DexClassLoader和PathClassLoader类加载机制

    0x00

    在DexClassLoader和PathClassLoader加载Dex流程一文中，我们分析了dex文件如何形成了DexFile结构体。本文中讲解类加载机制，实际上就是生成ClassObject对象。

    我们以DexClassLoader为例，讲解类加载机制，PathClassLoader是一样的。

    我们在加载类时通常会调用loadClass，那么我们就从loadClass来开始分析。

    0x01

    DexClassLoader类没有loadClass方法，所以调用的是父类ClassLoader类的loadClass方法，ClassLoader类的loadClass方法位于libcore\luni\src\main\java\java\lang\ClassLoader.java中。

    protected Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException {
        Class<?> clazz = findLoadedClass(className);

        if (clazz == null) {
            try {
                clazz = parent.loadClass(className, false);
            } catch (ClassNotFoundException e) {
                // Don‘t want to see this.
            }

            if (clazz == null) {
                clazz = findClass(className);
            }
        }

        return clazz;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        if (VERBOSE_DEBUG)
            System.out.println("DexClassLoader " + this
                + ": findClass ‘" + name + "‘");

        int length = mFiles.length;

        for (int i = 0; i < length; i++) {
            if (VERBOSE_DEBUG)
                System.out.println("  Now searching: " + mFiles[i].getPath());

            if (mDexs[i] != null) {
                String slashName = name.replace(‘.‘, ‘/‘);
                Class clazz = mDexs[i].loadClass(slashName, this);
                if (clazz != null) {
                    if (VERBOSE_DEBUG)
                        System.out.println("    found");
                    return clazz;
                }
            }
        }

        throw new ClassNotFoundException(name + " in loader " + this);
    }

    public Class loadClass(String name, ClassLoader loader) {
        String slashName = name.replace(‘.‘, ‘/‘);
        return loadClassBinaryName(slashName, loader);
    }

    public Class loadClassBinaryName(String name, ClassLoader loader) {
        return defineClass(name, loader, mCookie,
            null);
            //new ProtectionDomain(name) /*DEBUG ONLY*/);
    }

native private static Class defineClass(String name, ClassLoader loader,
        int cookie, ProtectionDomain pd);

const DalvikNativeMethod dvm_dalvik_system_DexFile[] = {
    { "openDexFile",        "(Ljava/lang/String;Ljava/lang/String;I)I",
        Dalvik_dalvik_system_DexFile_openDexFile },
    { "closeDexFile",       "(I)V",
        Dalvik_dalvik_system_DexFile_closeDexFile },
    { "defineClass",        "(Ljava/lang/String;Ljava/lang/ClassLoader;ILjava/security/ProtectionDomain;)Ljava/lang/Class;",
        Dalvik_dalvik_system_DexFile_defineClass },
    { "getClassNameList",   "(I)[Ljava/lang/String;",
        Dalvik_dalvik_system_DexFile_getClassNameList },
    { "isDexOptNeeded",     "(Ljava/lang/String;)Z",
        Dalvik_dalvik_system_DexFile_isDexOptNeeded },
    { NULL, NULL, NULL },
};

    defineClass对应的是Dalvik_dalvik_system_DexFile_defineClass方法。注意defineClass函数传递进来的参数有一个是mCookie，就是在DexClassLoader和PathClassLoader加载Dex流程一文中，openDexFile生成的，利用这个mCookie可以在native层找到openDexFile生成的DexFile结构体。

    Dalvik_dalvik_system_DexFile_defineClass代码位于dalvik\vm\native\dalvik_system_DexFile.c。

static void Dalvik_dalvik_system_DexFile_defineClass(const u4* args,
    JValue* pResult)
{
    StringObject* nameObj = (StringObject*) args[0];
    Object* loader = (Object*) args[1];
    int cookie = args[2];
    Object* pd = (Object*) args[3];
    ClassObject* clazz = NULL;
    DexOrJar* pDexOrJar = (DexOrJar*) cookie;
    DvmDex* pDvmDex;
    char* name;
    char* descriptor;

    name = dvmCreateCstrFromString(nameObj);
    descriptor = dvmDotToDescriptor(name);
    LOGV("--- Explicit class load ‘%s‘ 0x%08x\n", descriptor, 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);
    ......

    ......

    free(descriptor);
    RETURN_PTR(clazz);
}

    下面我们来分析核心函数dvmDefineClass，这个用来生成ClassObject。dvmDefineClass，findClassNoInit 方法都位于dalvik\vm\oo\Class.c。

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)
{
    Thread* self = dvmThreadSelf();
    ClassObject* clazz;
    bool profilerNotified = false;

    ......
    clazz = dvmLookupClass(descriptor, loader, true);
    if (clazz == NULL) {
        const DexClassDef* pClassDef;

        ......

        if (pDvmDex == NULL) {
            assert(loader == NULL);     /* shouldn‘t be here otherwise */
            pDvmDex = searchBootPathForClass(descriptor, &pClassDef);
        } else {
            pClassDef = dexFindClass(pDvmDex->pDexFile, descriptor);
        }

        ......

        /* found a match, try to load it */
        clazz = loadClassFromDex(pDvmDex, pClassDef, loader);
        ......
        if (!dvmAddClassToHash(clazz)) {
            ......
        }
		......
	}
    return clazz;
}

    我们假设没有对其加载过，然后调用dexFindClass方法找到DexClassDef结构体。我们首先来看下DexClassDef结构体，代码位于dalvik\vm\oo\Class.c。

typedef struct DexClassDef {
    u4  classIdx;           /* index into typeIds for this class */
    u4  accessFlags;
    u4  superclassIdx;      /* index into typeIds for superclass */
    u4  interfacesOff;      /* file offset to DexTypeList */
    u4  sourceFileIdx;      /* index into stringIds for source file name */
    u4  annotationsOff;     /* file offset to annotations_directory_item */
    u4  classDataOff;       /* file offset to class_data_item */
    u4  staticValuesOff;    /* file offset to DexEncodedArray */
} DexClassDef;

    dexFindClass方法也位于dalvik\vm\oo\Class.c。

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;
    }
}

    返回到findClassNoInit，继续执行loadClassFromDex方法，这是真正生成ClassObject对象的地方。代码位于dalvik\vm\oo\Class.c。

static ClassObject* loadClassFromDex(DvmDex* pDvmDex,
    const DexClassDef* pClassDef, Object* classLoader)
{
    ClassObject* result;
    DexClassDataHeader header;
    const u1* pEncodedData;
    const DexFile* pDexFile;

    assert((pDvmDex != NULL) && (pClassDef != NULL));
    pDexFile = pDvmDex->pDexFile;

    if (gDvm.verboseClass) {
        LOGV("CLASS: loading ‘%s‘...\n",
            dexGetClassDescriptor(pDexFile, pClassDef));
    }

    pEncodedData = dexGetClassData(pDexFile, pClassDef);

    if (pEncodedData != NULL) {
        dexReadClassDataHeader(&pEncodedData, &header);
    } else {
        // Provide an all-zeroes header for the rest of the loading.
        memset(&header, 0, sizeof(header));
    }

    result = loadClassFromDex0(pDvmDex, pClassDef, &header, pEncodedData,
            classLoader);

    if (gDvm.verboseClass && (result != NULL)) {
        LOGI("[Loaded %s from DEX %p (cl=%p)]\n",
            result->descriptor, pDvmDex, classLoader);
    }

    return result;
}

DEX_INLINE const u1* dexGetClassData(const DexFile* pDexFile,
    const DexClassDef* pClassDef)
{
    if (pClassDef->classDataOff == 0)
        return NULL;
    return (const u1*) (pDexFile->baseAddr + pClassDef->classDataOff);
}

static ClassObject* loadClassFromDex0(DvmDex* pDvmDex,
    const DexClassDef* pClassDef, const DexClassDataHeader* pHeader,
    const u1* pEncodedData, Object* classLoader)
{
    ClassObject* newClass = NULL;
    const DexFile* pDexFile;
    const char* descriptor;
    int i;

    pDexFile = pDvmDex->pDexFile;
    descriptor = dexGetClassDescriptor(pDexFile, pClassDef);

    /*
     * Make sure the aren‘t any "bonus" flags set, since we use them for
     * runtime state.
     */
    if ((pClassDef->accessFlags & ~EXPECTED_FILE_FLAGS) != 0) {
        LOGW("Invalid file flags in class %s: %04x\n",
            descriptor, pClassDef->accessFlags);
        return NULL;
    }

    /*
     * Allocate storage for the class object on the GC heap, so that other
     * objects can have references to it.  We bypass the usual mechanism
     * (allocObject), because we don‘t have all the bits and pieces yet.
     *
     * Note that we assume that java.lang.Class does not override
     * finalize().
     */
    /* TODO: Can there be fewer special checks in the usual path? */
    assert(descriptor != NULL);
    if (classLoader == NULL &&
        strcmp(descriptor, "Ljava/lang/Class;") == 0) {
        assert(gDvm.classJavaLangClass != NULL);
        newClass = gDvm.classJavaLangClass;
    } else {
        size_t size = classObjectSize(pHeader->staticFieldsSize);
        newClass = (ClassObject*) dvmMalloc(size, ALLOC_DEFAULT);
    }
    if (newClass == NULL)
        return NULL;

    DVM_OBJECT_INIT(&newClass->obj, gDvm.classJavaLangClass);
    dvmSetClassSerialNumber(newClass);
    newClass->descriptor = descriptor;
    assert(newClass->descriptorAlloc == NULL);
    newClass->accessFlags = pClassDef->accessFlags;
    dvmSetFieldObject((Object *)newClass,
                      offsetof(ClassObject, classLoader),
                      (Object *)classLoader);
    newClass->pDvmDex = pDvmDex;
    newClass->primitiveType = PRIM_NOT;
    newClass->status = CLASS_IDX;

    /*
     * Stuff the superclass index into the object pointer field.  The linker
     * pulls it out and replaces it with a resolved ClassObject pointer.
     * I‘m doing it this way (rather than having a dedicated superclassIdx
     * field) to save a few bytes of overhead per class.
     *
     * newClass->super is not traversed or freed by dvmFreeClassInnards, so
     * this is safe.
     */
    assert(sizeof(u4) == sizeof(ClassObject*)); /* 32-bit check */
    newClass->super = (ClassObject*) pClassDef->superclassIdx;

    /*
     * Stuff class reference indices into the pointer fields.
     *
     * The elements of newClass->interfaces are not traversed or freed by
     * dvmFreeClassInnards, so this is GC-safe.
     */
    const DexTypeList* pInterfacesList;
    pInterfacesList = dexGetInterfacesList(pDexFile, pClassDef);
    if (pInterfacesList != NULL) {
        newClass->interfaceCount = pInterfacesList->size;
        newClass->interfaces = (ClassObject**) dvmLinearAlloc(classLoader,
                newClass->interfaceCount * sizeof(ClassObject*));

        for (i = 0; i < newClass->interfaceCount; i++) {
            const DexTypeItem* pType = dexGetTypeItem(pInterfacesList, i);
            newClass->interfaces[i] = (ClassObject*)(u4) pType->typeIdx;
        }
        dvmLinearReadOnly(classLoader, newClass->interfaces);
    }

    /* load field definitions */

    /*
     * Over-allocate the class object and append static field info
     * onto the end.  It‘s fixed-size and known at alloc time.  This
     * seems to increase zygote sharing.  Heap compaction will have to
     * be careful if it ever tries to move ClassObject instances,
     * because we pass Field pointers around internally. But at least
     * now these Field pointers are in the object heap.
     */

    if (pHeader->staticFieldsSize != 0) {
        /* static fields stay on system heap; field data isn‘t "write once" */
        int count = (int) pHeader->staticFieldsSize;
        u4 lastIndex = 0;
        DexField field;

        newClass->sfieldCount = count;
        for (i = 0; i < count; i++) {
            dexReadClassDataField(&pEncodedData, &field, &lastIndex);
            loadSFieldFromDex(newClass, &field, &newClass->sfields[i]);
        }
    }

    if (pHeader->instanceFieldsSize != 0) {
        int count = (int) pHeader->instanceFieldsSize;
        u4 lastIndex = 0;
        DexField field;

        newClass->ifieldCount = count;
        newClass->ifields = (InstField*) dvmLinearAlloc(classLoader,
                count * sizeof(InstField));
        for (i = 0; i < count; i++) {
            dexReadClassDataField(&pEncodedData, &field, &lastIndex);
            loadIFieldFromDex(newClass, &field, &newClass->ifields[i]);
        }
        dvmLinearReadOnly(classLoader, newClass->ifields);
    }

    /*
     * Load method definitions.  We do this in two batches, direct then
     * virtual.
     *
     * If register maps have already been generated for this class, and
     * precise GC is enabled, we pull out pointers to them.  We know that
     * they were streamed to the DEX file in the same order in which the
     * methods appear.
     *
     * If the class wasn‘t pre-verified, the maps will be generated when
     * the class is verified during class initialization.
     */
    u4 classDefIdx = dexGetIndexForClassDef(pDexFile, pClassDef);
    const void* classMapData;
    u4 numMethods;

    if (gDvm.preciseGc) {
        classMapData =
            dvmRegisterMapGetClassData(pDexFile, classDefIdx, &numMethods);

        /* sanity check */
        if (classMapData != NULL &&
            pHeader->directMethodsSize + pHeader->virtualMethodsSize != numMethods)
        {
            LOGE("ERROR: in %s, direct=%d virtual=%d, maps have %d\n",
                newClass->descriptor, pHeader->directMethodsSize,
                pHeader->virtualMethodsSize, numMethods);
            assert(false);
            classMapData = NULL;        /* abandon */
        }
    } else {
        classMapData = NULL;
    }

    if (pHeader->directMethodsSize != 0) {
        int count = (int) pHeader->directMethodsSize;
        u4 lastIndex = 0;
        DexMethod method;

        newClass->directMethodCount = count;
        newClass->directMethods = (Method*) dvmLinearAlloc(classLoader,
                count * sizeof(Method));
        for (i = 0; i < count; i++) {
            dexReadClassDataMethod(&pEncodedData, &method, &lastIndex);
            loadMethodFromDex(newClass, &method, &newClass->directMethods[i]);
            if (classMapData != NULL) {
                const RegisterMap* pMap = dvmRegisterMapGetNext(&classMapData);
                if (dvmRegisterMapGetFormat(pMap) != kRegMapFormatNone) {
                    newClass->directMethods[i].registerMap = pMap;
                    /* TODO: add rigorous checks */
                    assert((newClass->directMethods[i].registersSize+7) / 8 ==
                        newClass->directMethods[i].registerMap->regWidth);
                }
            }
        }
        dvmLinearReadOnly(classLoader, newClass->directMethods);
    }

    if (pHeader->virtualMethodsSize != 0) {
        int count = (int) pHeader->virtualMethodsSize;
        u4 lastIndex = 0;
        DexMethod method;

        newClass->virtualMethodCount = count;
        newClass->virtualMethods = (Method*) dvmLinearAlloc(classLoader,
                count * sizeof(Method));
        for (i = 0; i < count; i++) {
            dexReadClassDataMethod(&pEncodedData, &method, &lastIndex);
            loadMethodFromDex(newClass, &method, &newClass->virtualMethods[i]);
            if (classMapData != NULL) {
                const RegisterMap* pMap = dvmRegisterMapGetNext(&classMapData);
                if (dvmRegisterMapGetFormat(pMap) != kRegMapFormatNone) {
                    newClass->virtualMethods[i].registerMap = pMap;
                    /* TODO: add rigorous checks */
                    assert((newClass->virtualMethods[i].registersSize+7) / 8 ==
                        newClass->virtualMethods[i].registerMap->regWidth);
                }
            }
        }
        dvmLinearReadOnly(classLoader, newClass->virtualMethods);
    }

    newClass->sourceFile = dexGetSourceFile(pDexFile, pClassDef);

    /* caller must call dvmReleaseTrackedAlloc */
    return newClass;
}

struct ClassObject {
    Object          obj;                /* MUST be first item */

    /* leave space for instance data; we could access fields directly if we
       freeze the definition of java/lang/Class */
    u4              instanceData[CLASS_FIELD_SLOTS];

    /* UTF-8 descriptor for the class; from constant pool, or on heap
       if generated ("[C") */
    const char*     descriptor;
    char*           descriptorAlloc;

    /* access flags; low 16 bits are defined by VM spec */
    u4              accessFlags;

    /* VM-unique class serial number, nonzero, set very early */
    u4              serialNumber;

    /* DexFile from which we came; needed to resolve constant pool entries */
    /* (will be NULL for VM-generated, e.g. arrays and primitive classes) */
    DvmDex*         pDvmDex;

    /* state of class initialization */
    ClassStatus     status;

    /* if class verify fails, we must return same error on subsequent tries */
    ClassObject*    verifyErrorClass;

    /* threadId, used to check for recursive <clinit> invocation */
    u4              initThreadId;

    /*
     * Total object size; used when allocating storage on gc heap.  (For
     * interfaces and abstract classes this will be zero.)
     */
    size_t          objectSize;

    /* arrays only: class object for base element, for instanceof/checkcast
       (for String[][][], this will be String) */
    ClassObject*    elementClass;

    /* arrays only: number of dimensions, e.g. int[][] is 2 */
    int             arrayDim;

    /* primitive type index, or PRIM_NOT (-1); set for generated prim classes */
    PrimitiveType   primitiveType;

    /* superclass, or NULL if this is java.lang.Object */
    ClassObject*    super;

    /* defining class loader, or NULL for the "bootstrap" system loader */
    Object*         classLoader;

    /* initiating class loader list */
    /* NOTE: for classes with low serialNumber, these are unused, and the
       values are kept in a table in gDvm. */
    InitiatingLoaderList initiatingLoaderList;

    /* array of interfaces this class implements directly */
    int             interfaceCount;
    ClassObject**   interfaces;

    /* static, private, and <init> methods */
    int             directMethodCount;
    Method*         directMethods;

    /* virtual methods defined in this class; invoked through vtable */
    int             virtualMethodCount;
    Method*         virtualMethods;

    /*
     * Virtual method table (vtable), for use by "invoke-virtual".  The
     * vtable from the superclass is copied in, and virtual methods from
     * our class either replace those from the super or are appended.
     */
    int             vtableCount;
    Method**        vtable;

    /*
     * Interface table (iftable), one entry per interface supported by
     * this class.  That means one entry for each interface we support
     * directly, indirectly via superclass, or indirectly via
     * superinterface.  This will be null if neither we nor our superclass
     * implement any interfaces.
     *
     * Why we need this: given "class Foo implements Face", declare
     * "Face faceObj = new Foo()".  Invoke faceObj.blah(), where "blah" is
     * part of the Face interface.  We can‘t easily use a single vtable.
     *
     * For every interface a concrete class implements, we create a list of
     * virtualMethod indices for the methods in the interface.
     */
    int             iftableCount;
    InterfaceEntry* iftable;

    /*
     * The interface vtable indices for iftable get stored here.  By placing
     * them all in a single pool for each class that implements interfaces,
     * we decrease the number of allocations.
     */
    int             ifviPoolCount;
    int*            ifviPool;

    /* instance fields
     *
     * These describe the layout of the contents of a DataObject-compatible
     * Object.  Note that only the fields directly defined by this class
     * are listed in ifields;  fields defined by a superclass are listed
     * in the superclass‘s ClassObject.ifields.
     *
     * All instance fields that refer to objects are guaranteed to be
     * at the beginning of the field list.  ifieldRefCount specifies
     * the number of reference fields.
     */
    int             ifieldCount;
    int             ifieldRefCount; // number of fields that are object refs
    InstField*      ifields;

    /* bitmap of offsets of ifields */
    u4 refOffsets;

    /* source file name, if known */
    const char*     sourceFile;

    /* static fields */
    int             sfieldCount;
    StaticField     sfields[]; /* MUST be last item */
}

struct Method {
    /* the class we are a part of */
    ClassObject*    clazz;

    /* access flags; low 16 bits are defined by spec (could be u2?) */
    u4              accessFlags;

    /*
     * For concrete virtual methods, this is the offset of the method
     * in "vtable".
     *
     * For abstract methods in an interface class, this is the offset
     * of the method in "iftable[n]->methodIndexArray".
     */
    u2             methodIndex;

    /*
     * Method bounds; not needed for an abstract method.
     *
     * For a native method, we compute the size of the argument list, and
     * set "insSize" and "registerSize" equal to it.
     */
    u2              registersSize;  /* ins + locals */
    u2              outsSize;
    u2              insSize;

    /* method name, e.g. "<init>" or "eatLunch" */
    const char*     name;

    /*
     * Method prototype descriptor string (return and argument types).
     *
     * TODO: This currently must specify the DexFile as well as the proto_ids
     * index, because generated Proxy classes don‘t have a DexFile.  We can
     * remove the DexFile* and reduce the size of this struct if we generate
     * a DEX for proxies.
     */
    DexProto        prototype;

    /* short-form method descriptor string */
    const char*     shorty;

    /*
     * The remaining items are not used for abstract or native methods.
     * (JNI is currently hijacking "insns" as a function pointer, set
     * after the first call.  For internal-native this stays null.)
     */

    /* the actual code */
    const u2*       insns;          /* instructions, in memory-mapped .dex */

    /* cached JNI argument and return-type hints */
    int             jniArgInfo;

    /*
     * Native method ptr; could be actual function or a JNI bridge.  We
     * don‘t currently discriminate between DalvikBridgeFunc and
     * DalvikNativeFunc; the former takes an argument superset (i.e. two
     * extra args) which will be ignored.  If necessary we can use
     * insns==NULL to detect JNI bridge vs. internal native.
     */
    DalvikBridgeFunc nativeFunc;

    /*
     * Register map data, if available.  This will point into the DEX file
     * if the data was computed during pre-verification, or into the
     * linear alloc area if not.
     */
    const RegisterMap* registerMap;

    /* set if method was called during method profiling */
    bool            inProfile;
}

    if (pHeader->directMethodsSize != 0) {
        int count = (int) pHeader->directMethodsSize;
        u4 lastIndex = 0;
        DexMethod method;

        newClass->directMethodCount = count;
        newClass->directMethods = (Method*) dvmLinearAlloc(classLoader,
                count * sizeof(Method));
        for (i = 0; i < count; i++) {
            dexReadClassDataMethod(&pEncodedData, &method, &lastIndex);
            loadMethodFromDex(newClass, &method, &newClass->directMethods[i]);
            if (classMapData != NULL) {
                const RegisterMap* pMap = dvmRegisterMapGetNext(&classMapData);
                if (dvmRegisterMapGetFormat(pMap) != kRegMapFormatNone) {
                    newClass->directMethods[i].registerMap = pMap;
                    /* TODO: add rigorous checks */
                    assert((newClass->directMethods[i].registersSize+7) / 8 ==
                        newClass->directMethods[i].registerMap->regWidth);
                }
            }
        }
        dvmLinearReadOnly(classLoader, newClass->directMethods);
    }

DEX_INLINE void dexReadClassDataMethod(const u1** pData, DexMethod* pMethod,
        u4* lastIndex) {
    u4 index = *lastIndex + readUnsignedLeb128(pData);

    pMethod->accessFlags = readUnsignedLeb128(pData);
    pMethod->codeOff = readUnsignedLeb128(pData);
    pMethod->methodIdx = index;
    *lastIndex = index;
}

static void loadMethodFromDex(ClassObject* clazz, const DexMethod* pDexMethod,
    Method* meth)
{
    DexFile* pDexFile = clazz->pDvmDex->pDexFile;
    const DexMethodId* pMethodId;
    const DexCode* pDexCode;

    pMethodId = dexGetMethodId(pDexFile, pDexMethod->methodIdx);

    meth->name = dexStringById(pDexFile, pMethodId->nameIdx);
    dexProtoSetFromMethodId(&meth->prototype, pDexFile, pMethodId);
    meth->shorty = dexProtoGetShorty(&meth->prototype);
    meth->accessFlags = pDexMethod->accessFlags;
    meth->clazz = clazz;
    meth->jniArgInfo = 0;

    if (dvmCompareNameDescriptorAndMethod("finalize", "()V", meth) == 0) {
        SET_CLASS_FLAG(clazz, CLASS_ISFINALIZABLE);
    }

    pDexCode = dexGetCode(pDexFile, pDexMethod);
    if (pDexCode != NULL) {
        /* integer constants, copy over for faster access */
        meth->registersSize = pDexCode->registersSize;
        meth->insSize = pDexCode->insSize;
        meth->outsSize = pDexCode->outsSize;

        /* pointer to code area */
        meth->insns = pDexCode->insns;
    } else {
        /*
         * We don‘t have a DexCode block, but we still want to know how
         * much space is needed for the arguments (so we don‘t have to
         * compute it later).  We also take this opportunity to compute
         * JNI argument info.
         *
         * We do this for abstract methods as well, because we want to
         * be able to substitute our exception-throwing "stub" in.
         */
        int argsSize = dvmComputeMethodArgsSize(meth);
        if (!dvmIsStaticMethod(meth))
            argsSize++;
        meth->registersSize = meth->insSize = argsSize;
        assert(meth->outsSize == 0);
        assert(meth->insns == NULL);

        if (dvmIsNativeMethod(meth)) {
            meth->nativeFunc = dvmResolveNativeMethod;
            meth->jniArgInfo = computeJniArgInfo(&meth->prototype);
        }
    }
}

typedef struct DexCode {
    u2  registersSize;
    u2  insSize;
    u2  outsSize;
    u2  triesSize;
    u4  debugInfoOff;       /* file offset to debug info stream */
    u4  insnsSize;          /* size of the insns array, in u2 units */
    u2  insns[1];
    /* followed by optional u2 padding */
    /* followed by try_item[triesSize] */
    /* followed by uleb128 handlersSize */
    /* followed by catch_handler_item[handlersSize] */
} DexCode;

DEX_INLINE const DexCode* dexGetCode(const DexFile* pDexFile,
    const DexMethod* pDexMethod)
{
    if (pDexMethod->codeOff == 0)
        return NULL;
    return (const DexCode*) (pDexFile->baseAddr + pDexMethod->codeOff);
}

    这样逐层返回就生成了ClassObject对象，并返回给Java层，在Java层表现为Class。参见这个Java方法protected Class<?> loadClass(String className, boolean resolve)。

DexClassLoader和PathClassLoader类加载机制

原文：http://blog.csdn.net/jltxgcy/article/details/50556239