考拉的性能测试中除了资源瓶颈之外,怕是慢请求这样的性能瓶颈最常见了。
说到javaagent,必须要讲的是一个叫做instrument的JVMTIAgent(Linux下对应的动态库是libinstrument.so),因为javaagent功能就是它来实现的,另外instrument agent还有个别名叫JPLISAgent(Java Programming Language Instrumentation Services Agent),这个名字也完全体现了其最本质的功能:就是专门为Java语言编写的插桩服务提供支持的。
instrument agent实现了Agent_OnLoad和Agent_OnAttach两方法,也就是说在使用时,agent既可以在启动时加载,也可以在运行时动态加载。其中启动时加载还可以通过类似-javaagent:myagent.jar的方式来间接加载instrument agent,运行时动态加载依赖的是JVM的attach机制(JVM Attach机制实现),通过发送load命令来加载agent。
instrument agent的核心数据结构如下:
struct _JPLISAgent {
JavaVM * mJVM; /* handle to the JVM */
JPLISEnvironment mNormalEnvironment; /* for every thing but retransform stuff */
JPLISEnvironment mRetransformEnvironment;/* for retransform stuff only */
jobject mInstrumentationImpl; /* handle to the Instrumentation instance */
jmethodID mPremainCaller; /* method on the InstrumentationImpl that does the premain stuff (cached to save lots of lookups) */
jmethodID mAgentmainCaller; /* method on the InstrumentationImpl for agents loaded via attach mechanism */
jmethodID mTransform; /* method on the InstrumentationImpl that does the class file transform */
jboolean mRedefineAvailable; /* cached answer to "does this agent support redefine" */
jboolean mRedefineAdded; /* indicates if can_redefine_classes capability has been added */
jboolean mNativeMethodPrefixAvailable; /* cached answer to "does this agent support prefixing" */
jboolean mNativeMethodPrefixAdded; /* indicates if can_set_native_method_prefix capability has been added */
char const * mAgentClassName; /* agent class name */
char const * mOptionsString; /* -javaagent options string */
};
struct _JPLISEnvironment {
jvmtiEnv * mJVMTIEnv; /* the JVM TI environment */
JPLISAgent * mAgent; /* corresponding agent */
jboolean mIsRetransformer; /* indicates if special environment */
};
这里解释一下几个重要项:
正如前面“概述”里提到的方式,就是启动时加载instrument agent,具体过程都在`InvocationAdapter.c`的`Agent_OnLoad`方法里,这里简单描述下过程:
在运行时加载的方式,大致按照下面的方式来操作:
VirtualMachine vm = VirtualMachine.attach(pid);
vm.loadAgent(agentPath, agentArgs);
上面会通过JVM的attach机制来请求目标JVM加载对应的agent,过程大致如下:
不管是启动时还是运行时加载的instrument agent,都关注着同一个jvmti事件——ClassFileLoadHook,这个事件是在读取字节码文件之后回调时用的,这样可以对原来的字节码做修改,那这里面究竟是怎样实现的呢?
void JNICALL
eventHandlerClassFileLoadHook( jvmtiEnv * jvmtienv,
JNIEnv * jnienv,
jclass class_being_redefined,
jobject loader,
const char* name,
jobject protectionDomain,
jint class_data_len,
const unsigned char* class_data,
jint* new_class_data_len,
unsigned char** new_class_data) {
JPLISEnvironment * environment = NULL;
environment = getJPLISEnvironment(jvmtienv);
/* if something is internally inconsistent (no agent), just silently return without touching the buffer */
if ( environment != NULL ) {
jthrowable outstandingException = preserveThrowable(jnienv);
transformClassFile( environment->mAgent,
jnienv,
loader,
name,
class_being_redefined,
protectionDomain,
class_data_len,
class_data,
new_class_data_len,
new_class_data,
environment->mIsRetransformer);
restoreThrowable(jnienv, outstandingException);
}
}
先根据jvmtiEnv取得对应的JPLISEnvironment,因为上面我已经说到其实有两个JPLISEnvironment(并且有两个jvmtiEnv),其中一个是专门做retransform的,而另外一个用来做其他事情,根据不同的用途,在注册具体的ClassFileTransformer时也是分开的,对于作为retransform用的ClassFileTransformer,我们会注册到一个单独的TransformerManager里。
接着调用transformClassFile方法,由于函数实现比较长,这里就不贴代码了,大致意思就是调用InstrumentationImpl对象的transform方法,根据最后那个参数来决定选哪个TransformerManager里的ClassFileTransformer对象们做transform操作。
private byte[]
transform( ClassLoader loader,
String classname,
Class classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer,
boolean isRetransformer) {
TransformerManager mgr = isRetransformer?
mRetransfomableTransformerManager :
mTransformerManager;
if (mgr == null) {
return null; // no manager, no transform
} else {
return mgr.transform( loader,
classname,
classBeingRedefined,
protectionDomain,
classfileBuffer);
}
}
public byte[]
transform( ClassLoader loader,
String classname,
Class classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer) {
boolean someoneTouchedTheBytecode = false;
TransformerInfo[] transformerList = getSnapshotTransformerList();
byte[] bufferToUse = classfileBuffer;
// order matters, gotta run 'em in the order they were added
for ( int x = 0; x < transformerList.length; x++ ) {
TransformerInfo transformerInfo = transformerList[x];
ClassFileTransformer transformer = transformerInfo.transformer();
byte[] transformedBytes = null;
try {
transformedBytes = transformer.transform( loader,
classname,
classBeingRedefined,
protectionDomain,
bufferToUse);
}
catch (Throwable t) {
// don't let any one transformer mess it up for the others.
// This is where we need to put some logging. What should go here? FIXME
}
if ( transformedBytes != null ) {
someoneTouchedTheBytecode = true;
bufferToUse = transformedBytes;
}
}
// if someone modified it, return the modified buffer.
// otherwise return null to mean "no transforms occurred"
byte [] result;
if ( someoneTouchedTheBytecode ) {
result = bufferToUse;
}
else {
result = null;
}
return result;
}
以上是最终调到的java代码,可以看到已经调用到我们自己编写的javaagent代码里了,我们一般是实现一个ClassFileTransformer类,然后创建一个对象注册到对应的TransformerManager里。
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