so 介绍¶
基本介绍¶
- 为什么会用到 Shared Object(SO)
- 开发效率
- 快速移植
- so 的版本
- 根据 CPU 平台有所不一样
加载方法¶
- System.loadLibrary
- 如果加载的文件名是 xxx ，那么其实加载的是项目中 libs 目录下的 libxxx.so文件。
- System.load
- 对应 lib 的绝对路径。
主要使用第一种方式，第二种方式主要用于在插件中加载 so 文件。
loadLibrary 加载流程¶
根据官方 API 介绍
The call
System.loadLibrary(name)is effectively equivalent to the callRuntime.getRuntime().loadLibrary(name)
可以看出该函数其实调用的是 Runtime.java（ libcore/luni/src/main/java/java/lang/Runtime.java ）中的函数 loadLibrary，继而会继续调用 loadLibrary 另一个重载函数，它包含两个参数
- libame，我们传入的库名字
- VMStack.getCallingClassLoader()，类加载器 ClassLoader，方便于去寻找相应的 library。
/**
* Loads and links the library with the specified name. The mapping of the
* specified library name to the full path for loading the library is
* implementation-dependent.
*
* @param libName
* the name of the library to load.
* @throws UnsatisfiedLinkError
* if the library can not be loaded.
*/
public void loadLibrary(String libName) {
loadLibrary(libName, VMStack.getCallingClassLoader());
}
/*
* Searches for a library, then loads and links it without security checks.
*/
void loadLibrary(String libraryName, ClassLoader loader) {
if (loader != null) {
String filename = loader.findLibrary(libraryName);
if (filename == null) {
throw new UnsatisfiedLinkError("Couldn't load " + libraryName +
" from loader " + loader +
": findLibrary returned null");
}
String error = doLoad(filename, loader);
if (error != null) {
throw new UnsatisfiedLinkError(error);
}
return;
}
String filename = System.mapLibraryName(libraryName);
List<String> candidates = new ArrayList<String>();
String lastError = null;
for (String directory : mLibPaths) {
String candidate = directory + filename;
candidates.add(candidate);
if (IoUtils.canOpenReadOnly(candidate)) {
String error = doLoad(candidate, loader);
if (error == null) {
return; // We successfully loaded the library. Job done.
}
lastError = error;
}
}
if (lastError != null) {
throw new UnsatisfiedLinkError(lastError);
}
throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates);
}
可以看出，程序主要的功能正如注释所说
Searches for a library, then loads and links it without security checks.
而其中所采用的加载函数是 doLoad 函数。在这里，我们先不继续分析，我们来看看 load 函数如何。
load 加载流程¶
根据官方 API 说明，如下
The call System.load(name) is effectively equivalent to the call:
java Runtime.getRuntime().load(name)
其同样也是调用 Runtime.java 中的函数，如下
/**
* Loads and links the dynamic library that is identified through the
* specified path. This method is similar to {@link #loadLibrary(String)},
* but it accepts a full path specification whereas {@code loadLibrary} just
* accepts the name of the library to load.
*
* @param pathName
* the absolute (platform dependent) path to the library to load.
* @throws UnsatisfiedLinkError
* if the library can not be loaded.
*/
public void load(String pathName) {
load(pathName, VMStack.getCallingClassLoader());
}
/*
* Loads and links the given library without security checks.
*/
void load(String pathName, ClassLoader loader) {
if (pathName == null) {
throw new NullPointerException("pathName == null");
}
String error = doLoad(pathName, loader);
if (error != null) {
throw new UnsatisfiedLinkError(error);
}
}
其同样也会调用load 的两个参数的重载函数，继而会调用doLoad函数。
无论是上面的哪一种加载方法，最后都会调用Runtime.java中的doLoad函数。
核心加载流程¶
doLoad¶
下面我们来分析一下 doLoad 函数，如下
private String doLoad(String name, ClassLoader loader) {
// Android apps are forked from the zygote, so they can't have a custom LD_LIBRARY_PATH,
// which means that by default an app's shared library directory isn't on LD_LIBRARY_PATH.
// The PathClassLoader set up by frameworks/base knows the appropriate path, so we can load
// libraries with no dependencies just fine, but an app that has multiple libraries that
// depend on each other needed to load them in most-dependent-first order.
// We added API to Android's dynamic linker so we can update the library path used for
// the currently-running process. We pull the desired path out of the ClassLoader here
// and pass it to nativeLoad so that it can call the private dynamic linker API.
// We didn't just change frameworks/base to update the LD_LIBRARY_PATH once at the
// beginning because multiple apks can run in the same process and third party code can
// use its own BaseDexClassLoader.
// We didn't just add a dlopen_with_custom_LD_LIBRARY_PATH call because we wanted any
// dlopen(3) calls made from a .so's JNI_OnLoad to work too.
// So, find out what the native library search path is for the ClassLoader in question...
String ldLibraryPath = null;
if (loader != null && loader instanceof BaseDexClassLoader) {
ldLibraryPath = ((BaseDexClassLoader) loader).getLdLibraryPath();
}
// nativeLoad should be synchronized so there's only one LD_LIBRARY_PATH in use regardless
// of how many ClassLoaders are in the system, but dalvik doesn't support synchronized
// internal natives.
synchronized (this) {
return nativeLoad(name, loader, ldLibraryPath);
}
}
虽然源代码很长，但是很多部分都是注释，也说明了为什么要使用这样的一个函数的原因，主要有以下原因
- Android App 都是由 zygote fork 生成的，因此他们的 LD_LIBRARY_PATH 就是 zygote 的LD_LIBRARY_PATH，这也说明 apk 中的 so 文件不在这个路径下。
- so 文件之间可能存在相互依赖，我们需要按照其按依赖关系的逆方向进行加载。
函数的基本思想就是找到库文件的路径，然后使用 synchronized 方式调用了 nativeLoad 函数。
nativeload¶
而 nativeload 函数其实就是一个原生层的函数
// TODO: should be synchronized, but dalvik doesn't support synchronized internal natives.
private static native String nativeLoad(String filename, ClassLoader loader,
String ldLibraryPath);
相应的文件路径为 dalvik/vm/native/java_lang_Runtime.cpp ，具体的 nativeLoad 函数如下
const DalvikNativeMethod dvm_java_lang_Runtime[] = {
{ "freeMemory", "()J",
Dalvik_java_lang_Runtime_freeMemory },
{ "gc", "()V",
Dalvik_java_lang_Runtime_gc },
{ "maxMemory", "()J",
Dalvik_java_lang_Runtime_maxMemory },
{ "nativeExit", "(I)V",
Dalvik_java_lang_Runtime_nativeExit },
{ "nativeLoad", "(Ljava/lang/String;Ljava/lang/ClassLoader;Ljava/lang/String;)Ljava/lang/String;",
Dalvik_java_lang_Runtime_nativeLoad },
{ "totalMemory", "()J",
Dalvik_java_lang_Runtime_totalMemory },
{ NULL, NULL, NULL },
};
可以看出在 native 层对应的函数是 Dalvik_java_lang_Runtime_nativeLoad，如下
/*
* static String nativeLoad(String filename, ClassLoader loader, String ldLibraryPath)
*
* Load the specified full path as a dynamic library filled with
* JNI-compatible methods. Returns null on success, or a failure
* message on failure.
*/
static void Dalvik_java_lang_Runtime_nativeLoad(const u4* args,
JValue* pResult)
{
StringObject* fileNameObj = (StringObject*) args[0];
Object* classLoader = (Object*) args[1];
StringObject* ldLibraryPathObj = (StringObject*) args[2];
assert(fileNameObj != NULL);
char* fileName = dvmCreateCstrFromString(fileNameObj);
if (ldLibraryPathObj != NULL) {
char* ldLibraryPath = dvmCreateCstrFromString(ldLibraryPathObj);
void* sym = dlsym(RTLD_DEFAULT, "android_update_LD_LIBRARY_PATH");
if (sym != NULL) {
typedef void (*Fn)(const char*);
Fn android_update_LD_LIBRARY_PATH = reinterpret_cast<Fn>(sym);
(*android_update_LD_LIBRARY_PATH)(ldLibraryPath);
} else {
ALOGE("android_update_LD_LIBRARY_PATH not found; .so dependencies will not work!");
}
free(ldLibraryPath);
}
StringObject* result = NULL;
char* reason = NULL;
bool success = dvmLoadNativeCode(fileName, classLoader, &reason);
if (!success) {
const char* msg = (reason != NULL) ? reason : "unknown failure";
result = dvmCreateStringFromCstr(msg);
dvmReleaseTrackedAlloc((Object*) result, NULL);
}
free(reason);
free(fileName);
RETURN_PTR(result);
}
根据注释，我们可以确定关键的代码在
bool success = dvmLoadNativeCode(fileName, classLoader, &reason);
这一行执行后会告诉我们加载对应的 so 是否成功。
dvmLoadNativeCode¶
其基本的代码如下，我们可以根据注释来简单判断一下该函数的功能：
- 程序根据指定的绝对路径加载相应的 native code，但是如果该 library 已经加载了，那么就不会再次进行加载。
此外，正如 JNI 中所说，我们不能将一个库加载到多个 class loader 中，也就是说，一个 library 只会和一个 class loader 关联。
函数的基本执行流程如下
- 利用 findSharedLibEntry 判断是否已经加载了这个库，以及如果已经加载的话，是不是采用的是同一个class loader。
/*
* Load native code from the specified absolute pathname. Per the spec,
* if we've already loaded a library with the specified pathname, we
* return without doing anything.
*
* TODO? for better results we should absolutify the pathname. For fully
* correct results we should stat to get the inode and compare that. The
* existing implementation is fine so long as everybody is using
* System.loadLibrary.
*
* The library will be associated with the specified class loader. The JNI
* spec says we can't load the same library into more than one class loader.
*
* Returns "true" on success. On failure, sets *detail to a
* human-readable description of the error or NULL if no detail is
* available; ownership of the string is transferred to the caller.
*/
bool dvmLoadNativeCode(const char* pathName, Object* classLoader,
char** detail)
{
SharedLib* pEntry;
void* handle;
bool verbose;
/* reduce noise by not chattering about system libraries */
verbose = !!strncmp(pathName, "/system", sizeof("/system")-1);
verbose = verbose && !!strncmp(pathName, "/vendor", sizeof("/vendor")-1);
if (verbose)
ALOGD("Trying to load lib %s %p", pathName, classLoader);
*detail = NULL;
/*
* See if we've already loaded it. If we have, and the class loader
* matches, return successfully without doing anything.
*/
pEntry = findSharedLibEntry(pathName);
if (pEntry != NULL) {
if (pEntry->classLoader != classLoader) {
ALOGW("Shared lib '%s' already opened by CL %p; can't open in %p",
pathName, pEntry->classLoader, classLoader);
return false;
}
if (verbose) {
ALOGD("Shared lib '%s' already loaded in same CL %p",
pathName, classLoader);
}
if (!checkOnLoadResult(pEntry))
return false;
return true;
}
- 如果没有加载的话，就会利用 dlopen 打开该共享库。
/*
* Open the shared library. Because we're using a full path, the system
* doesn't have to search through LD_LIBRARY_PATH. (It may do so to
* resolve this library's dependencies though.)
*
* Failures here are expected when java.library.path has several entries
* and we have to hunt for the lib.
*
* The current version of the dynamic linker prints detailed information
* about dlopen() failures. Some things to check if the message is
* cryptic:
* - make sure the library exists on the device
* - verify that the right path is being opened (the debug log message
* above can help with that)
* - check to see if the library is valid (e.g. not zero bytes long)
* - check config/prelink-linux-arm.map to ensure that the library
* is listed and is not being overrun by the previous entry (if
* loading suddenly stops working on a prelinked library, this is
* a good one to check)
* - write a trivial app that calls sleep() then dlopen(), attach
* to it with "strace -p <pid>" while it sleeps, and watch for
* attempts to open nonexistent dependent shared libs
*
* This can execute slowly for a large library on a busy system, so we
* want to switch from RUNNING to VMWAIT while it executes. This allows
* the GC to ignore us.
*/
Thread* self = dvmThreadSelf();
ThreadStatus oldStatus = dvmChangeStatus(self, THREAD_VMWAIT);
handle = dlopen(pathName, RTLD_LAZY);
dvmChangeStatus(self, oldStatus);
if (handle == NULL) {
*detail = strdup(dlerror());
ALOGE("dlopen(\"%s\") failed: %s", pathName, *detail);
return false;
}
其中的 dlopen 函数(bionic/linker/dlfcn.cpp)如下
void* dlopen(const char* filename, int flags) {
ScopedPthreadMutexLocker locker(&gDlMutex);
soinfo* result = do_dlopen(filename, flags);
if (result == NULL) {
__bionic_format_dlerror("dlopen failed", linker_get_error_buffer());
return NULL;
}
return result;
}
其会调用 do_dlopen 函数(bionic/linker/linker.cpp)，如下
soinfo* do_dlopen(const char* name, int flags) {
if ((flags & ~(RTLD_NOW|RTLD_LAZY|RTLD_LOCAL|RTLD_GLOBAL)) != 0) {
DL_ERR("invalid flags to dlopen: %x", flags);
return NULL;
}
set_soinfo_pool_protection(PROT_READ | PROT_WRITE);
soinfo* si = find_library(name); //判断是否有这个库，有的话，需要完成初始化工作
if (si != NULL) {
si->CallConstructors();
}
set_soinfo_pool_protection(PROT_READ);
return si;
}
在找到对应的库之后，会使用 si->CallConstructors(); 来构造相关信息，如下
void soinfo::CallConstructors() {
if (constructors_called) {
return;
}
// We set constructors_called before actually calling the constructors, otherwise it doesn't
// protect against recursive constructor calls. One simple example of constructor recursion
// is the libc debug malloc, which is implemented in libc_malloc_debug_leak.so:
// 1. The program depends on libc, so libc's constructor is called here.
// 2. The libc constructor calls dlopen() to load libc_malloc_debug_leak.so.
// 3. dlopen() calls the constructors on the newly created
// soinfo for libc_malloc_debug_leak.so.
// 4. The debug .so depends on libc, so CallConstructors is
// called again with the libc soinfo. If it doesn't trigger the early-
// out above, the libc constructor will be called again (recursively!).
constructors_called = true;
if ((flags & FLAG_EXE) == 0 && preinit_array != NULL) {
// The GNU dynamic linker silently ignores these, but we warn the developer.
PRINT("\"%s\": ignoring %d-entry DT_PREINIT_ARRAY in shared library!",
name, preinit_array_count);
}
if (dynamic != NULL) {
for (Elf32_Dyn* d = dynamic; d->d_tag != DT_NULL; ++d) {
if (d->d_tag == DT_NEEDED) {
const char* library_name = strtab + d->d_un.d_val;
TRACE("\"%s\": calling constructors in DT_NEEDED \"%s\"", name, library_name);
find_loaded_library(library_name)->CallConstructors(); //判断库是否已经加载
}
}
}
TRACE("\"%s\": calling constructors", name);
// DT_INIT should be called before DT_INIT_ARRAY if both are present.
CallFunction("DT_INIT", init_func);
CallArray("DT_INIT_ARRAY", init_array, init_array_count, false);
}
可以看出，正如注释所写的，如说 .init 函数与 init_array 存在的话，程序会依次调用 .init 函数与.init_array 中对应位置的代码。相关说明如下
#define DT_INIT 12 /* Address of initialization function */
#define DT_INIT_ARRAY 25 /* Address of initialization function array */
- 建立一个打开的共享库的 entry，并试图其加入到对应的 list 中，方便管理。如果加入失败的话，就会对其进行释放。
/* create a new entry */
SharedLib* pNewEntry;
pNewEntry = (SharedLib*) calloc(1, sizeof(SharedLib));
pNewEntry->pathName = strdup(pathName);
pNewEntry->handle = handle;
pNewEntry->classLoader = classLoader;
dvmInitMutex(&pNewEntry->onLoadLock);
pthread_cond_init(&pNewEntry->onLoadCond, NULL);
pNewEntry->onLoadThreadId = self->threadId;
/* try to add it to the list */
SharedLib* pActualEntry = addSharedLibEntry(pNewEntry);
if (pNewEntry != pActualEntry) {
ALOGI("WOW: we lost a race to add a shared lib (%s CL=%p)",
pathName, classLoader);
freeSharedLibEntry(pNewEntry);
return checkOnLoadResult(pActualEntry);
}
- 如果加载成功，就会利用 dlsym 来获取对应 so 文件中的 JNI_OnLoad 函数，如果存在该函数的话，就进行调用，否则，就会直接返回。
else {
if (verbose)
ALOGD("Added shared lib %s %p", pathName, classLoader);
bool result = false;
void* vonLoad;
int version;
vonLoad = dlsym(handle, "JNI_OnLoad");
if (vonLoad == NULL) {
ALOGD("No JNI_OnLoad found in %s %p, skipping init", pathName, classLoader);
result = true;
} else {
/*
* Call JNI_OnLoad. We have to override the current class
* loader, which will always be "null" since the stuff at the
* top of the stack is around Runtime.loadLibrary(). (See
* the comments in the JNI FindClass function.)
*/
OnLoadFunc func = (OnLoadFunc)vonLoad;
Object* prevOverride = self->classLoaderOverride;
self->classLoaderOverride = classLoader;
oldStatus = dvmChangeStatus(self, THREAD_NATIVE);
if (gDvm.verboseJni) {
ALOGI("[Calling JNI_OnLoad for \"%s\"]", pathName);
}
version = (*func)(gDvmJni.jniVm, NULL);
dvmChangeStatus(self, oldStatus);
self->classLoaderOverride = prevOverride;
if (version == JNI_ERR) {
*detail = strdup(StringPrintf("JNI_ERR returned from JNI_OnLoad in \"%s\"",
pathName).c_str());
} else if (dvmIsBadJniVersion(version)) {
*detail = strdup(StringPrintf("Bad JNI version returned from JNI_OnLoad in \"%s\": %d",
pathName, version).c_str());
/*
* It's unwise to call dlclose() here, but we can mark it
* as bad and ensure that future load attempts will fail.
*
* We don't know how far JNI_OnLoad got, so there could
* be some partially-initialized stuff accessible through
* newly-registered native method calls. We could try to
* unregister them, but that doesn't seem worthwhile.
*/
} else {
result = true;
}
if (gDvm.verboseJni) {
ALOGI("[Returned %s from JNI_OnLoad for \"%s\"]",
(result ? "successfully" : "failure"), pathName);
}
}
if (result)
pNewEntry->onLoadResult = kOnLoadOkay;
else
pNewEntry->onLoadResult = kOnLoadFailed;
pNewEntry->onLoadThreadId = 0;
/*
* Broadcast a wakeup to anybody sleeping on the condition variable.
*/
dvmLockMutex(&pNewEntry->onLoadLock);
pthread_cond_broadcast(&pNewEntry->onLoadCond);
dvmUnlockMutex(&pNewEntry->onLoadLock);
return result;
}
}
总结¶
这说明加载 .so 文件时，会按照执行如下顺序的函数（如果不存在的话，就会跳过）
- .init 函数
- .init_array 中的函数
- JNI_OnLoad 函数