Java Virtual Machine Tool Interface

The Java™ Virtual Machine Tool Interface (JVMTI) is a two-way interface that allows communication between the VM and a native agent. It replaces both the Java Virtual Machine Debug Interface (JVMDI) and Java Virtual Machine Profiler Interface (JVMPI).

Overview

The JVMTI allows third parties to develop debugging, profiling, and monitoring tools for the VM. The interface contains mechanisms for the agent to notify the VM about the kinds of information it requires, and also provides a means of receiving relevant notifications.

Several agents can be attached to a VM at any one time.

JVMTI agents can be loaded at startup using short or long forms of the command-line option:

-agentlib:<agent-lib-name>=<options>

or

-agentpath:<path-to-agent>=<options>

In the example that follows (see Sample JVMTI agent), the directory containing the jdwp library is assumed to be on the library path. If you require a specific library, such as jdwp, with your JVMTI agent, you can specify the path at startup, for example:

-agentlib:jdwp=<options>

For more information about -agentlib:jdwp, see -Xrunjdwp.

For more information about JVMTI, see https://docs.oracle.com/javase/8/docs/technotes/guides/management/index.html.

For a guide about writing a JVMTI agent, see http://www.oracle.com/technetwork/articles/javase/jvmti-136367.html.

Eclipse OpenJ9 extensions

Eclipse OpenJ9™ extensions to the JVMTI allow a JVMTI agent to query or automatically trigger operations in the VM, including the following tasks:

Task	OpenJ9 extensions
Get the OS thread ID	`GetOSThreadID`
Query, set, and reset the VM dump options	`QueryVmDump`, `SetVmDump`, `ResetVmDump`
Trigger a VM dump, and monitor JVMTI event functions when VM dumps start and end	`TriggerVmDump`, `VMDumpStart`, `VMDumpEnd`
Set VM trace options	`SetVmTrace`
Subscribe to and unsubscribe from VM tracepoints	`RegisterTracePointSubscriber`, `DeregisterTracePointSubscriber`
Query runtime environment native memory categories	`GetMemoryCategories`
Query and set VM log options	`QueryVmLogOptions`, `SetVmLogOptions`
Search for and remove a shared classes cache	`IterateSharedCaches`, `DestroySharedCache`
Subscribe to and unsubscribe from verbose garbage collection (GC) data logging	`RegisterVerboseGCSubscriber`, `DeregisterVerboseGCSubscriber`

The definitions that you need when you write a JVMTI agent are provided in the header files jvmti.h and ibmjvmti.h, in the include directory.

Sample JVMTI agent

The following sample shows you how to write a simple JVMTI agent that uses OpenJ9 extensions to the JVMTI.

Sample JVMTI agent written in C/C++, which uses the OpenJ9 extensions

/*
 * tiSample.c
 *
 * Sample JVMTI agent to demonstrate the OpenJ9 JVMTI dump extensions
 */

#include "jvmti.h"
#include "ibmjvmti.h"

/* Forward declarations for JVMTI callback functions */
void JNICALL VMInitCallback(jvmtiEnv *jvmti_env, JNIEnv* jni_env, jthread thread);
void JNICALL DumpStartCallback(jvmtiEnv *jvmti_env, char* label, char* event, char* detail, ...);


/*
 * Agent_Onload()
 *
 * JVMTI agent initialisation function, invoked as agent is loaded by the VM
 */
JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *jvm, char *options, void *reserved) {

  jvmtiEnv *jvmti = NULL;
  jvmtiError rc;
  jint extensionEventCount = 0;
  jvmtiExtensionEventInfo *extensionEvents = NULL;
  jint extensionFunctionCount = 0;
  jvmtiExtensionFunctionInfo *extensionFunctions = NULL;
  int i = 0, j = 0;

  printf("tiSample: Loading JVMTI sample agent\n");

    /* Get access to JVMTI */
    (*jvm)->GetEnv(jvm, (void **)&jvmti, JVMTI_VERSION_1_0);

     /* Look up all the JVMTI extension events and functions */
   (*jvmti)->GetExtensionEvents(jvmti, &extensionEventCount, &extensionEvents);
   (*jvmti)->GetExtensionFunctions(jvmti, &extensionFunctionCount, &extensionFunctions);

    printf("tiSample: Found %i JVMTI extension events, %i extension functions\n", extensionEventCount, extensionFunctionCount);

   /* Find the JVMTI extension event we want */
   while (i++ < extensionEventCount) {

        if (strcmp(extensionEvents->id, COM_IBM_VM_DUMP_START) == 0) {
            /* Found the dump start extension event, now set up a callback for it */
            rc = (*jvmti)->SetExtensionEventCallback(jvmti, extensionEvents->extension_event_index, &DumpStartCallback);
            printf("tiSample: Setting JVMTI event callback %s, rc=%i\n", COM_IBM_VM_DUMP_START, rc);
            break;
        }
        extensionEvents++; /* move on to the next extension event */
    }

    /* Find the JVMTI extension function we want */
    while (j++ < extensionFunctionCount) {
        jvmtiExtensionFunction function = extensionFunctions->func;

        if (strcmp(extensionFunctions->id, COM_IBM_SET_VM_DUMP) == 0) {
            /* Found the set dump extension function, now set a dump option to generate javadumps on
          thread starts */
            rc = function(jvmti, "java:events=thrstart");
            printf("tiSample: Calling JVMTI extension %s, rc=%i\n", COM_IBM_SET_VM_DUMP, rc);
            break;
        }
        extensionFunctions++; /* move on to the next extension function */
     }

     return JNI_OK;
}


/*
 * DumpStartCallback()
 * JVMTI callback for dump start event (IBM JVMTI extension)  */
void JNICALL
DumpStartCallback(jvmtiEnv *jvmti_env, char* label, char* event, char* detail, ...) {
      printf("tiSample: Received JVMTI event callback, for event %s\n", event);
}

The sample JVMTI agent consists of two functions, Agent_OnLoad() and DumpStartCallback():

`Agent_OnLoad()`

This function is called by the VM when the agent is loaded at VM startup, which allows the JVMTI agent to modify VM behavior before initialization is complete. The sample agent obtains access to the JVMTI interface by using the JNI Invocation API function GetEnv(). The agent calls the APIs GetExtensionEvents() and GetExtensionFunctions() to find the JVMTI extensions that are supported by the VM. These APIs provide access to the list of extensions available in the jvmtiExtensionEventInfo and jvmtiExtensionFunctionInfo structures. The sample uses an extension event and an extension function in the following way:

Extension event: The sample JVMTI agent searches for the extension event VmDumpStart in the list of jvmtiExtensionEventInfo structures, by using the identifier COM_IBM_VM_DUMP_START provided in ibmjvmti.h. When the event is found, the JVMTI agent calls the JVMTI interface SetExtensionEventCallback() to enable the event, providing a function DumpStartCallback() that is called when the event is triggered.

Extension function: Next, the sample JVMTI agent searches for the extension function SetVMDump in the list of jvmtiExtensionFunctionInfo structures, by using the identifier COM_IBM_SET_VM_DUMP provided in ibmjvmti.h. The JVMTI agent calls the function by using the jvmtiExtensionFunction pointer to set a VM dump option java:events=thrstart. This option requests the VM to trigger a Java dump every time a VM thread is started.

`DumpStartCallback()`

This callback function issues a message when the associated extension event is called. In the sample code, DumpStartCallback() is used when the VmDumpStart event is triggered.

Using the sample JVMTI agent

Build the sample JVMTI agent:

Windows:
```
cl /I<jre_path>\include /MD /FetiSample.dll tiSample.c /link /DLL
```
Linux, AIX®, and z/OS®:
```
gcc -I<jre_path>/include -o libtiSample.so -shared tiSample.c
```
where <jre_path> is the path to your Java runtime environment installation.
To run the sample JVMTI agent, use the command:
```
java -agentlib:tiSample -version
```
When the sample JVMTI agent loads, messages are generated. When the JVMTI agent initiates a Java dump, the message JVMDUMP010 is issued.

API reference

The following sections provide reference information for the OpenJ9 extensions to the JVMTI.

`GetOSThreadID`

You can get the OS thread ID by using the GetOSThreadID() API:

jvmtiError GetOSThreadID(jvmtiEnv* jvmti_env, jthread thread, jlong * threadid_ptr);

Parameters

jvmti_env: A pointer to the JVMTI environment.
thread: The thread for which the ID is required.
threadid_ptr: A pointer to a variable, used to return the thread ID that corresponds to the thread specified by the thread parameter.

Returns

JVMTI_ERROR_NONE: Success
JVMTI_ERROR_NULL_POINTER: The threadid_ptr parameter is null.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_INVALID_THREAD: The thread is not valid.
JVMTI_ERROR_THREAD_NOT_ALIVE: The VM state of the thread is not started or has died.
JVMTI_ERROR_UNATTACHED_THREAD: The current thread is not attached to the VM. JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI start or live phase.

Identifiers

JVMTI Extension Function identifier: com.ibm.GetOSThreadID
Macro declaration in the ibmjvmti.h file: COM_IBM_GET_OS_THREAD_ID

`QueryVmDump`

You can query the VM dump options that are set for a VM by using the QueryVmDump() API:

jvmtiError QueryVmDump(jvmtiEnv* jvmti_env, jint buffer_size, void* options_buffer, jint* data_size_ptr)

This extension returns a set of dump option specifications as ASCII strings. The syntax of the option string is the same as the -Xdump command-line option, with the initial -Xdump: omitted. See -Xdump. The option strings are separated by newline characters. If the memory buffer is too small to contain the current VM dump option strings, you can expect the following results:

The error message JVMTI_ERROR_ILLEGAL_ARGUMENT is returned.
The variable for data_size_ptr is set to the required buffer size.

Parameters

jvmti_env: A pointer to the JVMTI environment.
buffer_size: The size of the supplied memory buffer in bytes.
options_buffer: A pointer to the supplied memory buffer.
data_size_ptr: A pointer to a variable, used to return the total size of the option strings.

Returns

JVMTI_ERROR_NONE: Success
JVMTI_ERROR_NULL_POINTER: The options_buffer or data_size_ptr parameters are null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: The dump configuration is locked because a dump is in progress.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The supplied memory buffer in options_buffer is too small.

Identifiers

JVMTI Extension Function identifier: com.ibm.QueryVmDump
Macro declaration in the ibmjvmti.h file: COM_IBM_QUERY_VM_DUMP

`SetVmDump`

You can set VM dump options by using the SetVmDump() API:

jvmtiError SetVmDump(jvmtiEnv* jvmti_env, char* option)

The dump option is passed in as an ASCII character string. Use the same syntax as the -Xdump command-line option, with the initial -Xdump: omitted. See -Xdump.

When dumps are in progress, the dump configuration is locked, and calls to SetVmDump() fail with a return value of JVMTI_ERROR_NOT_AVAILABLE.

Parameters

jvmti_env: A pointer to the JVMTI environment.
option: The VM dump option string.
Note: On z/OS, you might need to convert the option string from EBCDIC to ASCII before using this JVMTI extension function.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: The parameter option is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: The dump configuration is locked because a dump is in progress.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The parameter option contains an invalid -Xdump string.

Identifiers

JVMTI Extension Function identifier: com.ibm.SetVmDump
Macro declaration in the ibmjvmti.h file: COM_IBM_SET_VM_DUMP

`TriggerVmDump`

You can trigger a VM dump and specify the type of dump you want by using the TriggerVmDump() API:

jvmtiError TriggerVmDump(jvmtiEnv* jvmti_env, char* option)

Choose the type of dump required by specifying an ASCII string that contains one of the supported dump agent types. See -Xdump. JVMTI events are provided at the start and end of the dump.

Parameters

jvmti_env: A pointer to the JVMTI environment.
option: A pointer to the dump type string, which can be one of the following types:

stack
java
system
console
tool
heap
snap
ceedump (z/OS only)

Note: On z/OS, you might need to convert the option string from EBCDIC to ASCII before using this JVMTI extension function.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: The option parameter is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: The dump configuration is locked because a dump is in progress.

Identifiers

JVMTI Extension Function identifier: com.ibm.TriggerVmDump
Macro declaration in the ibmjvmti.h file: COM_IBM_TRIGGER_VM_DUMP

`ResetVmDump`

You can reset VM dump options to the values at VM initialization by using the ResetVmDump() API:

jvmtiError ResetVmDump(jvmtiEnv* jvmti_env)

Parameters

jvmti_env: The JVMTI environment pointer.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: The dump configuration is locked because a dump is in progress.

Identifiers

JVMTI Extension Function identifier: com.ibm.ResetVmDump
Macro declaration in the ibmjvmti.h file: COM_IBM_RESET_VM_DUMP

`VMDumpStart`

The following JVMTI event function is called when a VM dump starts:

void JNICALL VMDumpStart(jvmtiEnv *jvmti_env, JNIEnv* jni_env, char* label, char* event, char* detail)

The event function provides the dump file name, the name of the JVMTI event, and the detail string from the dump event. The detail string provides additional information about the event that triggered the dump. This information is the same as the information detailed in the JVMDUMP039I message. For example:

JVMDUMP039I Processing dump event "systhrow", detail "java/lang/OutOfMemoryError" at 2014/10/17 13:31:03 - please wait."

Parameters

jvmti_env: JVMTI environment pointer.
jni_env: JNI environment pointer for the thread on which the event occurred.
label: The dump file name, including directory path.
event: The extension event name, such as com.ibm.VmDumpStart.
detail: The dump event detail string. The string can be empty.

Returns

None

`VMDumpEnd`

The following JVMTI event function is called when a VM dump ends:

void JNICALL VMDumpEnd(jvmtiEnv *jvmti_env, JNIEnv* jni_env, char* label, char* event, char* detail)

The event function provides the dump file name, the name of the JVMTI event, and the detail string from the dump event. The detail string provides additional information about the event that triggered the dump. This information is the same as the information detailed in the JVMDUMP039I message. For example:

JVMDUMP039I Processing dump event "systhrow", detail "java/lang/OutOfMemoryError" at 2014/10/17 13:31:03 - please wait.

Parameters

jvmti_env: JVMTI environment pointer.
jni_env: JNI environment pointer for the thread on which the event occurred.
label: The dump file name, including directory path.
event: The extension event name com.ibm.VmDumpEnd.
detail: The dump event detail string. The string can be empty.

Returns

None

`SetVmTrace`

You can set VM trace options by using the SetVmTrace() API:

jvmtiError SetVmTrace(jvmtiEnv* jvmti_env, char* option)

The trace option is passed in as an ASCII character string. Use the same syntax as the -Xtrace command-line option, with the initial -Xtrace: omitted. See -Xtrace.

Parameters

jvmti_env: JVMTI environment pointer.
option: Enter the VM trace option string.
Note: On z/OS, you might need to convert the option string from EBCDIC to ASCII before using this JVMTI extension function.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: The option parameter is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The option parameter contains an invalid -Xtrace string.

Identifiers

JVMTI Extension Function identifier: com.ibm.SetVmTrace
Macro declaration in the ibmjvmti.h file: COM_IBM_SET_VM_TRACE

`RegisterTracePointSubscriber`

You can subscribe to VM tracepoints by using the RegisterTracePointSubscriber() API:

jvmtiError RegisterTracePointSubscriber(jvmtiEnv* jvmti_env, char *description, jvmtiTraceSubscriber subscriber, jvmtiTraceAlarm alarm, void *userData, void **subscriptionID)

Parameters

jvmti_env: A pointer to the JVMTI environment.
description: An ASCII character string that describes the subscriber.
subscriber: A function of type jvmtiTraceSubscriber.
alarm: A function pointer of type jvmtiTraceAlarm.
user_data: A pointer to user data. This pointer is passed to the subscriber and alarm functions each time these functions are called. This pointer can be a null value.
subscription_id: A pointer to a subscription identifier. This pointer is returned by the RegisterTracePointSubscriber call if successful. The value must be supplied to a future call to the DeregisterTracePointSubscriber API, which is used to unsubscribe from the VM tracepoint.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: One of the supplied parameters is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is not valid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: VM trace is not available.
JVMTI_ERROR_INTERNAL: An internal error occurred.

Identifiers

JVMTI Extension Function identifier: com.ibm.RegisterTracePointSubscriber
Macro declaration in the ibmjvmti.h file: COM_IBM_REGISTER_TRACEPOINT_SUBSCRIBER

`jvmtiTraceSubscriber` function

The subscriber function type is defined as follows:

typedef jvmtiError (*jvmtiTraceSubscriber)(jvmtiEnv *jvmti_env, void *record, jlong length, void *user_data);

The subscriber function must be of type jvmtiTraceSubscriber, which is declared in ibmjvmti.h.

This function is called with each tracepoint record that is selected through the -Xtrace:external option.

The tracepoint record that is supplied to the subscriber function is valid only for the duration of the function. If the subscriber wants to save the data, the data must be copied elsewhere.

If the subscriber function returns an error, the alarm function is called, the subscription is disconnected, and no further tracepoints are sent to the subscriber.

Subscriber function parameters

jvmti_env: A pointer to the JVMTI environment.
record: A UTF-8 string that contains a tracepoint record.
length: The number of UTF-8 characters in the tracepoint record.
user_data: User data that is supplied when the subscriber is registered.

`jvmtiTraceAlarm` function

The alarm function type is defined as follows:

typedef jvmtiError (*jvmtiTraceAlarm)(jvmtiEnv *jvmti_env, void *subscription_id, void *user_data);

The alarm function must be of type jvmtiTraceAlarm, which is declared in ibmjvmti.h. This function is called if the subscriber function returns an error.

Alarm function parameters

jvmti_env: A pointer to the JVMTI environment.
subscription_id: The subscription identifier.
user_data: User data that is supplied when the subscriber is registered.

`DeregisterTracePointSubscriber`

You can unsubscribe from VM tracepoints by using the DeregisterTracePointSubscriber() API:

jvmtiError DeregisterTracePointSubscriber(jvmtiEnv* jvmti_env, void *userData, void *subscription_id)

After the DeregisterTracePointSubscriber() API is called, no further calls are made to the subscriber function.

Parameters

jvmti_env: A pointer to the JVMTI environment.
subscription_id: The subscription identifier that is returned by the call to the RegisterTracePointSubscriber API.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: The subscription_id parameter is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is not valid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.

Identifiers

JVMTI Extension Function identifier: com.ibm.DeregisterTracePointSubscriber
Macro declaration in the ibmjvmti.h file: COM_IBM_DEREGISTER_TRACEPOINT_SUBSCRIBER

`GetMemoryCategories`

You can query runtime environment native memory categories by using the GetMemoryCategories() API:

jvmtiError GetMemoryCategories(jvmtiEnv* env, jint version, jint max_categories, jvmtiMemoryCategory * categories_buffer, jint * written_count_ptr, jint * total_categories_ptr);

You can query the total native memory consumption of the runtime environment for each memory category by using this API. Native memory is memory requested from the operating system using library functions such as malloc() and mmap(). Runtime environment native memory use is grouped under high-level memory categories, as described in the NATIVEMEMINFO section of the Java dump topic. The data returned by the GetMemoryCategories() API is consistent with this format. See Java dump: NATIVEMEMINFO.

The extension writes native memory information to a memory buffer specified by the user. Each memory category is recorded as a jvmtiMemoryCategory structure, whose format is defined in ibmjvmti.h.

You can use the GetMemoryCategories() API to work out the buffer size you must allocate to hold all memory categories defined inside the VM. To calculate the size, call the API with a null categories_buffer argument and a non-null total_categories_ptr argument.

Parameters

env: A pointer to the JVMTI environment.
version: The version of the jvmtiMemoryCategory structure that you are using. Use COM_IBM_GET_MEMORY_CATEGORIES_VERSION_1 for this argument, unless you must work with an obsolete version of the jvmtiMemoryCategory structure.
max_categories: The number of jvmtiMemoryCategory structures that can fit in the categories_buffer memory buffer.
categories_buffer: A pointer to the memory buffer for holding the result of the GetMemoryCategories() call. The number of jvmtiMemoryCategory slots available in the categories_buffer memory buffer must be accurately specified with max_categories, otherwise GetMemoryCategories() can overflow the memory buffer. The value can be null.
written_count_ptr: A pointer to jint to store the number of jvmtiMemoryCategory structures to be written to the categories_buffer memory buffer. The value can be null.
total_categories_ptr: A pointer to jint to store the total number of memory categories declared in the VM. The value can be null.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_UNSUPPORTED_VERSION: Unrecognized value passed for version.
JVMTI_ERROR_ILLEGAL_ARGUMENT: Illegal argument; categories_buffer, count_ptr, and total_categories_ptr all have null values.
JVMTI_ERROR_INVALID_ENVIRONMENT: The env parameter is invalid.
JVMTI_ERROR_OUT_OF_MEMORY: Memory category data is truncated because max_categories is not large enough.

Identifiers

JVMTI Extension Function identifier: com.ibm.GetMemoryCategories
Macro declaration in the ibmjvmti.h file: COM_IBM_GET_MEMORY_CATEGORIES

`QueryVmLogOptions`

You can query VM log options by using the QueryVmLogOptions() API:

jvmtiError QueryVmLogOptions(jvmtiEnv* jvmti_env, jint buffer_size, void* options, jint* data_size_ptr)

This extension returns the current log options as an ASCII string. The syntax of the string is the same as the -Xsyslog command-line option, with the initial -Xsyslog: omitted. For example, the string "error,warn" indicates that the VM is set to log error and warning messages only. For more information, see -Xsyslog.

Parameters

jvmti_env: A pointer to the JVMTI environment.
buffer_size: The size of the supplied memory buffer in bytes. If the memory buffer is too small to contain the current VM log option string, the JVMTI_ERROR_ILLEGAL_ARGUMENT error message is returned.
options_buffer: A pointer to the supplied memory buffer.
data_size_ptr: A pointer to a variable, used to return the total size of the option string.

Returns

JVMTI_ERROR_NONE: Success
JVMTI_ERROR_NULL_POINTER: The options or data_size_ptr parameters are null.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The supplied memory buffer is too small.

Identifiers

JVMTI Extension Function identifier: com.ibm.QueryVmLogOptions
Macro declaration in the ibmjvmti.h file: COM_IBM_QUERY_VM_LOG_OPTIONS

`SetVmLogOptions`

You can set VM log options by using the SetVmLogOptions() API:

jvmtiError SetVmLogOptions(jvmtiEnv* jvmti_env, char* options_buffer)

The log option is passed in as an ASCII character string. Use the same syntax as the -Xsyslog command-line option, with the initial -Xsyslog: omitted. For example, to set the VM to log error and warning messages, pass in a string containing "error,warn". For more information, see -Xsyslog.

Parameters

jvmti_env: A pointer to the JVMTI environment.
options_buffer: A pointer to memory containing the log option.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: The parameter option is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is invalid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The parameter option contains an invalid -Xsyslog string.

Identifiers

JVMTI Extension Function identifier: com.ibm.SetVmLogOptions
Macro declaration in the ibmjvmti.h file: COM_IBM_SET_VM_LOG_OPTIONS

`IterateSharedCaches`

You can search for shared classes caches that exist in a specified cache directory by using the IterateSharedCaches() API:

jvmtiError IterateSharedCaches(jvmtiEnv* env, jint version, const char *cacheDir, jint flags, jboolean useCommandLineValues, jvmtiIterateSharedCachesCallback callback, void *user_data);

Information about the caches is returned in a structure that is populated by a user-specified callback function. You can specify the search directory in two ways:

Set the value of useCommandLineValues to true and specify the directory on the command line. If the directory is not specified on the command line, the default location for the platform is used.
Set the value of useCommandLineValues to false and use the cacheDir parameter. To accept the default location for the platform, specify cacheDir with a null value.

Parameters

env: A pointer to the JVMTI environment.
version: Version information for IterateSharedCaches, which describes the jvmtiSharedCacheInfo structure passed to the jvmtiIterateSharedCachesCallback function. The values allowed are:
- COM_IBM_ITERATE_SHARED_CACHES_VERSION_1
- COM_IBM_ITERATE_SHARED_CACHES_VERSION_2
- COM_IBM_ITERATE_SHARED_CACHES_VERSION_3
- COM_IBM_ITERATE_SHARED_CACHES_VERSION_4
- COM_IBM_ITERATE_SHARED_CACHES_VERSION_5
cacheDir: When the value of useCommandLineValues is false, specify the absolute path of the directory for the shared classes cache. If the value is null, the platform-dependent default is used.
flags: Reserved for future use. The only value allowed is COM_IBM_ITERATE_SHARED_CACHES_NO_FLAGS.
useCommandLineValues: Set this value to true when you want to specify the cache directory on the command line. Set this value to false when you want to use the cacheDir parameter.
callback: A function pointer to a user provided callback routine jvmtiIterateSharedCachesCallback.

user_data: User supplied data, passed as an argument to the callback function.

jint (JNICALL *jvmtiIterateSharedCachesCallback)(jvmtiEnv *env,jvmtiSharedCacheInfo *cache_info, void *user_data);

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The env parameter is not valid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_UNSUPPORTED_VERSION: The version parameter is not valid.
JVMTI_ERROR_NULL_POINTER: The callback parameter is null.
JVMTI_ERROR_NOT_AVAILABLE: The shared classes feature is not enabled in the VM.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The flags parameter is not valid.
JVMTI_ERROR_INTERNAL: This error is returned when the jvmtiIterateSharedCachesCallback returns JNI_ERR.

Identifiers

JVMTI Extension Function identifier: com.ibm.IterateSharedCaches
Macro declaration in the ibmjvmti.h file: COM_IBM_ITERATE_SHARED_CACHES

`jvmtiIterateSharedCachesCallback` function

Callback function parameters

env: A pointer to the JVMTI environment when calling COM_IBM_ITERATE_SHARED_CACHES.
cache_info: A jvmtiSharedCacheInfo structure containing information about a shared cache.
user_data: User-supplied data, passed as an argument to IterateSharedCaches.

Callback function returns

JNI_OK: Continue iterating.
JNI_ERR: Stop iterating, which causes IterateSharedCaches to return JVMTI_ERROR_INTERNAL

`jvmtiSharedCacheInfo` structure

The structure of jvmtiSharedCacheInfo

typedef struct jvmtiSharedCacheInfo {
const char *name;      // the name of the shared cache
jboolean isCompatible; // if the shared cache is compatible with this VM
jboolean isPersistent; // true if the shared cache is persistent, false if its non-persistent
jint os_shmid;         // the OS shared memory ID associated with a non-persistent cache, -1 otherwise
jint os_semid;         // the OS shared semaphore ID associated with a non-persistent cache, -1 otherwise
jint modLevel;         // one of:
                       //   COM_IBM_SHARED_CACHE_MODLEVEL_JAVA5
                       //   COM_IBM_SHARED_CACHE_MODLEVEL_JAVA6
                       //   COM_IBM_SHARED_CACHE_MODLEVEL_JAVA7
                       //   COM_IBM_SHARED_CACHE_MODLEVEL_JAVA8
                       //   COM_IBM_SHARED_CACHE_MODLEVEL_JAVA9
                       //   from Java 10: the version number of the Java level on which the shared cache is created
jint addrMode;         // the address mode of the VM creating the shared cache: includes additional
                       // information on whether it is a 64-bit compressedRefs cache when
                       // COM_IBM_ITERATE_SHARED_CACHES_VERSION_3 or later is specified.
jboolean isCorrupt;    // if the cache is corrupted
jlong cacheSize;       // the total usable shared classes cache size, or -1 when isCompatible is false
jlong freeBytes;       // the number of free bytes in the shared classes cache, or -1 when isCompatible is false
jlong lastDetach;      // the last detach time specified in milliseconds since 00:00:00 on 1 January 1970 UTC,
                       // or -1 when the last detach time is not available
jint cacheType;        // the type of the cache
jlong softMaxBytes;    // the soft limit for the available space in the cache
jint layer;            // the shared cache layer number
} jvmtiSharedCacheInfo;

Notes:

The field cacheType is included when COM_IBM_ITERATE_SHARED_CACHES_VERSION_2 or later is specified.
jvmtiSharedCacheInfo.addrMode encodes both address mode and the compressed reference mode when COM_IBM_ITERATE_SHARED_CACHES_VERSION_3 or later is specified. In this case, use the following set of macros to access the address mode and compressed reference mode:

To get the address mode, use:

COM_IBM_ITERATE_SHARED_CACHES_GET_ADDR_MODE(jvmtiSharedCacheInfo.addrMode)

This macro returns one of the following values:
COM_IBM_SHARED_CACHE_ADDRMODE_32
COM_IBM_SHARED_CACHE_ADDRMODE_64

To get the compressed references mode, use:

COM_IBM_ITERATE_SHARED_CACHES_GET_CMPRSSREF_MODE(jvmtiSharedCacheInfo.addrMode)

This macro returns one of the following values:
COM_IBM_ITERATE_SHARED_CACHES_UNKNOWN_COMPRESSED_POINTERS_MODE
COM_IBM_ITERATE_SHARED_CACHES_COMPRESSED_POINTERS_MODE
COM_IBM_ITERATE_SHARED_CACHES_NON_COMPRESSED_POINTERS_MODE

The field softMaxBytes is included when COM_IBM_ITERATE_SHARED_CACHES_VERSION_4 or later is specified.
The field layer is included when COM_IBM_ITERATE_SHARED_CACHES_VERSION_5 or later is specified. If the shared cache does not have a layer number, the value for layer is -1.

`DestroySharedCache`

You can remove a shared classes cache by using the DestroySharedCache() API:

jvmtiError DestroySharedCache(jvmtiEnv *env, const char *cacheDir, const char *name, jint persistence, jboolean useCommandLineValues, jint *internalErrorCode);

This extension removes a named shared classes cache of a given persistence type, in a given directory. You can specify the cache name, persistence type, and directory in one of these ways:

Set useCommandLineValues to true and specify the values on the command line. If a value is not available, the default values for the platform are used.
Set useCommandLineValues to false and use the cacheDir, persistence and cacheName parameters to identify the cache to be removed. To accept the default value for cacheDir or cacheName, specify the parameter with a null value.

Parameters

env: A pointer to the JVMTI environment.
cacheDir: When the value of useCommandLineValues is false, specify the absolute path of the directory for the shared classes cache. If the value is null, the platform-dependent default is used.
cacheName: When the value of useCommandLineValues is false, specify the name of the cache to be removed. If the value is null, the platform-dependent default is used.
persistence: When the value of useCommandLineValues is false, specify the type of cache to remove. This parameter must have one of the following values:
PERSISTENCE_DEFAULT (The default value for the platform).
PERSISTENT
NONPERSISTENT
useCommandLineValues: Set this value to true when you want to specify the shared classes cache name, persistence type, and directory on the command line. Set this value to false when you want to use the cacheDir, persistence, and cacheName parameters instead.
internalErrorCode: If not null, this value is set to one of the following constants when JVMTI_ERROR_INTERNAL is returned:
- COM_IBM_DESTROYED_ALL_CACHE: Set when JVMTI_ERROR_NONE is returned.
- COM_IBM_DESTROYED_NONE: Set when the function fails to remove any caches.
- COM_IBM_DESTROY_FAILED_CURRENT_GEN_CACHE: Set when the function fails to remove the existing current generation cache, irrespective of the state of older generation caches.
- COM_IBM_DESTROY_FAILED_OLDER_GEN_CACHE: Set when the function fails to remove any older generation caches. The current generation cache does not exist or is successfully removed.

Returns

JVMTI_ERROR_NONE: Success. No cache exists or all existing caches of all generations are removed.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The env parameter is not valid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: The shared classes feature is not enabled in the VM.
JVMTI_ERROR_ILLEGAL_ARGUMENT: The persistence parameter is not valid. JVMTI_ERROR_INTERNAL: Failed to remove any existing cache with the given name. See the value of the internalErrorCode parameter for more information about the failure.

Identifiers

JVMTI Extension Function identifier: com.ibm.DestroySharedCache
Macro declaration in the ibmjvmti.h file: COM_IBM_DESTROY_SHARED_CACHE

`RegisterVerboseGCSubscriber`

You can subscribe to verbose garbage collection (GC) data logging by using the RegisterVerboseGCSubscriber() API:

jvmtiError RegisterVerboseGCSubscriber(jvmtiEnv* jvmti_env, char *description, jvmtiVerboseGCSubscriber subscriber, jvmtiVerboseGCAlarm alarm, void *user_data, void **subscription_id)

Parameters

jvmti_env: A pointer to the JVMTI environment.
description: An ASCII character string that describes the subscriber.
subscriber: A function of type jvmtiVerboseGCSubscriber.
alarm: A function pointer of type jvmtiVerboseGCAlarm.
user_data: A pointer to user data. This pointer is passed to the subscriber and alarm functions each time these functions are called. This pointer can be a null value.
subscription_id: A pointer to a subscription identifier. This pointer is returned by the RegisterVerboseGCSubscriber call if successful. The value must be supplied to a future call to DeregisterVerboseGCSubscriber API, which is used to unsubscribe from verbose GC data logging.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: One of the supplied parameters is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is not valid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.
JVMTI_ERROR_NOT_AVAILABLE: GC verbose logging is not available.
JVMTI_ERROR_INTERNAL: An internal error has occurred.

Identifiers

JVMTI Extension Function identifier: com.ibm.RegisterVerboseGCSubscriber
Macro declaration in the ibmjvmti.h file: COM_IBM_REGISTER_VERBOSEGC_SUBSCRIBER

`jvmtiVerboseGCSubscriber` function

The subscriber function type is defined as follows:

typedef jvmtiError (*jvmtiVerboseGCSubscriber)(jvmtiEnv *jvmti_env, const char *record, jlong length, void *user_data);

The subscriber function must be of type jvmtiVerboseGCSubscriber, which is declared in ibmjvmti.h.

This function is called with each record of verbose logging data produced by the VM.

This function runs under the same restrictions as the GarbageCollectionStart and GarbageCollectionFinish events in the standard JVMTI specification. For more information about these events, see the JVMTI Event Index for your OpenJDK version.

The verbose logging record supplied to the subscriber function is valid only for the duration of the function. If the subscriber wants to save the data, the data must be copied elsewhere.

If the subscriber function returns an error, the alarm function is called, and the subscription is deregistered.

Subscriber function parameters

jvmti_env: A pointer to the JVMTI environment.
record: An ASCII string that contains a verbose log record.
length: The number of ASCII characters in the verbose log record.
user_data: User data supplied when the subscriber is registered.

`jvmtiVerboseGCAlarm` function

The alarm function type is defined as follows:

typedef jvmtiError (*jvmtiVerboseGCAlarm)(jvmtiEnv *jvmti_env, void *subscription_id, void *user_data);

The alarm function must be of type jvmtiVerboseGCAlarm, which is declared in ibmjvmti.h. This function is called if the subscriber function returns an error.

This function runs under the same restrictions as the GarbageCollectionStart and GarbageCollectionFinish events in the standard JVMTI specification. For more information about these events, see the JVMTI Event Index for your OpenJDK version.

Alarm function parameters

jvmti_env: A pointer to the JVMTI environment.
user_data: User data supplied when the subscriber is registered.
subscription_id: The subscription identifier.

`DeregisterVerboseGCSubscriber`

You can unsubscribe from verbose Garbage Collection (GC) data logging by using the DeregisterVerboseGCSubscriber() API:

jvmtiError DeregisterVerboseGCSubscriber(jvmtiEnv* jvmti_env, void *userData, void *subscription_id)

After the DeregisterVerboseGCSubscriber() API is called, no further calls are made to the previously registered subscriber function.

Parameters

jvmti_env: A pointer to the JVMTI environment.
subscription_id: The subscription identifier that is returned by the call to the RegisterVerboseGCSubscriber() API.

Returns

JVMTI_ERROR_NONE: Success.
JVMTI_ERROR_NULL_POINTER: The subscription_id parameter is null.
JVMTI_ERROR_OUT_OF_MEMORY: There is insufficient system memory to process the request.
JVMTI_ERROR_INVALID_ENVIRONMENT: The jvmti_env parameter is not valid.
JVMTI_ERROR_WRONG_PHASE: The extension has been called outside the JVMTI live phase.

Identifiers

JVMTI Extension Function identifier: com.ibm.DeregisterVerboseGCSubscriber
Macro declaration in the ibmjvmti.h file: COM_IBM_DEREGISTER_VERBOSEGC_SUBSCRIBER

Java Virtual Machine Tool Interface

Overview

Eclipse OpenJ9 extensions

Sample JVMTI agent

Agent_OnLoad()

DumpStartCallback()

Using the sample JVMTI agent

API reference

GetOSThreadID

QueryVmDump

SetVmDump

TriggerVmDump

ResetVmDump

VMDumpStart

VMDumpEnd

SetVmTrace

RegisterTracePointSubscriber

jvmtiTraceSubscriber function

jvmtiTraceAlarm function

DeregisterTracePointSubscriber

GetMemoryCategories

QueryVmLogOptions

SetVmLogOptions

IterateSharedCaches

jvmtiIterateSharedCachesCallback function

jvmtiSharedCacheInfo structure

DestroySharedCache

RegisterVerboseGCSubscriber

jvmtiVerboseGCSubscriber function

jvmtiVerboseGCAlarm function

DeregisterVerboseGCSubscriber

`Agent_OnLoad()`

`DumpStartCallback()`

`GetOSThreadID`

`QueryVmDump`

`SetVmDump`

`TriggerVmDump`

`ResetVmDump`

`VMDumpStart`

`VMDumpEnd`

`SetVmTrace`

`RegisterTracePointSubscriber`

`jvmtiTraceSubscriber` function

`jvmtiTraceAlarm` function

`DeregisterTracePointSubscriber`

`GetMemoryCategories`

`QueryVmLogOptions`

`SetVmLogOptions`

`IterateSharedCaches`

`jvmtiIterateSharedCachesCallback` function

`jvmtiSharedCacheInfo` structure

`DestroySharedCache`

`RegisterVerboseGCSubscriber`

`jvmtiVerboseGCSubscriber` function

`jvmtiVerboseGCAlarm` function

`DeregisterVerboseGCSubscriber`