细说So动态库的加载流程

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细说So动态库的加载流程

sossai 看雪学院 2021-03-08

本文为看雪论坛优秀文章

看雪论坛作者ID：sossai

dlopen之内存装载

dlopen用来打开一个动态链接库，并将其装入内存。它的定义在Android源码中的路径为/bionic/linker/dlfcn.cpp，执行流程如下：

其核心代码在do_dlopen中实现，根据传入的路径或文件名去查找一个动态库，并执行该动态链接库的初始化代码。

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;
}

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);
  if (si != NULL) {
    si->CallConstructors();
  }
  set_soinfo_pool_protection(PROT_READ);
  return si;
}

再来看find_library这个方法，它会先在solist（已经加载的动态链接库链表）里进行查找，如果找到了就返回对应的soinfo结构体指针。否则，就调用load_library进行加载。然后，调用soinfo_link_image方法，根据soinfo结构体解析相应的Section。

static soinfo *find_loaded_library(const char *name)
{
 soinfo *si;
 const char *bname;

 // TODO: don't use basename only for determining libraries
 // http://code.google.com/p/android/issues/detail?id=6670

 bname = strrchr(name, '/');
 bname = bname ? bname + 1 : name;

 for (si = solist; si != NULL; si = si->next) {
 if (!strcmp(bname, si->name)) {
 return si;
 }
 }
 return NULL;
}

static soinfo* find_library_internal(const char* name) {
 if (name == NULL) {
 return somain;
 }

 soinfo* si = find_loaded_library(name);
 if (si != NULL) {
 if (si->flags & FLAG_LINKED) {
 return si;
 }
 DL_ERR("OOPS: recursive link to \"%s\"", si->name);
 return NULL;
 }

 TRACE("[ '%s' has not been loaded yet. Locating...]", name);
 si = load_library(name);
 if (si == NULL) {
 return NULL;
 }

 // At this point we know that whatever is loaded @ base is a valid ELF
 // shared library whose segments are properly mapped in.
 TRACE("[ init_library base=0x%08x sz=0x%08x name='%s' ]",
 si->base, si->size, si->name);

 if (!soinfo_link_image(si)) {
 munmap(reinterpret_cast<void*>(si->base), si->size);
 soinfo_free(si);
 return NULL;
 }

 return si;
}

static soinfo* find_library(const char* name) {
 soinfo* si = find_library_internal(name);
 if (si != NULL) {
 si->ref_count++;
 }
 return si;
}

load_library调用open_library打开一个动态链接库，返回一个句柄，将其与共享库所在的路径作为参数，对ElfReader进行初始化。

ElfReader作用域中的Load函数，会执行以下操作：

1. 读取并校验ELF文件头

2. 读ELF程序头并映射至内存

3. 将Load Segment加载进内存

4. 在内存中找到程序的起始地址

``` C++
bool ElfReader::Load() {
return ReadElfHeader() &&
VerifyElfHeader() &&
ReadProgramHeader() &&
ReserveAddressSpace() &&
LoadSegments() &&
FindPhdr();
}

bool ElfReader::ReadElfHeader() {
ssize_t rc = TEMP_FAILURERETRY(read(fd, &header, sizeof(header)));
if (rc < 0) {
DLERR("can't read file \"%s\": %s", name, strerror(errno));
return false;
}
if (rc != sizeof(header_)) {
DLERR("\"%s\" is too small to be an ELF executable", name);
return false;
}
return true;
}

<center>**读ELF文件头**</center>
``` C++
// Loads the program header table from an ELF file into a read-only private
// anonymous mmap-ed block.
bool ElfReader::ReadProgramHeader() {
 phdr_num_ = header_.e_phnum;

 // Like the kernel, we only accept program header tables that
 // are smaller than 64KiB.
 if (phdr_num_ < 1 || phdr_num_ > 65536/sizeof(Elf32_Phdr)) {
 DL_ERR("\"%s\" has invalid e_phnum: %d", name_, phdr_num_);
 return false;
 }

 Elf32_Addr page_min = PAGE_START(header_.e_phoff); //页的起始地址
 Elf32_Addr page_max = PAGE_END(header_.e_phoff + (phdr_num_ * sizeof(Elf32_Phdr))); //页的结束地址
 Elf32_Addr page_offset = PAGE_OFFSET(header_.e_phoff); //程序头部在页中的偏移

 phdr_size_ = page_max - page_min;

 void* mmap_result = mmap(NULL, phdr_size_, PROT_READ, MAP_PRIVATE, fd_, page_min); //将程序头映射到内存
 if (mmap_result == MAP_FAILED) {
 DL_ERR("\"%s\" phdr mmap failed: %s", name_, strerror(errno));
 return false;
 }

 phdr_mmap_ = mmap_result;
 phdr_table_ = reinterpret_cast<Elf32_Phdr*>(reinterpret_cast<char*>(mmap_result) + page_offset); //程序头表在内存中的地址
 return true;
}

<center>读ELF程序头，并映射到内存</center>

// Reserve a virtual address range big enough to hold all loadable
// segments of a program header table. This is done by creating a
// private anonymous mmap() with PROT_NONE.
bool ElfReader::ReserveAddressSpace() {
 Elf32_Addr min_vaddr;
 load_size_ = phdr_table_get_load_size(phdr_table_, phdr_num_, &min_vaddr); //根据页对齐来计算Load段所占用的大小
 if (load_size_ == 0) {
 DL_ERR("\"%s\" has no loadable segments", name_);
 return false;
 }

 uint8_t* addr = reinterpret_cast<uint8_t*>(min_vaddr);
 int mmap_flags = MAP_PRIVATE | MAP_ANONYMOUS; //匿名私有
 void* start = mmap(addr, load_size_, PROT_NONE, mmap_flags, -1, 0); //调用mmap为动态库分配一块内存空间
 if (start == MAP_FAILED) {
 DL_ERR("couldn't reserve %d bytes of address space for \"%s\"", load_size_, name_);
 return false;
 }

 load_start_ = start;
 load_bias_ = reinterpret_cast<uint8_t*>(start) - addr; //真实的加载地址与计算出来的（读ELF程序头中的p_vaddr）加载地址之差
 return true;
}

<center>调用mmap申请一块足够大的内存空间，为后面进行映射Load段的映射做准备</center>

// Map all loadable segments in process' address space.
// This assumes you already called phdr_table_reserve_memory to
// reserve the address space range for the library.
// TODO: assert assumption.
bool ElfReader::LoadSegments() {
 for (size_t i = 0; i < phdr_num_; ++i) {
 const Elf32_Phdr* phdr = &phdr_table_[i];

 if (phdr->p_type != PT_LOAD) {
 continue;
 }

 // Segment addresses in memory.
 Elf32_Addr seg_start = phdr->p_vaddr + load_bias_;
 Elf32_Addr seg_end = seg_start + phdr->p_memsz;

 Elf32_Addr seg_page_start = PAGE_START(seg_start);
 Elf32_Addr seg_page_end = PAGE_END(seg_end);

 Elf32_Addr seg_file_end = seg_start + phdr->p_filesz;

 // File offsets.
 Elf32_Addr file_start = phdr->p_offset;
 Elf32_Addr file_end = file_start + phdr->p_filesz;

 Elf32_Addr file_page_start = PAGE_START(file_start);
 Elf32_Addr file_length = file_end - file_page_start;

 if (file_length != 0) {
 void* seg_addr = mmap((void*)seg_page_start, //将Load Segment映射到内存，大小为在ELF文件中所占用的长度
 file_length,
 PFLAGS_TO_PROT(phdr->p_flags),
 MAP_FIXED|MAP_PRIVATE,
 fd_,
 file_page_start);
 if (seg_addr == MAP_FAILED) {
 DL_ERR("couldn't map \"%s\" segment %d: %s", name_, i, strerror(errno));
 return false;
 }
 }

 // if the segment is writable, and does not end on a page boundary,
 // zero-fill it until the page limit.
 if ((phdr->p_flags & PF_W) != 0 && PAGE_OFFSET(seg_file_end) > 0) {
 memset((void*)seg_file_end, 0, PAGE_SIZE - PAGE_OFFSET(seg_file_end)); //如果这块Segment是可写的，且在内存中的结束地址不在页的边界上，则将后面的数据都填充0
 }

 seg_file_end = PAGE_END(seg_file_end);

 // seg_file_end is now the first page address after the file
 // content. If seg_end is larger, we need to zero anything
 // between them. This is done by using a private anonymous
 // map for all extra pages.
 if (seg_page_end > seg_file_end) {
 void* zeromap = mmap((void*)seg_file_end, //如果seg_end大于它在文件中的长度，则继续为多出的那部分申请内存空间，并填充0。这里应该是主要针对bss段
 seg_page_end - seg_file_end,
 PFLAGS_TO_PROT(phdr->p_flags),
 MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE,
 -1,
 0);
 if (zeromap == MAP_FAILED) {
 DL_ERR("couldn't zero fill \"%s\" gap: %s", name_, strerror(errno));
 return false;
 }
 }
 }
 return true;
}

<center>将类型为Load的Segment映射到内存</center>

接下来，soinfo_alloc方法会为该库在共享库链表中分配一个soinfo节点，并初始化其数据结构。

static soinfo* load_library(const char* name) {
    // Open the file.
    int fd = open_library(name);
    if (fd == -1) {
        DL_ERR("library \"%s\" not found", name);
        return NULL;
    }

    // Read the ELF header and load the segments.
    ElfReader elf_reader(name, fd);
    if (!elf_reader.Load()) {
        return NULL;
    }

    const char* bname = strrchr(name, '/');
    soinfo* si = soinfo_alloc(bname ? bname + 1 : name);
    if (si == NULL) {
        return NULL;
    }
    si->base = elf_reader.load_start();
    si->size = elf_reader.load_size();
    si->load_bias = elf_reader.load_bias();
    si->flags = 0;
    si->entry = 0;
    si->dynamic = NULL;
    si->phnum = elf_reader.phdr_count();
    si->phdr = elf_reader.loaded_phdr();
    return si;
}

static soinfo* soinfo_alloc(const char* name) {
  if (strlen(name) >= SOINFO_NAME_LEN) {
    DL_ERR("library name \"%s\" too long", name);
    return NULL;
  }

  if (!ensure_free_list_non_empty()) {
    DL_ERR("out of memory when loading \"%s\"", name);
    return NULL;
  }

  // Take the head element off the free list.
  soinfo* si = gSoInfoFreeList;
  gSoInfoFreeList = gSoInfoFreeList->next;

  // Initialize the new element.
  memset(si, 0, sizeof(soinfo));
  strlcpy(si->name, name, sizeof(si->name));
  sonext->next = si;
  sonext = si;

  TRACE("name %s: allocated soinfo @ %p", name, si);
  return si;
}

再回过头来看下soinfo_link_image这个方法，它主要实现了动态链接库中section信息的解析：

1. 先解析dynamic section动态节区，进而实现各个Section的定位：

2. 获取其他Section的信息：

3. 待所有section信息解析完毕后，对HASH、STRTAB、SYMTAB节是否正常解析做校验：

4. 若标志位有FLAG_EXE，则表示当前程序执行的是一个可执行文件。到这里可以确定，linker不仅负责加载so，也负责解析加载一个可执行的ELF文件：

5. 加载所需要的其他共享库，其中find_library会递归调用这个so_link_image函数，直到某个so库没有DT_NEEDED段：

6. 完成rel节的重定位：

最后，CallConstructors函数会根据动态节区中的信息，获取该共享库所依赖的所有so文件名，并在已加载的动态链接库链表中进行查找、递归调用它们的初始化函数。

当运行所需的依赖库都初始化完成后，再执行init_func、init_array方法初始化该动态库。

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();
   }
}
}

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);
}

loadlibrary之加载调用

Java层通过System.load或System.loadLibrary来加载一个so文件，它的定义在Android源码中的路径为/libcore/luni/src/main/java/java/lang/System.java，执行流程如下：

接下来，让我们具体看下System.loadLibrary这个方法的实现。可以发现它实际是先通过VMStack.getCallingClassLoader()获取到ClassLoader，然后调用运行时的loadLibrary。

/**
* 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);
}

以上代码块的主要功能为：

1. 若ClassLoader非空，则利用ClassLoader的findLibrary方法来获取library的path；

2. 若ClassLoader为空，则根据传递进来的libraryName，获取到library file的name（比如传递“test”进来，经过System.mapLibraryName方法的调用，返回的会是“libtest.so”）。然后再在一个path list（即下面代码截图中的mLibPaths）中查找到这个library file，并最终确定library 的path；

3. 调用nativeLoad这个jni方法来load library。

然而，这里其实又牵扯出了几个问题：

首先，可用的library path都是哪些？这实际上也决定了我们的so文件放在哪些目录下，才可以真正的被load起来。

其次，在native层的nativeLoad又是如何实现加载的？下面会对这两个问题，逐一分析介绍。

>>>>
So的加载路径

先来看看当传入的ClassLoader为空的情况(执行System.loadLibrary时并不会发生)，那么就需要关注下mLibPaths的赋值，相应代码如下：

private static final Runtime mRuntime = new Runtime();

/**
* Holds the library paths, used for native library lookup.
*/
private final String[] mLibPaths = initLibPaths();

private static String[] initLibPaths() {
 String javaLibraryPath = System.getProperty("java.library.path");
 if (javaLibraryPath == null) {
 return EmptyArray.STRING;
 }
 String[] paths = javaLibraryPath.split(":");
 // Add a '/' to the end of each directory so we don't have to do it every time.
 for (int i = 0; i < paths.length; ++i) {
 if (!paths[i].endsWith("/")) {
 paths[i] += "/";
 }
 }
 return paths;
}

这里library path list实际上读取自一个system property，直接到System.java下查看初始化代码，它其实是LD_LIBRARY_PATH环境变量的值，具体内容可以查看注释，为"/vendor/lib:/system/lib"。

然后再来看下传入的ClassLoader非空的情况，也就是ClassLoader的findLibrary的执行过程。

/**
* Returns the absolute path of the native library with the specified name,
* or {@code null}. If this method returns {@code null} then the virtual
* machine searches the directories specified by the system property
* "java.library.path".
* 
* This implementation always returns {@code null}.
* 
*
* @param libName
* the name of the library to find.
* @return the absolute path of the library.
*/
protected String findLibrary(String libName) {
return null;
}

结果发现竟然是一个空函数，而ClassLoader本身也只是个抽象类，那系统中实际运行的ClassLoader是哪个呢？这里可以写个小程序，将实际运行的ClassLoader输出：

于是，得知android系统中ClassLoader真正的实现在dalvik.system.PathClassLoader。此外，在这条日志中，还顺带将PathClassLoader初始化的参数一同打印了出来。其中，libraryPath为"/data/app-lib/elf.xuexi-1"..

不过PathClassLoader只是继承 BaseDexClassLoader，并没有实际内容。

继续到BaseDexClassLoader下看findLibrary的实现。

可以看到，这里又是在调用DexPathList类下的findLibrary。关注splitLibraryPath方法，它返回了需要加载的动态库所在目录。

这里简单说下splitLibraryPath方法的作用，它是根据传进来的libraryPath和system property中"java.library.path"的属性值即“/vendor/lib:/system/lib”来构造出要加载的动态库所在目录列表。

/**
* Splits the given library directory path string into elements
* using the path separator ({@code File.pathSeparator}, which
* defaults to {@code ":"} on Android, appending on the elements
* from the system library path, and pruning out any elements that
* do not refer to existing and readable directories.
*/
private static File[] splitLibraryPath(String path) {
 // Native libraries may exist in both the system and
 // application library paths, and we use this search order:
 //
 // 1. this class loader's library path for application libraries
 // 2. the VM's library path from the system property for system libraries
 //
 // This order was reversed prior to Gingerbread; see http://b/2933456.
 ArrayList<File> result = splitPaths(path, System.getProperty("java.library.path"), true);
 return result.toArray(new File[result.size()]);
}

/**
* Splits the given path strings into file elements using the path
* separator, combining the results and filtering out elements
* that don't exist, aren't readable, or aren't either a regular
* file or a directory (as specified). Either string may be empty
* or {@code null}, in which case it is ignored. If both strings
* are empty or {@code null}, or all elements get pruned out, then
* this returns a zero-element list.
*/
private static ArrayList<File> splitPaths(String path1, String path2,
 boolean wantDirectories) {
 ArrayList<File> result = new ArrayList<File>();
 splitAndAdd(path1, wantDirectories, result);
 splitAndAdd(path2, wantDirectories, result);
 return result;
}

/**
* Helper for {@link #splitPaths}, which does the actual splitting
* and filtering and adding to a result.
*/
private static void splitAndAdd(String searchPath, boolean directoriesOnly,
 ArrayList<File> resultList) {
 if (searchPath == null) {
 return;
 }
 for (String path : searchPath.split(":")) {
 try {
 StructStat sb = Libcore.os.stat(path);
 if (!directoriesOnly || S_ISDIR(sb.st_mode)) {
 resultList.add(new File(path));
 }
 } catch (ErrnoException ignored) {
 }
 }
}

现在可以对动态链接库的加载路径做个总结了，系统默认的目录为"/vendor/lib"和"/system/lib"。当使用System.loadLibrary或System.load来加载一个共享库的时候，会将VM栈中的ClassLoader传入。之后调用findLibrary方法，在两个目录中去寻找指定的so文件：一个是构造ClassLoader时，传进来的那个libraryPath；另一个则是system property中"java.library.path"的属性值。也就是说，实际上是会在如下的3个目录中进行查找：

1. "/vendor/lib"

2. "/system/lib"

3. "/data/app-lib/包名-n"

对于"/data/app-lib/包名-n"这个路径，大家可能会比较陌生，但应该都知道"/data/data/包名/lib"目录，这里就简单讲解下apk安装过程中的一点细节，以说明二者之间的关系（在Android源码中的路径为"/frameworks/native/cmds/installd/commands.c"）

int install(const char *pkgname, uid_t uid, gid_t gid, const char *seinfo)
{
 char pkgdir[PKG_PATH_MAX];
 char libsymlink[PKG_PATH_MAX];
 char applibdir[PKG_PATH_MAX];
 struct stat libStat;

 if ((uid < AID_SYSTEM) || (gid < AID_SYSTEM)) {
 ALOGE("invalid uid/gid: %d %d\n", uid, gid);
 return -1;
 }

 if (create_pkg_path(pkgdir, pkgname, PKG_DIR_POSTFIX, 0)) { //创建包路径，"/data/data/包名"
 ALOGE("cannot create package path\n");
 return -1;
 }

 if (create_pkg_path(libsymlink, pkgname, PKG_LIB_POSTFIX, 0)) { //创建库路径，"/data/data/包名/lib"
 ALOGE("cannot create package lib symlink origin path\n");
 return -1;
 }

 if (create_pkg_path_in_dir(applibdir, &android_app_lib_dir, pkgname, PKG_DIR_POSTFIX)) { //创建"/data/app-lib/包名"目录
 ALOGE("cannot create package lib symlink dest path\n");
 return -1;
 }

 if (mkdir(pkgdir, 0751) < 0) {
 ALOGE("cannot create dir '%s': %s\n", pkgdir, strerror(errno));
 return -1;
 }
 if (chmod(pkgdir, 0751) < 0) {
 ALOGE("cannot chmod dir '%s': %s\n", pkgdir, strerror(errno));
 unlink(pkgdir);
 return -1;
 }

 if (lstat(libsymlink, &libStat) < 0) {
 if (errno != ENOENT) {
 ALOGE("couldn't stat lib dir: %s\n", strerror(errno));
 return -1;
 }
 } else {
 if (S_ISDIR(libStat.st_mode)) {
 if (delete_dir_contents(libsymlink, 1, 0) < 0) {
 ALOGE("couldn't delete lib directory during install for: %s", libsymlink);
 return -1;
 }
 } else if (S_ISLNK(libStat.st_mode)) {
 if (unlink(libsymlink) < 0) {
 ALOGE("couldn't unlink lib directory during install for: %s", libsymlink);
 return -1;
 }
 }
 }

 if (symlink(applibdir, libsymlink) < 0) {
 ALOGE("couldn't symlink directory '%s' -> '%s': %s\n", libsymlink, applibdir,
 strerror(errno));
 unlink(pkgdir);
 return -1;
 }

 if (selinux_android_setfilecon2(pkgdir, pkgname, seinfo, uid) < 0) {
 ALOGE("cannot setfilecon dir '%s': %s\n", pkgdir, strerror(errno));
 unlink(libsymlink);
 unlink(pkgdir);
 return -errno;
 }

 if (chown(pkgdir, uid, gid) < 0) {
 ALOGE("cannot chown dir '%s': %s\n", pkgdir, strerror(errno));
 unlink(libsymlink);
 unlink(pkgdir);
 return -1;
 }

 return 0;
}

以上代码会先构造几个目录名：pkgdir为"/data/data/包名"，libsymlink为"/data/data/包名/lib"，applibdir为"/data/app-lib/包名"。然后创建相应目录，并赋权限。之后，建立"/data/data/包名/lib"指向"/data/app-lib/包名"的符号链接。

现在再回过头来说明下"/data/app-lib/包名-n"、"/data/data/包名/lib"这二者之间的关系。在"/data/data/包名/"目录下执行ls –l命令，就会发现lib是一个链接，So其实是放在"/data/app-lib/包名-n"路径下的。

>>>>
Native层的加载实现

doLoad实际上是调用本地的nativeLoad方法，nativeLoad会先更新LD_LIBRARY_PATH，然后执行dvmLoadNativeCode函数，真正实现so文件的加载。

/*
* 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);
}

dvmLoadNativeCode定义在Android源码中的路径为/dalvik/vm/Native.cpp，它的主要功能如下：

1. 调用findSharedLibEntry方法，遍历查找已加载的lib。具体来说，就是先用待加载的lib路径名计算出一个32位hash值，然后遍历gDvm中的nativeLibs（其结构为HashTable用来保存加载的本地库），如果找到则返回一个SharedLib结构。这里如果LIB已被加载，则会对其加载的ClassLoader进行比较，JNI只允许同一个LIB只被一个ClassLoader加载。

2. 调用dlopen打开一个so。

3. 将新加载的LIB插入到gDvm保存的链表中，执行JNI_OnLoad的调用。

总结

在了解So在内存中的加载原理后，可以得知以下几点：

So的加载路径为："/vendor/lib"、"/system/lib"、"/data/app-lib/包名-n"。
So的入口为init_array、init_func这些初始化函数。这部分在dlopen的过程中就会执行，再之后的是JNI_Onload方法的调用。这里面可以注册一些本地方法，也可以继续做些变量的初始化等操作。
在So的加载流程中，最终会被存放到SharedLib这个结构体中，并添加到nativeLibs这个hash表下。

- End -

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