涉及源码（Android 4.4.2）：
/libcore/libart/src/main/java/java/lang/Class.java
/dalvik/vm/native/java_lang_Class.cpp
/dalvik/vm/native/InternalNative.cpp
/dalvik/vm/oo/Class.cpp
/libcore/libart/src/main/java/java/lang/Class.java
/libcore/libart/src/main/java/java/lang/ClassLoader.java
/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java
/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java
/libcore/dalvik/src/main/java/dalvik/system/DexFile.java
/dalvik/vm/native/dalvik_system_DexFile.cpp
/dalvik/vm/oo/Class.cpp
/dalvik/libdex/DexFile.cpp

从Class.forName()函数看起。

public static Class<?> forName(String className) throws ClassNotFoundException {
    return forName(className, true, VMStack.getCallingClassLoader());
}

可以看到默认使用的classLoader是调用该函数的类的classLoader。

public static Class<?> forName(String className, boolean shouldInitialize,
        ClassLoader classLoader) throws ClassNotFoundException {

    if (classLoader == null) {
        classLoader = ClassLoader.getSystemClassLoader();
    }

    Class<?> result;
    try {
        result = classForName(className, shouldInitialize, classLoader);
    } catch (ClassNotFoundException e) {
        Throwable cause = e.getCause();
        if (cause instanceof LinkageError) {
            throw (LinkageError) cause;
        }
        throw e;
    }
    return result;
}

static native Class<?> classForName(String className, boolean shouldInitialize,
            ClassLoader classLoader) throws ClassNotFoundException;

最终会调用到native方法

/dalvik/vm/native/java_lang_Class.cpp

static void Dalvik_java_lang_Class_classForName(const u4* args, JValue* pResult)
{
    StringObject* nameObj = (StringObject*) args[0];
    bool initialize = (args[1] != 0);
    Object* loader = (Object*) args[2];

    RETURN_PTR(dvmFindClassByName(nameObj, loader, initialize));
}

/dalvik/vm/native/InternalNative.cpp

ClassObject* dvmFindClassByName(StringObject* nameObj, Object* loader,
    bool doInit)
{
    ClassObject* clazz = NULL;
    char* name = NULL;
    char* descriptor = NULL;

    if (nameObj == NULL) {
        dvmThrowNullPointerException("name == null");
        goto bail;
    }
    name = dvmCreateCstrFromString(nameObj);

    if (!dexIsValidClassName(name, true)) {
        ALOGW("dvmFindClassByName rejecting '%s'", name);
        dvmThrowClassNotFoundException(name);
        goto bail;
    }

    descriptor = dvmDotToDescriptor(name);
    if (descriptor == NULL) {
        goto bail;
    }

    if (doInit)
        clazz = dvmFindClass(descriptor, loader);
    else
        clazz = dvmFindClassNoInit(descriptor, loader);

    if (clazz == NULL) {
        LOGVV("FAIL: load %s (%d)", descriptor, doInit);
        Thread* self = dvmThreadSelf();
        Object* oldExcep = dvmGetException(self);
        dvmAddTrackedAlloc(oldExcep, self);     /* don't let this be GCed */
        dvmClearException(self);
        dvmThrowChainedClassNotFoundException(name, oldExcep);
        dvmReleaseTrackedAlloc(oldExcep, self);
    } else {
        LOGVV("GOOD: load %s (%d) --> %p ldr=%p",
            descriptor, doInit, clazz, clazz->classLoader);
    }

bail:
    free(name);
    free(descriptor);
    return clazz;
}

重点关注dvmFindClassNoInit方法。

/dalvik/vm/oo/Class.cpp

ClassObject* dvmFindClassNoInit(const char* descriptor,
        Object* loader)
{
    assert(descriptor != NULL);
    //assert(loader != NULL);

    LOGVV("FindClassNoInit '%s' %p", descriptor, loader);

    if (*descriptor == '[') {
        /*
         * Array class.  Find in table, generate if not found.
         */
        return dvmFindArrayClass(descriptor, loader);
    } else {
        /*
         * Regular class.  Find in table, load if not found.
         */
        if (loader != NULL) {
            return findClassFromLoaderNoInit(descriptor, loader);
        } else {
            return dvmFindSystemClassNoInit(descriptor);
        }
    }
}

进入到findClassFromLoaderNoInit方法

static ClassObject* findClassFromLoaderNoInit(const char* descriptor,
    Object* loader)
{

    Thread* self = dvmThreadSelf();

  
     // 1、首先进行查找是否以及进行了加载
    ClassObject* clazz = dvmLookupClass(descriptor, loader, false);
    if (clazz != NULL) {
        LOGVV("Already loaded: %s %p", descriptor, loader);
        return clazz;
    } else {
        LOGVV("Not already loaded: %s %p", descriptor, loader);
    }

    char* dotName = NULL;
    StringObject* nameObj = NULL;

    /* convert "Landroid/debug/Stuff;" to "android.debug.Stuff" */
    dotName = dvmDescriptorToDot(descriptor);
    if (dotName == NULL) {
        dvmThrowOutOfMemoryError(NULL);
        return NULL;
    }
    nameObj = dvmCreateStringFromCstr(dotName);
    if (nameObj == NULL) {
        assert(dvmCheckException(self));
        goto bail;
    }

    dvmMethodTraceClassPrepBegin();

   
    LOGVV("--- Invoking loadClass(%s, %p)", dotName, loader);
    {
        // 2、如果没有加载，这里调用java/lang/ClassLoader的loadClass函数来加载类
        const Method* loadClass =
            loader->clazz->vtable[gDvm.voffJavaLangClassLoader_loadClass];
        JValue result;
        dvmCallMethod(self, loadClass, loader, &result, nameObj);
        clazz = (ClassObject*) result.l;

        dvmMethodTraceClassPrepEnd();
        Object* excep = dvmGetException(self);
        if (excep != NULL) {
#if DVM_SHOW_EXCEPTION >= 2
            ALOGD("NOTE: loadClass '%s' %p threw exception %s",
                 dotName, loader, excep->clazz->descriptor);
#endif
            dvmAddTrackedAlloc(excep, self);
            dvmClearException(self);
            dvmThrowChainedNoClassDefFoundError(descriptor, excep);
            dvmReleaseTrackedAlloc(excep, self);
            clazz = NULL;
            goto bail;
        } else if (clazz == NULL) {
            ALOGW("ClassLoader returned NULL w/o exception pending");
            dvmThrowNullPointerException("ClassLoader returned null");
            goto bail;
        }
    }

    /* not adding clazz to tracked-alloc list, because it's a ClassObject */

    dvmAddInitiatingLoader(clazz, loader);

    LOGVV("--- Successfully loaded %s %p (thisldr=%p clazz=%p)",
        descriptor, clazz->classLoader, loader, clazz);

bail:
    dvmReleaseTrackedAlloc((Object*)nameObj, NULL);
    free(dotName);
    return clazz;
}

1、首先调用dvmLookupClass方法在gDvm.loadedClasses中查找是否存在该类，加载过的类都会存放到gDvm.loadedClasses中。

2、如果没有加载过。可以看到最终是调用java/lang/ClassLoader的loadClass函数来加载类，下面来看看ClassLoader的loadClass方法。

/libcore/libart/src/main/java/java/lang/ClassLoader.java

public Class<?> loadClass(String name) throws ClassNotFoundException {
    return loadClass(name, false);
}

protected Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException {
    // 查看该类是否以及加载
    Class<?> clazz = findLoadedClass(className);
    // 委托机制
    if (clazz == null) {
        ClassNotFoundException suppressed = null;
        try {
            clazz = parent.loadClass(className, false);
        } catch (ClassNotFoundException e) {
            suppressed = e;
        }
        // 最终如果没有找到该类，就自己来查找
        if (clazz == null) {
            try {
                clazz = findClass(className);
            } catch (ClassNotFoundException e) {
                e.addSuppressed(suppressed);
                throw e;
            }
        }
    }

    return clazz;
}

查看BaseDexClassLoader的findClass方法。

/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java

@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
    List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
    Class c = pathList.findClass(name, suppressedExceptions);
    if (c == null) {
        ClassNotFoundException cnfe = new ClassNotFoundException("Didn't find class \"" + name + "\" on path: " + pathList);
        for (Throwable t : suppressedExceptions) {
            cnfe.addSuppressed(t);
        }
        throw cnfe;
    }
    return c;
}

/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java

public Class findClass(String name, List<Throwable> suppressed) {
    // 遍历dexElements数组
    for (Element element : dexElements) {
        DexFile dex = element.dexFile;

        if (dex != null) {
            Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
            if (clazz != null) {
                return clazz;
            }
        }
    }
    if (dexElementsSuppressedExceptions != null) {
        suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
    }
    return null;
}

/libcore/dalvik/src/main/java/dalvik/system/DexFile.java

public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
    return defineClass(name, loader, mCookie, suppressed);
}

private static Class defineClass(String name, ClassLoader loader, int cookie,
                                 List<Throwable> suppressed) {
    Class result = null;
    try {
        result = defineClassNative(name, loader, cookie);
    } catch (NoClassDefFoundError e) {
        if (suppressed != null) {
            suppressed.add(e);
        }
    } catch (ClassNotFoundException e) {
        if (suppressed != null) {
            suppressed.add(e);
        }
    }
    return result;
}

private static native Class defineClassNative(String name, ClassLoader loader, int cookie)
        throws ClassNotFoundException, NoClassDefFoundError;

/dalvik/vm/native/dalvik_system_DexFile.cpp

static void Dalvik_dalvik_system_DexFile_defineClassNative(const u4* args,
    JValue* pResult)
{
    StringObject* nameObj = (StringObject*) args[0];
    Object* loader = (Object*) args[1];
    int cookie = args[2];
    ClassObject* clazz = NULL;
    DexOrJar* pDexOrJar = (DexOrJar*) cookie;
    DvmDex* pDvmDex;
    char* name;
    char* descriptor;

    name = dvmCreateCstrFromString(nameObj);
    descriptor = dvmDotToDescriptor(name);
    ALOGV("--- Explicit class load '%s' l=%p c=0x%08x",
        descriptor, loader, cookie);
    free(name);

    if (!validateCookie(cookie))
        RETURN_VOID();

    if (pDexOrJar->isDex)
        pDvmDex = dvmGetRawDexFileDex(pDexOrJar->pRawDexFile);
    else
        pDvmDex = dvmGetJarFileDex(pDexOrJar->pJarFile);

    /* once we load something, we can't unmap the storage */
    pDexOrJar->okayToFree = false;

    clazz = dvmDefineClass(pDvmDex, descriptor, loader);
    Thread* self = dvmThreadSelf();
    if (dvmCheckException(self)) {
        /*
         * If we threw a "class not found" exception, stifle it, since the
         * contract in the higher method says we simply return null if
         * the class is not found.
         */
        Object* excep = dvmGetException(self);
        if (strcmp(excep->clazz->descriptor,
                   "Ljava/lang/ClassNotFoundException;") == 0 ||
            strcmp(excep->clazz->descriptor,
                   "Ljava/lang/NoClassDefFoundError;") == 0)
        {
            dvmClearException(self);
        }
        clazz = NULL;
    }

    free(descriptor);
    RETURN_PTR(clazz);
}

重点看看dvmDefineClass方法

/dalvik/vm/oo/Class.cpp

ClassObject* dvmDefineClass(DvmDex* pDvmDex, const char* descriptor,
    Object* classLoader)
{
    assert(pDvmDex != NULL);

    return findClassNoInit(descriptor, classLoader, pDvmDex);
}


static ClassObject* findClassNoInit(const char* descriptor, Object* loader,
    DvmDex* pDvmDex)
{
    ClassObject* clazz;
    // 查看是否以及加载
    clazz = dvmLookupClass(descriptor, loader, true);
    // 如果没有加载，则进行查找加载
     if (clazz == NULL) {
         if (pDvmDex == NULL) {
            pDvmDex = searchBootPathForClass(descriptor, &pClassDef);
        } else {
            pClassDef = dexFindClass(pDvmDex->pDexFile, descriptor);
        }
        
        clazz = loadClassFromDex(pDvmDex, pClassDef, loader);
        // 进行缓存
        dvmAddClassToHash(clazz);
     }
     
     
}

下面看看dexFindClass方法。

/dalvik/libdex/DexFile.cpp

const DexClassDef* dexFindClass(const DexFile* pDexFile,
    const char* descriptor)
{
    const DexClassLookup* pLookup = pDexFile->pClassLookup;
    u4 hash;
    int idx, mask;

    hash = classDescriptorHash(descriptor);
    mask = pLookup->numEntries - 1;
    idx = hash & mask;

    /*
     * Search until we find a matching entry or an empty slot.
     */
    while (true) {
        int offset;

        offset = pLookup->table[idx].classDescriptorOffset;
        if (offset == 0)
            return NULL;

        if (pLookup->table[idx].classDescriptorHash == hash) {
            const char* str;

            str = (const char*) (pDexFile->baseAddr + offset);
            if (strcmp(str, descriptor) == 0) {
                return (const DexClassDef*)
                    (pDexFile->baseAddr + pLookup->table[idx].classDefOffset);
            }
        }

        idx = (idx + 1) & mask;
    }
}

从DexFile的pClassLookup中进行查找

/dalvik/vm/oo/Class.cpp

bool dvmAddClassToHash(ClassObject* clazz)
{
    void* found;
    u4 hash;

    hash = dvmComputeUtf8Hash(clazz->descriptor);

    dvmHashTableLock(gDvm.loadedClasses);
    found = dvmHashTableLookup(gDvm.loadedClasses, hash, clazz,
                hashcmpClassByClass, true);
    dvmHashTableUnlock(gDvm.loadedClasses);

    ALOGV("+++ dvmAddClassToHash '%s' %p (isnew=%d) --> %p",
        clazz->descriptor, clazz->classLoader,
        (found == (void*) clazz), clazz);

    //dvmCheckClassTablePerf();

    /* can happen if two threads load the same class simultaneously */
    return (found == (void*) clazz);
}

已经被加载过的类会存放到gDvm.loadedClasses里面去。

整个流程图基本如下：

dexload3.png