Contents
Transports
Connectors
VM Invocation
Options
Connecting with
JDB
Service Provider
Interfaces
Transports
A Java Platform Debugger Architecture (JPDA) Transport is a method of communication between a debugger and the virtual machine that is being debugged (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 endpoint 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. See the section Connectors. 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. See the section Service Provider Interfaces.
Socket Transport
The JPDA reference implementation provides a socket transport for the Linux, macOS, 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. Because 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. See the section VM Invocation Options. 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 Windows. 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. See the section VM Invocation Options. 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 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, which allows 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 allows 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. See the section Connecting with JDB.
The JDI reference implementations provides several connectors that
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 the 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
A debugger application can use this connector to launch any VM that supports the same invocation debugging options as described in the section VM Invocation Options. 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 Windows, the shared memory transport is used. On Linux and macOS, the socket transport is used.
This connector is uniquely identified by the name
com.sun.jdi.CommandLineLaunch.
| 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. See VM Invocation Options. |
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
A debugger application can use this connector 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 Windows, the shared memory transport is used. On Linux and macOS, the socket transport is used.
This connector is uniquely identified by the name
com.sun.jdi.RawCommandLineLaunch.
| 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. See the section Transports. This value is typically part of the raw command-line 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. The section VM Invocation Options describes the required options.
This connector is uniquely identified by the name
com.sun.jdi.SocketAttach.
| 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 Windows. The target VM must have been invoked with options consistent with this connectors arguments described in the following table. The section VM Invocation Options describes the required options.
This connector is uniquely identified by the name
com.sun.jdi.SharedMemoryAttach.
| Name | Required? | Default Value | Description |
|---|---|---|---|
name
|
yes | "" | The shared memory transport address at which the target VM is listening. See the section Transports. |
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. The section VM Invocation Options describes the required options.
This connector can accept connections from multiple target VMs.
This connector is uniquely identified by the name
com.sun.jdi.SocketListen.
| 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 Windows. The target VM must be invoked with options consistent with this connector's arguments, which are described in the following table. The section VM Invocation Options describes the required options.
This connector can accept connections from multiple target VMs.
This connector is uniquely identified by the name
com.sun.jdi.SharedMemoryListen.
| 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 suboption described in the section 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.
| 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. |
VM Invocation Options
This section describes the options required to invoke a 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=<suboptions>
- Loads the JPDA reference implementation of JDWP. This library resides in the target VM and uses the Java Virtual Machine Tool Interface (JVM TI) and Java Native Interface (JNI) to interact with it. It uses a transport and the JDWP protocol to communicate with a separate debugger application. The section -agentlib:jdwp and -Xrunjdwp Suboptions describes specific suboptions.
-agentlib:jdwp and -Xrunjdwp Suboptions
The -agentlib:jdwp and -Xrunjdwp options
can be further qualified with suboptions. The suboptions 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:
| 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. See the section Transports. |
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. See the
section Transports.
|
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
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.
|
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. |
includevirtualthreads
|
no | "n" | If "y", virtual threads are included when the debugger requests the list of all running threads. Virtual threads created before attaching to the debugger may not be included. If the number of virtual threads is very large, this can overwhelm the debugger. If "y", also causes the JDWP library to remember all created virtual threads until their death, which can overwhelm the JDWP library if the number is large. |
|
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 addresses separated by the plus ( Each entry in the list can be:
|
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=yby 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=yby 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.IOExceptionto be thrown in this VM. Suspend the VM (suspend=yby 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. -
Connecting with JDB
The example implementation of the Java Debugger (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
Windows, sockets on Linux and macOS). The -listen option
provides access to one of the listening connectors in the reference
implementation (shared memory on Windows, sockets on Linux and macOS). 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 on macOS), 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 previous command lines 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 to 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.
See Java Platform Debugger Architecture - Service Provider Interfaces for more information.