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Connection and Invocation Details


Connection and Invocation Details

Contents

Transports
Connectors
Oracle VM Invocation Options
Debugging Plug-in Applets
Connecting with JDB
Service Provider Interfaces


Transports

A JPDA Transport is a method of communication between a debugger and the virtual machine that is being debugged (hereafter the target VM). The communication is connection oriented - one side acts as a server, listening for a connection. The other side acts as a client and connects to the server. JPDA allows either the debugger application or the target VM to act as the server. Transport implementations can allow communications between processes running on a single machine, on different machines, or either. When establishing a connection a transport address is used to identify the end-point of the connection. The format of a transport address depends on the type of transport.

Within JPDA the debugger application uses the Java Debug Interface (JDI) interface and the Connector abstraction to establish a connection to the target VM. The Connector used by the debugger application encapsulates the transport. On the target VM an agent supporting the Java Debug Wire Protocol is used to communicate with the debugger. This agent (which may be built into the target VM or loaded from a runtime library) encapsulates the transport to communicate with the debugger.

Two transport implementations are shipped with the reference implementation: A socket transport based on TCP/IP, and a shared memory transport. The specifications do not require any specific transport implementation to exist. In addition to transports provided with the implementation the architecture includes service provider interfaces to allow additional transports be developed and deployed.

Socket Transport

The JPDA reference implementation provides a socket transport for the Linux and Microsoft Windows platforms. With the socket transport, the debugger application and the target VM can reside either on the same machine or on different machines. The socket transport uses a single stream TCP/IP connection between the debugger application and the target VM. Both IPv4 and IPv6 are supported on the JDI side and the target VM side of the socket transport.

Command and reply packets are written to the stream in accordance with the JDWP specification using the JDWP Transport Interface. Since many small packets can be sent over JDWP, the TCP_NO_DELAY socket option can improve performance in some socket implementations by avoiding delays that could occur if the socket implementation buffers small packets before sending them. Sockets are closed gracefully so that unsent data is sent if possible.

The socket transport is identified through a unique string, dt_socket. This name can be used to select the socket transport when invoking the target VM. Within the debugger application a corresponding Connector is used which encapsulates the socket transport.

Socket transport addresses have the format "<host>:<port>" where <host> is the host name or the IP address (may be enclosed in square brackets) and <port> is the socket port number to attach to or listen on. If <host> is empty, the local loopback address is used. If <host> equals "*" in contexts where a server is waiting for a client to attach, the server listens on all network interfaces.

Shared Memory Transport

In addition to the socket transport, the JPDA reference implementation provides a shared memory transport on the Microsoft Windows platform. The shared memory transport uses a shared memory region to exchange JDWP packets between the debugger application and the target VM. With the shared memory transport, the debugger application and target VM must reside on the same machine.

The shared memory transport is identified through a unique string, dt_shmem. This name can be used to select the socket transport when invoking the target VM. Within the debugger application a corresponding Connector is used which encapsulates the shared memory transport.

Shared memory transport addresses are simply names that can be used as Microsoft Windows file-mapping object names. The name string can consist of any combination of characters, excluding the backslash.

Connectors

A connector is a JDI abstraction that is used in establishing a connection between a debugger application (written to the JDI) and a target VM. Different JDI implementations are free to provide different connector implementations to match the transports and VMs they support. The connector interfaces are very general, to allow JDI to be used with varying connector implementations. Connectors are configured through a set of name/value pairs. Specific connectors accept different name/value pairs.

A good JDI client application will allow users to choose and configure any connector that may be present, but it can be beneficial to incorporate knowledge of specific connectors into the debugger to make their configuration a more pleasant user experience. The example JDB implementation provided with the JPDA illustrates this approach.

The JDI reference implementations provides several connectors which map to the available transport types and the modes of connection (launching, listening, and attaching). These connectors are described in the following sections. A List containing these connectors is returned by JDI method VirtualMachineManager.allConnectors(). In addition, each attaching, listening, and launching connector is contained in the lists returned by the corresponding VirtualMachineManager methods attachingConnectors(), listeningConnectors(), and launchingConnectors().

Command Line Launching Connector

This connector can be used by a debugger application to launch an Oracle VM or any other VM which supports the same invocation options with respect to debugging. The details of launching the VM and specifying the necessary debug options are handled by the connector. The underlying transport used by this connector depends on the platform. On Microsoft Windows, the shared memory transport is used. On Linux the socket transport is used.

This connector is uniquely identified by the name "com.sun.jdi.CommandLineLaunch".


Command Line Launching Connector Arguments
name required? default value description
home no current java.home property value Location of the Java Runtime Environment (JRE) used to invoke the Target VM.
options no "" Options, in addition to the standard debugging options, with which to invoke the VM.
main yes "" The debugged application's main class and command line arguments.
suspend no true True if the target VM is to be suspended immediately before the main class is loaded; false otherwise.
quote yes "\"" The character used to combine space-delimited text on the command line.
vmexec yes "java" The VM launcher executable. This can be changed to javaw or to java_g for debugging, if that launcher is available.

Raw Command Line Launching Connector

This connector can be used by a debugger application to launch any VM. The entire command line must be specified and it is not edited in any way. The details of launching the VM with the given command line are handled by the connector. The underlying transport used by this connector depends on the platform. On Microsoft Windows, the shared memory transport is used. On Linux, the socket transport is used.

This connector is uniquely identified by the name "com.sun.jdi.RawCommandLineLaunch".


Raw Command Line Launching Connector Arguments
name required? default value description
command yes "" Full command line to invoke the target VM with the application to be debugged..
address yes "" Transport address at which to listen for the newly launched target VM to connect. This value is typically part of the raw command argument as well, but this is not required if the target VM has some other means of determining the transport address to which it should connect.
quote yes "\"" The character used to combine space-delimited text on the command line.

Socket Attaching Connector

This connector can be used by a debugger application to attach to a currently running target VM through the socket transport. The target VM must have been invoked with options consistent with this connector's arguments described in the following table. For an Oracle VM, the necessary options are described in Oracle Invocation VM Options.

This connector is uniquely identified by the name "com.sun.jdi.SocketAttach".


Socket Attaching Connector Arguments
name required? default value description
hostname no local host name Name of host machine to connect to.
port yes "" Port number on the host machine to connect to.
timeout no "" The timeout, in milliseconds, to use when attaching to the target VM.

Shared Memory Attaching Connector

This connector can be used by a debugger application to attach to a currently running target VM through the shared memory transport. It is available only on the Microsoft Windows platform. The target VM must have been invoked with options consistent with this connectors arguments described in the following table. For an Oracle VM, the necessary options are described in Oracle VM Invocation Options.

This connector is uniquely identified by the name "com.sun.jdi.SharedMemoryAttach".


Shared Memory Attaching Connector Arguments
name required? default value description
name yes "" The shared memory transport address at which the target VM is listening.
timeout no "" The timeout, in milliseconds, to use when attaching to the target VM

Socket Listening Connector

This connector can be used by a debugger application to accept a connection from a separately invoked target VM through the socket transport.. The target VM must be invoked with options consistent with this connector's arguments described in the following table. For an Oracle VM, the necessary options are described in Oracle VM Invocation Options.

This connector can accept connections from multiple target VMs.

This connector is uniquely identified by the name "com.sun.jdi.SocketListen".


Socket Listening Connector Arguments
name required? default value description
port no Ephemeral port number (port assigned by the TCP/IP stack) Port number on which to listen for a connection
localAddress no Loopback address Host name or IP address on which to listen for a connection
timeout no "" The timeout, in milliseconds, to use while waiting for the target VM to connect

Shared Memory Listening Connector

This connector can be used by a debugger application to accept a connection from a separately invoked target VM through the shared memory transport. It is available only on the Microsoft Windows platform. The target VM must be invoked with options consistent with this connector's arguments described in the following table. For an Oracle VM, the necessary options are described in Oracle VM Invocation Options.

This connector can accept connections from multiple target VMs.

This connector is uniquely identified by the name "com.sun.jdi.SharedMemoryListen".


Shared Memory Listening Connector Arguments
name required? default value description
name yes "" A shared memory transport address at which to listen for the target VM connection.
timeout no "" The timeout, in milliseconds, to use while waiting for the target VM to connect

Process Attaching Connector

This connector can be used by a debugger application to attach to a currently running target VM that was started with the "server=y" debugging sub-option described in Oracle VM Invocation Options. The target VM must be Java SE 6 or newer.

The Process Attaching Connector does not have an associated transport. Instead, the transport is determined dynamically when an attach actually occurs. Because of this, the transport().name() method for this connector returns "local".

This connector is uniquely identified by the name "com.sun.jdi.ProcessAttach".

Process Attaching Connector Arguments
name required? default value description
pid yes "" The Process ID of a process to be debugged.
timeout no "" The timeout, in milliseconds, to use when attaching to the target VM.



Oracle VM Invocation Options

This section describes the options necessary to invoke an Oracle VM for debugging.

Oracle's VM implementation requires command line options to load the JDWP agent for debugging. The -agentlib:jdwp option is used to load and specify options to the JDWP agent.

The -agentlib:jdwp option is specified as follows:

-agentlib:jdwp=<sub-options>
Loads the JPDA reference implementation of JDWP. This library resides in the target VM and uses JVM TI and JNI to interact with it. It uses a transport and the JDWP protocol to communicate with a separate debugger application. Specific sub-options are described in -Xrunjdwp Sub-options.

-agentlib:jdwp and -Xrunjdwp sub-options

The -agentlib:jdwp and -Xrunjdwp option can be further qualified with sub-options. The sub-options are specified as follows:

    -agentlib:jdwp=<name1>[=<value1>],<name2>[=<value2>]...

or

    -Xrunjdwp:<name1>[=<value1>],<name2>[=<value2>]...

The following table describes the options that can be used:

-Xrunjdwp Sub-options
name required? default value description
help no N/A Prints a brief help message and exits the VM.
transport yes none Name of the transport to use in connecting to debugger application.
server no "n"

If "y", listen for a debugger application to attach; otherwise, attach to the debugger application at the specified address.

If "y" and no address is specified, choose a transport address at which to listen for a debugger application, and print the address to the standard output stream.

address yes, if server=n no, otherwise "" Transport address for the connection. If server=n, attempt to attach to debugger application at this address. If server=y, listen for a connection at this address.
timeout no "" If server=y specifies the timeout, in milliseconds, to wait for the debugger to attach. If server=n specifies the timeout, in milliseconds, to use when attaching to the debugger. Note that the timeout option may be ignored by some transport implementations.
launch no none

At completion of JDWP initialization, launch the process given in this string. This option is used in combination with onthrow and/or onuncaught to provide "Just-In-Time debugging" in which a debugger process is launched when a particular event occurs in this VM.

Note that the launched process is not started in its own window. In most cases the launched process should be a small application which in turns launches the debugger application in its own window.

The following strings are appended to the string given in this argument (space-delimited). They can aid the launched debugger in establishing a connection with this VM. The resulting string is executed.

  • The value of the transport sub-option.
  • The value of the address sub-option (or the generated address if one is not given)
onthrow no none Delay initialization of the JDWP library until an exception of the given class is thrown in this VM. The exception class name must be package-qualified. Connection establishment is included in JDWP initialization, so it will not begin until the exception is thrown.
onuncaught no "n" If "y", delay initialization of the JDWP library until an uncaught exception is thrown in this VM. Connection establishment is included in JDWP initialization, so it will not begin until the exception is thrown. See the JDI specification for com.sun.jdi.ExceptionEvent for a definition of uncaught exceptions.
suspend no "y" If "y", VMStartEvent has a suspendPolicy of SUSPEND_ALL. If "n", VMStartEvent has a suspendPolicy of SUSPEND_NONE.

Additionally dt_socket transport supports the following option:

allow no "*"

If server=y, allows connections only from the addresses/subnets specified.

The value may be either "*" (which allows connections from any address), or a list of '+' separated addresses.

Each entry in the list can be:

  • A single IP address (e.g. 127.0.0.1 or ::1).
  • A group of addresses (subnet) defined by subnet address and prefix length (e.g. 192.168.1.0/24 defines address range 192.168.1.0 - 192.168.1.255).

    For IPv4, the prefix length must be in range 1..31; for IPv6, the range is 1..127.

Examples

-agentlib:jdwp=transport=dt_socket,server=y,address=8000
Listen for a socket connection on port 8000 at the loopback address only. Suspend this VM before main class loads (suspend=y by default). Once the debugger application connects, it can send a JDWP command to resume the VM.

-agentlib:jdwp=transport=dt_socket,server=y,address=*:8000,allow=192.168.1.0/24+::1,timeout=5000
Listen for a socket connection on port 8000 on all network interfaces. Allow the debugger to connect only from addresses 192.168.1.00 - 192.168.1.255 and from the local machine by IPv6 address (::1). Terminate if the debugger does not attach within 5 seconds. Suspend this VM before main class loads (suspend=y by default). Once the debugger application connects, it can send a JDWP command to resume the VM.

-agentlib:jdwp=transport=dt_shmem,server=y,suspend=n
Choose an available shared memory transport address and print it to stdout. Listen for a shared memory connection at that address. Allow the VM to begin executing before the debugger application attaches.

-agentlib:jdwp=transport=dt_socket,address=myhost:8000
Attach to a running debugger application via socket on host myhost on port 8000. Suspend this VM before the main class loads.

-agentlib:jdwp=transport=dt_shmem,address=mysharedmemory
Attach to a running debugger application via shared memory at transport address "mysharedmemory". Suspend this VM before the main class loads.

-agentlib:jdwp=transport=dt_socket,server=y,address=192.168.1.18:8000,allow=*,onthrow=java.io.IOException,launch=/usr/local/bin/debugstub
Wait for an instance of java.io.IOException to be thrown in this VM. Suspend the VM (suspend=y by default). Listen for a socket connection at address 192.168.1.18 on port 8000. Allow the debugger to connect from any address. Execute the following: "/usr/local/bin/debugstub dt_socket myhost:8000".This program can launch a debugger process in a separate window which will attach to this VM and begin debugging it.

-agentlib:jdwp=transport=dt_shmem,server=y,onuncaught=y,launch=d:\bin\debugstub.exe
Wait for an uncaught exception to be thrown in this VM. Suspend the VM. Select a shared memory transport address and listen for a connection at that address. Execute the following: "d:\bin\debugstub.exe dt_shmem <address>", where <address> is the selected shared memory address. This program can launch a debugger process in a separate window which will attach to this VM and begin debugging it.

Debugging Plug-in Applets

Applets running under Java Plug-in can be debugged. The required VM options described above can be specified in the Plug-In control panel, under the "Basic" tab, in the "Java Run-Time Parameters". The debug options under the "Advanced" tab should not be used as they will enable the old sun.tools.debug debug support. In future versions of Java Plug-in, these debug options will be changed to use JPDA.

JDI launching connectors cannot be used to debug Plug-in applets.

Connecting with JDB

The example implementation of JDB provided with the JPDA, provides an illustration of the usage of JDI connectors. There are "shortcut" options to JDB which assume the use of connectors known to it (that is, connectors present in the reference implementation). It also provides a way to establish a general connection using any connector. While JDB is hardly an example of a good debugger interface, it does provide a simple example of connectors in use.

In JDB, the -attach option provides access to one of the attaching connectors in the reference implementation (shared memory on Microsoft Windows, sockets on the Linux platform). The -listen option provides access to one of the listening connectors in the reference implementation (shared memory on Microsoft Windows, sockets on the Linux platform). A class name and arguments specified directly on the command line provide access to the command line launching connector.

For example:

jdb -attach myhost:8000

is an easy way to attach to a target VM with the Socket Attaching Connector (on Linux), and

jdb Hello 1 2 3

is an easy way to launch a target VM with the Command Line Launching Connector.

However, the -connect option is also provided by JDB to handle any connector by taking an connector name and a set of arbitrary name/value argument pairs. For example the command lines above have the following equivalents.

jdb -connect com.sun.jdi.SocketAttach:hostname=myhost,port=8000
jdb -connect "com.sun.jdi.CommandLineLaunch:main=Hello 1 2 3"

These command lines are more cumbersome than the ones above, but the -connect option can be used with any connector. This kind of operation is a primitive example of how a JDI debugger can deal with any kind of connector while providing a simplified interface for dealing with common, well-known connectors.


Service Provider Interfaces

JPDA includes service provider interfaces to allow the development and deployment of connector and transport implementations. These service provider interfaces allow debugger and other tool vendors develop new connector implementations and provide addition transport mechanisms over and beyond the socket and shared memory transport provided by Oracle. The service provider interfaces in JDI are specified in the com.sun.jdi.connect.spi package.

In addition to the service provider interfaces in JDI the Oracle implementation also includes a transport library interface called the Java Debug Wire Protocol Transport Interface. A transport library is loaded by the JDWP agent in the target VM and is used to establish a connection to the debugger and to transport JDWP packets between the debugger and the VM.

Further information on the service provider interfaces can be in the document Java Platform Debugger Architecture - Service Provider Interfaces.