最近想看下HotSpotVM是怎么找到一个native方法的实现的，例如Thread.start0和FileChannelImpl.transferTo0，最后发现是两种不同的方式。
nativeLookup
通过JNI规范可以知道，JVM通过加载动态链接库，将native方法链接过去，具体native方法解析成哪个符号，是按下面的约定来的，
Dynamic linkers resolve entries based on their names. A native method name is concatenated from the following components:
- the prefix Java_
- a mangled fully-qualified class name
- an underscore (“_”) separator
- a mangled method name
- for overloaded native methods, two underscores (“__”) followed by the mangled argument signature
HotspotVM对这部分的实现在nativeLookup.cpp中，
address NativeLookup::lookup(methodHandle method, bool& in_base_library, TRAPS) {
// 调用has_native_function
if (!method->has_native_function()) {
// lookup_base
address entry = lookup_base(method, in_base_library, CHECK_NULL);
// 调用set_native_function
method->set_native_function(entry,
methodOopDesc::native_bind_event_is_interesting);
// -verbose:jni printing
if (PrintJNIResolving) {
ResourceMark rm(THREAD);
tty->print_cr("[Dynamic-linking native method %s.%s ... JNI]",
Klass::cast(method->method_holder())->external_name(),
method->name()->as_C_string());
}
}
return method->native_function();
}address NativeLookup::lookup_base(methodHandle method, bool& in_base_library, TRAPS) {
address entry = NULL;
ResourceMark rm(THREAD);
entry = lookup_entry(method, in_base_library, THREAD);
if (entry != NULL) return entry;
// standard native method resolution has failed. Check if there are any
// JVM TI prefixes which have been applied to the native method name.
entry = lookup_entry_prefixed(method, in_base_library, THREAD);
if (entry != NULL) return entry;
// Native function not found, throw UnsatisfiedLinkError
THROW_MSG_0(vmSymbols::java_lang_UnsatisfiedLinkError(),
method->name_and_sig_as_C_string());
}// Check all the formats of native implementation name to see if there is one
// for the specified method.
address NativeLookup::lookup_entry(methodHandle method, bool& in_base_library, TRAPS) {
address entry = NULL;
in_base_library = false;
// Compute pure name
char* pure_name = pure_jni_name(method);
// Compute argument size
int args_size = 1 // JNIEnv
+ (method->is_static() ? 1 : 0) // class for static methods
+ method->size_of_parameters(); // actual parameters
// 1) Try JNI short style
entry = lookup_style(method, pure_name, "", args_size, true, in_base_library, CHECK_NULL);
if (entry != NULL) return entry;
// Compute long name
char* long_name = long_jni_name(method);
// 2) Try JNI long style
entry = lookup_style(method, pure_name, long_name, args_size, true, in_base_library, CHECK_NULL);
if (entry != NULL) return entry;
// 3) Try JNI short style without os prefix/suffix
entry = lookup_style(method, pure_name, "", args_size, false, in_base_library, CHECK_NULL);
if (entry != NULL) return entry;
// 4) Try JNI long style without os prefix/suffix
entry = lookup_style(method, pure_name, long_name, args_size, false, in_base_library, CHECK_NULL);
return entry; // NULL indicates not found
}char* NativeLookup::pure_jni_name(methodHandle method) {
stringStream st;
// Prefix
st.print("Java_");
// Klass name
mangle_name_on(&st, method->klass_name());
st.print("_");
// Method name
mangle_name_on(&st, method->name());
return st.as_string();
}
char* NativeLookup::long_jni_name(methodHandle method) {
// Signature ignore the wrapping parenteses and the trailing return type
stringStream st;
Symbol* signature = method->signature();
st.print("__");
// find ')'
int end;
for (end = 0; end < signature->utf8_length() && signature->byte_at(end) != ')'; end++);
// skip first '('
mangle_name_on(&st, signature, 1, end);
return st.as_string();
}address NativeLookup::lookup_style(methodHandle method, char* pure_name, const char* long_name, int args_size, bool os_style, bool& in_base_library, TRAPS) {
address entry;
// Compute complete JNI name for style
stringStream st;
if (os_style) os::print_jni_name_prefix_on(&st, args_size);
st.print_raw(pure_name);
st.print_raw(long_name);
if (os_style) os::print_jni_name_suffix_on(&st, args_size);
char* jni_name = st.as_string();
// If the loader is null we have a system class, so we attempt a lookup in
// the native Java library. This takes care of any bootstrapping problems.
// Note: It is critical for bootstrapping that Java_java_lang_ClassLoader_00024NativeLibrary_find
// gets found the first time around - otherwise an infinite loop can occure. This is
// another VM/library dependency
Handle loader(THREAD,
instanceKlass::cast(method->method_holder())->class_loader());
if (loader.is_null()) {
entry = lookup_special_native(jni_name);
if (entry == NULL) {
// 查找本地动态链接库
entry = (address) os::dll_lookup(os::native_java_library(), jni_name);
}
if (entry != NULL) {
in_base_library = true;
return entry;
}
}
// Otherwise call static method findNative in ClassLoader
KlassHandle klass (THREAD, SystemDictionary::ClassLoader_klass());
Handle name_arg = java_lang_String::create_from_str(jni_name, CHECK_NULL);
JavaValue result(T_LONG);
JavaCalls::call_static(&result,
klass,
vmSymbols::findNative_name(),
vmSymbols::classloader_string_long_signature(),
// Arguments
loader,
name_arg,
CHECK_NULL);
entry = (address) (intptr_t) result.get_jlong();
if (entry == NULL) {
// findNative didn't find it, if there are any agent libraries look in them
AgentLibrary* agent;
for (agent = Arguments::agents(); agent != NULL; agent = agent->next()) {
entry = (address) os::dll_lookup(agent->os_lib(), jni_name);
if (entry != NULL) {
return entry;
}
}
}
return entry;
}最后便是找到本地动态链接库，调用os::dll_lookup查找符号表。先看查找本地动态链接库的逻辑，os::native_java_library，
static void* _native_java_library = NULL;
void* os::native_java_library() {
if (_native_java_library == NULL) {
char buffer[JVM_MAXPATHLEN];
char ebuf[1024];
// Try to load verify dll first. In 1.3 java dll depends on it and is not
// always able to find it when the loading executable is outside the JDK.
// In order to keep working with 1.2 we ignore any loading errors.
dll_build_name(buffer, sizeof(buffer), Arguments::get_dll_dir(), "verify");
dll_load(buffer, ebuf, sizeof(ebuf));
// Load java dll
dll_build_name(buffer, sizeof(buffer), Arguments::get_dll_dir(), "java");
_native_java_library = dll_load(buffer, ebuf, sizeof(ebuf));
if (_native_java_library == NULL) {
vm_exit_during_initialization("Unable to load native library", ebuf);
}
#if defined(__OpenBSD__)
// Work-around OpenBSD's lack of $ORIGIN support by pre-loading libnet.so
// ignore errors
dll_build_name(buffer, sizeof(buffer), Arguments::get_dll_dir(), "net");
dll_load(buffer, ebuf, sizeof(ebuf));
#endif
}
static jboolean onLoaded = JNI_FALSE;
if (onLoaded) {
// We may have to wait to fire OnLoad until TLS is initialized.
if (ThreadLocalStorage::is_initialized()) {
// The JNI_OnLoad handling is normally done by method load in
// java.lang.ClassLoader$NativeLibrary, but the VM loads the base library
// explicitly so we have to check for JNI_OnLoad as well
const char *onLoadSymbols[] = JNI_ONLOAD_SYMBOLS;
JNI_OnLoad_t JNI_OnLoad = CAST_TO_FN_PTR(
JNI_OnLoad_t, dll_lookup(_native_java_library, onLoadSymbols[0]));
if (JNI_OnLoad != NULL) {
JavaThread* thread = JavaThread::current();
ThreadToNativeFromVM ttn(thread);
HandleMark hm(thread);
jint ver = (*JNI_OnLoad)(&main_vm, NULL);
onLoaded = JNI_TRUE;
if (!Threads::is_supported_jni_version_including_1_1(ver)) {
vm_exit_during_initialization("Unsupported JNI version");
}
}
}
}
return _native_java_library;
}dll_build_name的时候，因为各个平台的动态链接库文件格式不同，例如上面加载的java动态链接库，在Win下是java.dll文件，而Linux下则是libjava.so，Linux平台下的这个逻辑在os_linux.cpp中实现，
void os::dll_build_name(char* buffer, size_t buflen,
const char* pname, const char* fname) {
// Copied from libhpi
const size_t pnamelen = pname ? strlen(pname) : 0;
// Quietly truncate on buffer overflow. Should be an error.
if (pnamelen + strlen(fname) + 10 > (size_t) buflen) {
*buffer = '\0';
return;
}
if (pnamelen == 0) {
snprintf(buffer, buflen, "lib%s.so", fname);
} else if (strchr(pname, *os::path_separator()) != NULL) {
int n;
char** pelements = split_path(pname, &n);
for (int i = 0 ; i < n ; i++) {
// Really shouldn't be NULL, but check can't hurt
if (pelements[i] == NULL || strlen(pelements[i]) == 0) {
continue; // skip the empty path values
}
snprintf(buffer, buflen, "%s/lib%s.so", pelements[i], fname);
if (file_exists(buffer)) {
break;
}
}
// release the storage
for (int i = 0 ; i < n ; i++) {
if (pelements[i] != NULL) {
FREE_C_HEAP_ARRAY(char, pelements[i], mtInternal);
}
}
if (pelements != NULL) {
FREE_C_HEAP_ARRAY(char*, pelements, mtInternal);
}
} else {
snprintf(buffer, buflen, "%s/lib%s.so", pname, fname);
}
}动态链接库文件格式不同，dll_lookup查找符号表的实现肯定也不同，Linux下的实现还是在os_linux.cpp中，
/* * glibc-2.0 libdl is not MT safe. If you are building with any glibc, * chances are you might want to run the generated bits against glibc-2.0 * libdl.so, so always use locking for any version of glibc. */
void* os::dll_lookup(void* handle, const char* name) {
pthread_mutex_lock(&dl_mutex);
void* res = dlsym(handle, name);
pthread_mutex_unlock(&dl_mutex);
return res;
}dlsym是Linux的系统调用，参考man。
Thread.start0
在Thread.java的开头有这么几行代码，
/* Make sure registerNatives is the first thing <clinit> does. */
private static native void registerNatives();
static {
registerNatives();
}根据上面的规则，在libjava.so中应该存在Java_java_lang_Thread_registerNatives这样的方法，那么这个方法在哪？不是在OpenJDK的hotspot子项目下，而是在jdk子项目下，src/share/native/java/lang/Thread.c，
static JNINativeMethod methods[] = {
{"start0", "()V", (void *)&JVM_StartThread},
{"stop0", "(" OBJ ")V", (void *)&JVM_StopThread},
{"isAlive", "()Z", (void *)&JVM_IsThreadAlive},
{"suspend0", "()V", (void *)&JVM_SuspendThread},
{"resume0", "()V", (void *)&JVM_ResumeThread},
{"setPriority0", "(I)V", (void *)&JVM_SetThreadPriority},
{"yield", "()V", (void *)&JVM_Yield},
{"sleep", "(J)V", (void *)&JVM_Sleep},
{"currentThread", "()" THD, (void *)&JVM_CurrentThread},
{"countStackFrames", "()I", (void *)&JVM_CountStackFrames},
{"interrupt0", "()V", (void *)&JVM_Interrupt},
{"isInterrupted", "(Z)Z", (void *)&JVM_IsInterrupted},
{"holdsLock", "(" OBJ ")Z", (void *)&JVM_HoldsLock},
{"getThreads", "()[" THD, (void *)&JVM_GetAllThreads},
{"dumpThreads", "([" THD ")[[" STE, (void *)&JVM_DumpThreads},
{"setNativeName", "(" STR ")V", (void *)&JVM_SetNativeThreadName},
};
JNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{
(*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}看代码发现，并没有一个Java_java_lang_Thread_start0这样的方法，但是在Java_java_lang_Thread_registerNatives中调用了RegisterNatives，这货干嘛用的？先看它的定义，在jni.h中，是JNINativeInterface_中的一个函数指针（C中都是通过这种方式实现接口抽象的），
struct JNINativeInterface_ {
...
jint (JNICALL *RegisterNatives)
(JNIEnv *env, jclass clazz, const JNINativeMethod *methods,
jint nMethods);
...
}具体实现在jni.cpp中，
struct JNINativeInterface_ jni_NativeInterface = {
...
jni_RegisterNatives,
...
}JNI_ENTRY(jint, jni_RegisterNatives(JNIEnv *env, jclass clazz,
const JNINativeMethod *methods,
jint nMethods))
JNIWrapper("RegisterNatives");
#ifndef USDT2
DTRACE_PROBE4(hotspot_jni, RegisterNatives__entry, env, clazz, methods, nMethods);
#else /* USDT2 */
HOTSPOT_JNI_REGISTERNATIVES_ENTRY(
env, clazz, (void *) methods, nMethods);
#endif /* USDT2 */
jint ret = 0;
DT_RETURN_MARK(RegisterNatives, jint, (const jint&)ret);
KlassHandle h_k(thread, java_lang_Class::as_klassOop(JNIHandles::resolve_non_null(clazz)));
for (int index = 0; index < nMethods; index++) {
const char* meth_name = methods[index].name;
const char* meth_sig = methods[index].signature;
int meth_name_len = (int)strlen(meth_name);
// The class should have been loaded (we have an instance of the class
// passed in) so the method and signature should already be in the symbol
// table. If they're not there, the method doesn't exist.
TempNewSymbol name = SymbolTable::probe(meth_name, meth_name_len);
TempNewSymbol signature = SymbolTable::probe(meth_sig, (int)strlen(meth_sig));
if (name == NULL || signature == NULL) {
ResourceMark rm;
stringStream st;
st.print("Method %s.%s%s not found", Klass::cast(h_k())->external_name(), meth_name, meth_sig);
// Must return negative value on failure
THROW_MSG_(vmSymbols::java_lang_NoSuchMethodError(), st.as_string(), -1);
}
// 注册本地方法
bool res = register_native(h_k, name, signature,
(address) methods[index].fnPtr, THREAD);
if (!res) {
ret = -1;
break;
}
}
return ret;
JNI_ENDstatic bool register_native(KlassHandle k, Symbol* name, Symbol* signature, address entry, TRAPS) {
methodOop method = Klass::cast(k())->lookup_method(name, signature);
if (method == NULL) {
ResourceMark rm;
stringStream st;
st.print("Method %s name or signature does not match",
methodOopDesc::name_and_sig_as_C_string(Klass::cast(k()), name, signature));
THROW_MSG_(vmSymbols::java_lang_NoSuchMethodError(), st.as_string(), false);
}
if (!method->is_native()) {
// trying to register to a non-native method, see if a JVM TI agent has added prefix(es)
method = find_prefixed_native(k, name, signature, THREAD);
if (method == NULL) {
ResourceMark rm;
stringStream st;
st.print("Method %s is not declared as native",
methodOopDesc::name_and_sig_as_C_string(Klass::cast(k()), name, signature));
THROW_MSG_(vmSymbols::java_lang_NoSuchMethodError(), st.as_string(), false);
}
}
if (entry != NULL) {
// 调用set_native_function
method->set_native_function(entry,
methodOopDesc::native_bind_event_is_interesting);
} else {
method->clear_native_function();
}
if (PrintJNIResolving) {
ResourceMark rm(THREAD);
tty->print_cr("[Registering JNI native method %s.%s]",
Klass::cast(method->method_holder())->external_name(),
method->name()->as_C_string());
}
return true;
}有没有发现最后调用set_native_function似曾相识，是的在nativeLookup::lookup中也会做相同的事情。回去看lookup的代码，你会发现，假如我们先使用了RegisterNatives的方式注册了本地方法，那么是不需要走到lookup_base里面去查找动态链接库了。set_native_function的定义是在methodOop.hpp，
class methodOopDesc : public oopDesc {
friend class methodKlass;
...
// native function (used for native methods only)
enum {
native_bind_event_is_interesting = true
};
address native_function() const { return *(native_function_addr()); }
address critical_native_function();
// Must specify a real function (not NULL).
// Use clear_native_function() to unregister.
void set_native_function(address function, bool post_event_flag);
bool has_native_function() const;
void clear_native_function();
...
}也就是说每一个Java的native方法，最后会调用哪个本地方法，其实是被放到methodOopDesc::native_function中的，不管是按约定名称解析的方式，还是RegisterNatives的方式。其实在JNI规范中已经给出了RegisterNatives方式的说明，
The programmer can also call the JNI function RegisterNatives() to register the native methods associated with a class. The RegisterNatives() function is particularly useful with statically linked functions.
相对于查找动态链接库的方式，这是一种静态链接的方式。
回过头去看下给Thread.start0注册的本地方法JVM_StartThread，在jvm.cpp中实现，
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
JVMWrapper("JVM_StartThread");
JavaThread *native_thread = NULL;
// We cannot hold the Threads_lock when we throw an exception,
// due to rank ordering issues. Example: we might need to grab the
// Heap_lock while we construct the exception.
bool throw_illegal_thread_state = false;
// We must release the Threads_lock before we can post a jvmti event
// in Thread::start.
{
// Ensure that the C++ Thread and OSThread structures aren't freed before
// we operate.
MutexLocker mu(Threads_lock);
// Since JDK 5 the java.lang.Thread threadStatus is used to prevent
// re-starting an already started thread, so we should usually find
// that the JavaThread is null. However for a JNI attached thread
// there is a small window between the Thread object being created
// (with its JavaThread set) and the update to its threadStatus, so we
// have to check for this
if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {
throw_illegal_thread_state = true;
} else {
// We could also check the stillborn flag to see if this thread was already stopped, but
// for historical reasons we let the thread detect that itself when it starts running
jlong size =
java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread));
// Allocate the C++ Thread structure and create the native thread. The
// stack size retrieved from java is signed, but the constructor takes
// size_t (an unsigned type), so avoid passing negative values which would
// result in really large stacks.
size_t sz = size > 0 ? (size_t) size : 0;
native_thread = new JavaThread(&thread_entry, sz);
// At this point it may be possible that no osthread was created for the
// JavaThread due to lack of memory. Check for this situation and throw
// an exception if necessary. Eventually we may want to change this so
// that we only grab the lock if the thread was created successfully -
// then we can also do this check and throw the exception in the
// JavaThread constructor.
if (native_thread->osthread() != NULL) {
// Note: the current thread is not being used within "prepare".
native_thread->prepare(jthread);
}
}
}
if (throw_illegal_thread_state) {
THROW(vmSymbols::java_lang_IllegalThreadStateException());
}
assert(native_thread != NULL, "Starting null thread?");
if (native_thread->osthread() == NULL) {
// No one should hold a reference to the 'native_thread'.
delete native_thread;
if (JvmtiExport::should_post_resource_exhausted()) {
JvmtiExport::post_resource_exhausted(
JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_THREADS,
"unable to create new native thread");
}
THROW_MSG(vmSymbols::java_lang_OutOfMemoryError(),
"unable to create new native thread");
}
Thread::start(native_thread);
JVM_ENDFileChannelImpl.transferTo0
FileChannelImpl.transferTo0则是通过动态链接的方式实现，具体代码实现在src/solaris/native/sun/nio/ch/FileChannelImpl.c中，
JNIEXPORT jlong JNICALL
Java_sun_nio_ch_FileChannelImpl_transferTo0(JNIEnv *env, jobject this,
jint srcFD,
jlong position, jlong count,
jint dstFD)
{
#if defined(__linux__)
off64_t offset = (off64_t)position;
jlong n = sendfile64(dstFD, srcFD, &offset, (size_t)count);
if (n < 0) {
if (errno == EAGAIN)
return IOS_UNAVAILABLE;
if ((errno == EINVAL) && ((ssize_t)count >= 0))
return IOS_UNSUPPORTED_CASE;
if (errno == EINTR) {
return IOS_INTERRUPTED;
}
JNU_ThrowIOExceptionWithLastError(env, "Transfer failed");
return IOS_THROWN;
}
return n;
#elif defined (__solaris__)
sendfilevec64_t sfv;
size_t numBytes = 0;
jlong result;
sfv.sfv_fd = srcFD;
sfv.sfv_flag = 0;
sfv.sfv_off = (off64_t)position;
sfv.sfv_len = count;
result = sendfilev64(dstFD, &sfv, 1, &numBytes);
/* Solaris sendfilev() will return -1 even if some bytes have been * transferred, so we check numBytes first. */
if (numBytes > 0)
return numBytes;
if (result < 0) {
if (errno == EAGAIN)
return IOS_UNAVAILABLE;
if (errno == EOPNOTSUPP)
return IOS_UNSUPPORTED_CASE;
if ((errno == EINVAL) && ((ssize_t)count >= 0))
return IOS_UNSUPPORTED_CASE;
if (errno == EINTR)
return IOS_INTERRUPTED;
JNU_ThrowIOExceptionWithLastError(env, "Transfer failed");
return IOS_THROWN;
}
return result;
#elif defined(__APPLE__)
off_t numBytes;
int result;
numBytes = count;
#ifdef __APPLE__
result = sendfile(srcFD, dstFD, position, &numBytes, NULL, 0);
#endif
if (numBytes > 0)
return numBytes;
if (result == -1) {
if (errno == EAGAIN)
return IOS_UNAVAILABLE;
if (errno == EOPNOTSUPP || errno == ENOTSOCK || errno == ENOTCONN)
return IOS_UNSUPPORTED_CASE;
if ((errno == EINVAL) && ((ssize_t)count >= 0))
return IOS_UNSUPPORTED_CASE;
if (errno == EINTR)
return IOS_INTERRUPTED;
JNU_ThrowIOExceptionWithLastError(env, "Transfer failed");
return IOS_THROWN;
}
return result;
#else
return IOS_UNSUPPORTED_CASE;
#endif
}