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 suppressedExceptions = new ArrayList();
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 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 suppressed) {
return defineClass(name, loader, mCookie, suppressed);
}
private static Class defineClass(String name, ClassLoader loader, int cookie,
List 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