Class 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

Constructors

Modifier Constructor Description
protected VirtualMachine​(AttachProvider provider, String id)

Initializes a new instance of this class.

Methods

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.
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.
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 java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Constructors

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.

Methods

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:
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.

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:
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.

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(java.lang.String, java.lang.String), loadAgent(java.lang.String, java.lang.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 satifies 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(java.lang.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.

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https://docs.oracle.com/en/java/javase/11/docs/api/jdk.attach/com/sun/tools/attach/VirtualMachine.html