "management-agent.jar";

      // load agent into target VM
      vm.loadAgent(agent, "com.sun.management.jmxremote.port=5000");

      // detach
      vm.detach();

In this example we attach to a Java virtual machine that is identified by the process identifier 2177. The system properties from the target VM are then used to construct the path to a management agent which is then loaded into the target VM. Once loaded the client detaches from the target VM.

A VirtualMachine is safe for use by multiple concurrent threads.

Summary

Protected Constructors
VirtualMachine(AttachProvider provider, String id)
Initializes a new instance of this class.
Public Methods
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.
String toString()
Returns the string representation of the VirtualMachine.
[Expand]
Inherited Methods
From class java.lang.Object

Protected Constructors

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.

Public Methods

public static VirtualMachine attach (VirtualMachineDescriptor vmd)

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
SecurityException If a security manager has been installed and it denies AttachPermission ("attachVirtualMachine"), or another permission required by the implementation.
AttachNotSupportedException If the attach provider's attachVirtualmachine throws AttachNotSupportedException.
IOException If an I/O error occurs
NullPointerException If vmd is null.

public static VirtualMachine attach (String id)

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
SecurityException If a security manager has been installed and it denies AttachPermission ("attachVirtualMachine"), or another permission required by the implementation.
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.

public abstract void detach ()

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

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.

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.

public abstract Properties getAgentProperties ()

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
IOException If an I/O error occurs

public abstract Properties getSystemProperties ()

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
IOException If an I/O error occurs
See Also

public int hashCode ()

Returns a hash-code value for this VirtualMachine. The hash code is based upon the VirtualMachine's components, and satifies the general contract of the Object.hashCode method.

Returns
  • A hash-code value for this virtual machine

public final String id ()

Returns the identifier for this Java virtual machine.

Returns
  • The identifier for this Java virtual machine.

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.

public void loadAgent (String agent)

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.

public abstract void loadAgent (String agent, String options)

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.

public void loadAgentLibrary (String agentLibrary)

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, or cannot be loaded for another reason.
AgentInitializationException If the Agent_OnAttach function returns an error
IOException If an I/O error occurs
NullPointerException If agentLibrary is null.

public abstract void loadAgentLibrary (String agentLibrary, String options)

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. This method causes the given agent library to be loaded into the target VM (if not already loaded). It then causes the target VM to invoke the Agent_OnAttach function as specified in the JVM Tools Interface specification. Note that the Agent_OnAttach 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 foo might be expanded to libfoo.so, and located using the search path specified by the LD_LIBRARY_PATH environment variable.

If the Agent_OnAttach function in the agent library returns an error then an AgentInitializationException is thrown. The return value from the Agent_OnAttach 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 function (can be null).
Throws
AgentLoadException If the agent library does not exist, or cannot be loaded for another reason.
AgentInitializationException If the Agent_OnAttach function returns an error
IOException If an I/O error occurs
NullPointerException If agentLibrary is null.
See Also

public void loadAgentPath (String agentPath)

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, or cannot be loaded for another reason.
AgentInitializationException If the Agent_OnAttach function returns an error
IOException If an I/O error occurs
NullPointerException If agentPath is null.

public abstract void loadAgentPath (String agentPath, String options)

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. This method causes the given agent library to be loaded into the target VM (if not already loaded). It then causes the target VM to invoke the Agent_OnAttach function as specified in the JVM Tools Interface specification. Note that the Agent_OnAttach 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 function in the agent library returns an error then an AgentInitializationException is thrown. The return value from the Agent_OnAttach 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 function (can be null).
Throws
AgentLoadException If the agent library does not exist, or cannot be loaded for another reason.
AgentInitializationException If the Agent_OnAttach function returns an error
IOException If an I/O error occurs
NullPointerException If agentPath is null.
See Also

public final AttachProvider provider ()

Returns the provider that created this virtual machine.

Returns
  • The provider that created this virtual machine.

public String toString ()

Returns the string representation of the VirtualMachine.

Returns
  • a string representation of the object.