在启动和运行期都可以加载agent代理，在启动的时候可通过-javaagent参数来执行agent代理，而在运行期就是通过attach这种机制动态load了。
如果在vm启动过程中加载agent，那么会在vm初始化过程中先执行libinstrument.dylib里InvocationAdapter.c的Agent_OnLoad方法，这里主要是实例化agent，解析agent的MF文件，将相关属性取出来，并注册jvmti的一些回调函数，在vm初始化完成之后，会通过回调函数去实例化Instrumentation实现对象，设置ClassFileLoadHook函数，并调用Pre-Main指定类的premain方法。
JNIEXPORT jint JNICALL
Agent_OnLoad(JavaVM *vm, char *tail, void * reserved) {
JPLISInitializationError initerror = JPLIS_INIT_ERROR_NONE;
jint result = JNI_OK;
JPLISAgent * agent = NULL;
initerror = createNewJPLISAgent(vm, &agent);
if ( initerror == JPLIS_INIT_ERROR_NONE ) {
int oldLen, newLen;
char * jarfile;
char * options;
jarAttribute* attributes;
char * premainClass;
char * agentClass;
char * bootClassPath;
/*
* Parse [=options] into jarfile and options
*/
if (parseArgumentTail(tail, &jarfile, &options) != 0) {
fprintf(stderr, "-javaagent: memory allocation failure.\n");
return JNI_ERR;
}
/*
* Agent_OnLoad is specified to provide the agent options
* argument tail in modified UTF8. However for 1.5.0 this is
* actually in the platform encoding - see 5049313.
*
* Open zip/jar file and parse archive. If can't be opened or
* not a zip file return error. Also if Premain-Class attribute
* isn't present we return an error.
*/
attributes = readAttributes(jarfile);
if (attributes == NULL) {
fprintf(stderr, "Error opening zip file or JAR manifest missing : %s\n", jarfile);
free(jarfile);
if (options != NULL) free(options);
return JNI_ERR;
}
premainClass = getAttribute(attributes, "Premain-Class");
if (premainClass == NULL) {
fprintf(stderr, "Failed to find Premain-Class manifest attribute in %s\n",
jarfile);
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return JNI_ERR;
}
/*
* Add to the jarfile
*/
appendClassPath(agent, jarfile);
/*
* The value of the Premain-Class attribute becomes the agent
* class name. The manifest is in UTF8 so need to convert to
* modified UTF8 (see JNI spec).
*/
oldLen = (int)strlen(premainClass);
newLen = modifiedUtf8LengthOfUtf8(premainClass, oldLen);
if (newLen == oldLen) {
premainClass = strdup(premainClass);
} else {
char* str = (char*)malloc( newLen+1 );
if (str != NULL) {
convertUtf8ToModifiedUtf8(premainClass, oldLen, str, newLen);
}
premainClass = str;
}
if (premainClass == NULL) {
fprintf(stderr, "-javaagent: memory allocation failed\n");
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return JNI_ERR;
}
/*
* If the Boot-Class-Path attribute is specified then we process
* each relative URL and add it to the bootclasspath.
*/
bootClassPath = getAttribute(attributes, "Boot-Class-Path");
if (bootClassPath != NULL) {
appendBootClassPath(agent, jarfile, bootClassPath);
}
/*
* Convert JAR attributes into agent capabilities
*/
convertCapabilityAtrributes(attributes, agent);
/*
* Track (record) the agent class name and options data
*/
initerror = recordCommandLineData(agent, premainClass, options);
/*
* Clean-up
*/
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
free(premainClass);
}
switch (initerror) {
case JPLIS_INIT_ERROR_NONE:
result = JNI_OK;
break;
case JPLIS_INIT_ERROR_CANNOT_CREATE_NATIVE_AGENT:
result = JNI_ERR;
fprintf(stderr, "java.lang.instrument/-javaagent: cannot create native agent.\n");
break;
case JPLIS_INIT_ERROR_FAILURE:
result = JNI_ERR;
fprintf(stderr, "java.lang.instrument/-javaagent: initialization of native agent failed.\n");
break;
case JPLIS_INIT_ERROR_ALLOCATION_FAILURE:
result = JNI_ERR;
fprintf(stderr, "java.lang.instrument/-javaagent: allocation failure.\n");
break;
case JPLIS_INIT_ERROR_AGENT_CLASS_NOT_SPECIFIED:
result = JNI_ERR;
fprintf(stderr, "-javaagent: agent class not specified.\n");
break;
default:
result = JNI_ERR;
fprintf(stderr, "java.lang.instrument/-javaagent: unknown error\n");
break;
}
return result;
}
如果在运行期通过attach api来load agent，那么会在收到load指令之后，会调用InvocationAdapter.c的Agent_OnAttach方法，其实现基本和Agent_OnLoad一致，只是还会调用Agent-Class的agentmain方法，还有点不同就是对vmint事件没有再关注（都运行期了，关注也没用），而是直接对ClassFileLoad关注，也不会再调用Pre-Main指定的类的premain方法（顾名思义，是在执行main方法之前执行的，所以运行期搞执行Pre-Main的class也不妥）。
JNIEXPORT jint JNICALL
Agent_OnAttach(JavaVM* vm, char *args, void * reserved) {
JPLISInitializationError initerror = JPLIS_INIT_ERROR_NONE;
jint result = JNI_OK;
JPLISAgent * agent = NULL;
JNIEnv * jni_env = NULL;
/*
* Need JNIEnv - guaranteed to be called from thread that is already
* attached to VM
*/
result = (*vm)->GetEnv(vm, (void**)&jni_env, JNI_VERSION_1_2);
jplis_assert(result==JNI_OK);
initerror = createNewJPLISAgent(vm, &agent);
if ( initerror == JPLIS_INIT_ERROR_NONE ) {
int oldLen, newLen;
char * jarfile;
char * options;
jarAttribute* attributes;
char * agentClass;
char * bootClassPath;
jboolean success;
/*
* Parse [=options] into jarfile and options
*/
if (parseArgumentTail(args, &jarfile, &options) != 0) {
return JNI_ENOMEM;
}
/*
* Open the JAR file and parse the manifest
*/
attributes = readAttributes( jarfile );
if (attributes == NULL) {
fprintf(stderr, "Error opening zip file or JAR manifest missing: %s\n", jarfile);
free(jarfile);
if (options != NULL) free(options);
return AGENT_ERROR_BADJAR;
}
agentClass = getAttribute(attributes, "Agent-Class");
if (agentClass == NULL) {
fprintf(stderr, "Failed to find Agent-Class manifest attribute from %s\n",
jarfile);
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return AGENT_ERROR_BADJAR;
}
/*
* Add the jarfile to the system class path
*/
if (appendClassPath(agent, jarfile)) {
fprintf(stderr, "Unable to add %s to system class path "
"- not supported by system class loader or configuration error!\n",
jarfile);
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return AGENT_ERROR_NOTONCP;
}
/*
* The value of the Agent-Class attribute becomes the agent
* class name. The manifest is in UTF8 so need to convert to
* modified UTF8 (see JNI spec).
*/
oldLen = strlen(agentClass);
newLen = modifiedUtf8LengthOfUtf8(agentClass, oldLen);
if (newLen == oldLen) {
agentClass = strdup(agentClass);
} else {
char* str = (char*)malloc( newLen+1 );
if (str != NULL) {
convertUtf8ToModifiedUtf8(agentClass, oldLen, str, newLen);
}
agentClass = str;
}
if (agentClass == NULL) {
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return JNI_ENOMEM;
}
/*
* If the Boot-Class-Path attribute is specified then we process
* each URL - in the live phase only JAR files will be added.
*/
bootClassPath = getAttribute(attributes, "Boot-Class-Path");
if (bootClassPath != NULL) {
appendBootClassPath(agent, jarfile, bootClassPath);
}
/*
* Convert JAR attributes into agent capabilities
*/
convertCapabilityAtrributes(attributes, agent);
/*
* Create the java.lang.instrument.Instrumentation instance
*/
success = createInstrumentationImpl(jni_env, agent);
jplis_assert(success);
/*
* Turn on the ClassFileLoadHook.
*/
if (success) {
success = setLivePhaseEventHandlers(agent);
jplis_assert(success);
}
/*
* Start the agent
*/
if (success) {
success = startJavaAgent(agent,
jni_env,
agentClass,
options,
agent->mAgentmainCaller);
}
if (!success) {
fprintf(stderr, "Agent failed to start!\n");
result = AGENT_ERROR_STARTFAIL;
}
/*
* Clean-up
*/
free(jarfile);
if (options != NULL) free(options);
free(agentClass);
freeAttributes(attributes);
}
return result;
}