Android系統採用OpenGL繪製3D圖形，使用skia來繪製二維圖形；OpenGL原始碼位於：

frameworks/native/opengl

frameworks/base/opengl

本文簡單介紹OpenGL庫的載入過程。OpenGL以動態庫的方式提供，因此在使用OpenGL的介面函式繪圖前，需要載入OpenGL庫，並得到介面函式指標。函式EGLBoolean egl_init_drivers()就是負責OpenGL庫的載入。

EGLBoolean egl_init_drivers() {
    EGLBoolean res;
    pthread_mutex_lock(&sInitDriverMutex);
    res = egl_init_drivers_locked();
    pthread_mutex_unlock(&sInitDriverMutex);
    return res;
}
 

為保證多執行緒訪問的安全性，使用執行緒鎖來放完另一個介面函式egl_init_drivers_locked

static EGLBoolean egl_init_drivers_locked() {
    if (sEarlyInitState) {
        // initialized by static ctor. should be set here.
        return EGL_FALSE;
    }
    // 得到Loader物件例項
    Loader& loader(Loader::getInstance());
    //載入EGL庫
    egl_connection_t* cnx = &gEGLImpl;
    if (cnx->dso == 0) {
        cnx->hooks[egl_connection_t::GLESv1_INDEX] =&gHooks[egl_connection_t::GLESv1_INDEX];
        cnx->hooks[egl_connection_t::GLESv2_INDEX] =&gHooks[egl_connection_t::GLESv2_INDEX];
        cnx->dso = loader.open(cnx);
    }
    return cnx->dso ? EGL_TRUE : EGL_FALSE;
}
 

函式首先定義指標cnx指向全域性變數gEGLImpl，並且將cnx的域hooks指向gHooks，最後通過loader物件的open函式開啟EGL動態庫，因此最後從EGL庫中載入的介面函式指標都儲存到了變數gEGLImpl和gHooks中了。

frameworks\native\opengl\libs\EGL\Loader.cpp

Loader::Loader()
{
    char line[256];
    char tag[256];
    /* Special case for GLES emulation 針對模擬器處理*/
    if (checkGlesEmulationStatus() == 0) {
        ALOGD("Emulator without GPU support detected. ""Fallback to software renderer.");
        mDriverTag.setTo("android");
        return;
    }
    /*開啟/system/lib/egl/egl.cfg檔案 */
    FILE* cfg = fopen("/system/lib/egl/egl.cfg", "r");
    if (cfg == NULL) {
        //如果開啟失敗，mDriverTag ="android"
        ALOGD("egl.cfg not found, using default config");
        mDriverTag.setTo("android");
    } else {//否則讀取檔案內容
        while (fgets(line, 256, cfg)) {
            int dpy, impl;
            if (sscanf(line, "%u %u %s", &dpy, &impl, tag) == 3) {
                // /system/lib/egl/egl.cfg檔案內容為：0 1 mali
                if (tag != String8("android")) {
                    mDriverTag = tag; //mDriverTag = "mali"
                }
            }
        }
        fclose(cfg);
    }
}
 

如果/system/lib/egl/egl.cfg檔案不存在，則預設配置為0 0 android，否則就讀取/system/lib/egl/egl.cfg檔案內容，內容定義如下：

因此變數dpy=0, impl = 1, tag = mali

void* Loader::open(egl_connection_t* cnx)
{
    void* dso;
    driver_t* hnd = 0;
    char const* tag = mDriverTag.string();//tag="mali"
    if (tag) {
		//載入GLES庫函式
        dso = load_driver("GLES", tag, cnx, EGL | GLESv1_CM | GLESv2);
        if (dso) {
            hnd = new driver_t(dso);
        } else {//如果載入GLES庫失敗，則載入EGL，GLESv1_CM，GLESv2三個庫
            dso = load_driver("EGL", tag, cnx, EGL);
            if (dso) {//只有EGL庫載入成功，才載入GLESv1_CM，GLESv2庫
                hnd = new driver_t(dso);
                // TODO: make this more automated
                hnd->set(load_driver("GLESv1_CM", tag, cnx, GLESv1_CM), GLESv1_CM );
                hnd->set(load_driver("GLESv2",    tag, cnx, GLESv2),    GLESv2 );
            }
        }
    }
    return (void*)hnd;
}

這個函式首先去載入/system/lib/egl/libGLES_mali.so，如果載入成功，那麼對EGL | GLESv1_CM | GLESv2三個函式庫，進行初始化。如果載入不成功，那麼就載入libEGL_mali.so，libGLESv1_CM_mali.so，libGLESv2_mali.so這三個庫，/system/lib/egl目錄下的庫如下：

由於libGLES_mali.so庫不存在，因此最終載入的是libEGL_mali.so，libGLESv1_CM_mali.so，libGLESv2_mali.so三個庫

void *Loader::load_driver(const char* kind, const char *tag,
        egl_connection_t* cnx, uint32_t mask)
{
    char driver_absolute_path[PATH_MAX];
    const char* const search1 = "/vendor/lib/egl/lib%s_%s.so";
    const char* const search2 = "/system/lib/egl/lib%s_%s.so";
	//首先從/vendor/lib/egl/目錄下查詢對應的庫，如果該目錄下沒有，則查詢/system/lib/egl/目錄
    snprintf(driver_absolute_path, PATH_MAX, search1, kind, tag);
	//driver_absolute_path="/vendor/lib/egl/libEGL_mali.so"
	//driver_absolute_path="/vendor/lib/egl/libGLESv1_CM_mali.so"
	//driver_absolute_path="/vendor/lib/egl/libGLESv2_mali.so"
	//判斷該路徑下的動態庫是否可以訪問
    if (access(driver_absolute_path, R_OK)) {
        snprintf(driver_absolute_path, PATH_MAX, search2, kind, tag);
		//driver_absolute_path="/system/lib/egl/libEGL_mali.so"
		//driver_absolute_path="/system/lib/egl/libGLESv1_CM_mali.so"
		//driver_absolute_path="/system/lib/egl/libGLESv2_mali.so"
        if (access(driver_absolute_path, R_OK)) {
            return 0;
        }
    }
	//開啟so動態庫到cnx->egl中
    void* dso = dlopen(driver_absolute_path, RTLD_NOW | RTLD_LOCAL);
    if (dso == 0) {
        const char* err = dlerror();
        ALOGE("load_driver(%s): %s", driver_absolute_path, err?err:"unknown");
        return 0;
    }
    ALOGD("loaded %s", driver_absolute_path);
	//從動態庫中載入EGL函式庫到cnx->egl中，EGL庫函式定義在egl_names陣列中
    if (mask & EGL) {
		//讀取動態庫中eglGetProcAddress函式指標
        getProcAddress = (getProcAddressType)dlsym(dso, "eglGetProcAddress");
        ALOGE_IF(!getProcAddress, "can't find eglGetProcAddress() in %s", driver_absolute_path);
#ifdef SYSTEMUI_PBSIZE_HACK
#warning "SYSTEMUI_PBSIZE_HACK enabled"
        /*
         * Here we adjust the PB size from its default value to 512KB which
         * is the minimum acceptable for the systemui process.
         */
        const char *cmdline = getProcessCmdline();
        if (strstr(cmdline, "systemui")) {
            void *imgegl = dlopen("/vendor/lib/libIMGegl.so", RTLD_LAZY);
            if (imgegl) {
                unsigned int *PVRDefaultPBS =(unsigned int *)dlsym(imgegl, "PVRDefaultPBS");
                if (PVRDefaultPBS) {
                    ALOGD("setting default PBS to 512KB, was %d KB", *PVRDefaultPBS / 1024);
                    *PVRDefaultPBS = 512*1024;
                }
            }
        }
#endif
        egl_t* egl = &cnx->egl;
        __eglMustCastToProperFunctionPointerType* curr =
            (__eglMustCastToProperFunctionPointerType*)egl;
	   /*定義指向egl_names陣列的指標
		* char const * const egl_names[] = {
		*	#include "egl_entries.in"
		*	NULL
		* };
		* 檔案frameworks/native/opengl/libs/EGL/egl_entries.in聲明瞭EGL庫中的所有函式
		*/
        char const * const * api = egl_names;
		//遍歷陣列egl_names，將陣列中定義的函式指標儲存到cnx->egl中
        while (*api) {
            char const * name = *api;
			//根據函式名稱從動態庫EGL中取得函式指標
            __eglMustCastToProperFunctionPointerType f = 
                (__eglMustCastToProperFunctionPointerType)dlsym(dso, name);
            if (f == NULL) {
                //如果在動態庫中查詢不到該函式指標，則使用getProcAddress函式來獲取
                f = getProcAddress(name);
                if (f == NULL) {//如果依然得不到該函式指標，則設定為0
                    f = (__eglMustCastToProperFunctionPointerType)0;
                }
            }
            *curr++ = f;
            api++;
        }
    }
	//char const * const gl_names[] = {
    // #include "entries.in"
    // NULL
	//};
	//從動態庫中載入GLESv1_CM函式庫到cnx->hooks[egl_connection_t::GLESv1_INDEX]->gl中，GLESv1_CM庫函式定義在gl_names陣列中
    if (mask & GLESv1_CM) {
        init_api(dso, gl_names,(__eglMustCastToProperFunctionPointerType*)
                &cnx->hooks[egl_connection_t::GLESv1_INDEX]->gl,
            getProcAddress);
    }
	//從動態庫中載入GLESv2函式庫到cnx->hooks[egl_connection_t::GLESv2_INDEX]->gl中，GLESv2庫函式定義在gl_names陣列中
    if (mask & GLESv2) {
      init_api(dso, gl_names,(__eglMustCastToProperFunctionPointerType*)
                &cnx->hooks[egl_connection_t::GLESv2_INDEX]->gl,
            getProcAddress);
    }
    return dso;
}

該函式就是從動態庫libEGL_mali.so中查詢egl_names陣列，即frameworks/native/opengl/libs/EGL/egl_entries.in檔案中宣告的介面函式，及從動態庫libGLESv1_CM_mali.so，libGLESv2_mali.so中查詢gl_names陣列，即frameworks/native/opengl/libs/entries.in檔案中宣告的介面函式，函式宣告如下：

void Loader::init_api(void* dso, char const * const * api, 
        __eglMustCastToProperFunctionPointerType* curr, 
        getProcAddressType getProcAddress) 
{
    const ssize_t SIZE = 256;
    char scrap[SIZE];
    while (*api) {
        char const * name = *api;
		//根據函式名稱從動態庫中查詢函式指標
        __eglMustCastToProperFunctionPointerType f = (__eglMustCastToProperFunctionPointerType)dlsym(dso, name);
        if (f == NULL) {
            //使用eglGetProcAddress()得到指定函式指標
            f = getProcAddress(name);
        }
        if (f == NULL) {
            //將函式名稱去掉字尾OES
            ssize_t index = ssize_t(strlen(name)) - 3;
            if ((index>0 && (index<SIZE-1)) && (!strcmp(name+index, "OES"))) {
                strncpy(scrap, name, index);
                scrap[index] = 0;
                f = (__eglMustCastToProperFunctionPointerType)dlsym(dso, scrap);
            }
        }
        if (f == NULL) {
            //將函式名稱增加字尾OES 
            ssize_t index = ssize_t(strlen(name)) - 3;
            if (index>0 && strcmp(name+index, "OES")) {
                snprintf(scrap, SIZE, "%sOES", name);
                f = (__eglMustCastToProperFunctionPointerType)dlsym(dso, scrap);
            }
        }
        if (f == NULL) {
            f = (__eglMustCastToProperFunctionPointerType)gl_unimplemented;
            if (!strcmp(name, "glInsertEventMarkerEXT")) {
                f = (__eglMustCastToProperFunctionPointerType)gl_noop;
            } else if (!strcmp(name, "glPushGroupMarkerEXT")) {
                f = (__eglMustCastToProperFunctionPointerType)gl_noop;
            } else if (!strcmp(name, "glPopGroupMarkerEXT")) {
                f = (__eglMustCastToProperFunctionPointerType)gl_noop;
            }
        }
        *curr++ = f;
        api++;
    }
}

load_driver函式所做的工作：首先通過dlopen載入/system/lib/egl/libGLES_mali.so庫，然後從/system/lib/egl/libGLES_mali.so庫中提取EGL的各個API函式的地址放到cnx->egl中，從libGLES_mali.so獲取GLESv1_CM的API儲存到cnx->hooks[GLESv1_INDEX]->gl中，從libGLES_mali.so獲取GLESv1_CM的API儲存到cnx->hooks[GLESv2_INDEX]->gl。提取EGLAPI地址的方法是首先通過dlsym函式獲得一個獲取函式地址的函式eglGetProcAddress的地址，然後遍歷EGL的API所在檔案frameworks/base/opengl/libs/EGL/egl_entries.in。先通過dlsym獲取各個API地址，如果返回NULL再利用eglGetProcAddress去獲得，如果依舊為空就把函式地址賦值為0；提取GLESv1_CM和GLESv1_CM庫中函式地址方法和提取EGL差不多，只是他們的函式檔案儲存在frameworks/base/opengl/libs/entries.in中。還有它們把函式地址複製給了cnx->hooks[GLESv1_INDEX]->gl和cnx->hooks[GLESv2_INDEX]->gl。