关于Java中几种loadClass的讨论
java中有几类加载class的方法，本文针对这几个方法，就行源码导读。
本文的native源码来源于android4.1.2源码。
1. Class.forName(className, true, classLoader);
clazz = Class.forName(className, true, classLoader);
className:要加载的类的全路径名。
classLoader：类加载器。
这个函数会调用native方法，源码：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));
}
这里调用的是dvmFindClassByName(nameObj, loader, initialize)，重点在这句，由于此函数代码比较多，我只罗列了关键语句：
ClassObject* dvmFindClassByName(StringObject* nameObj, Object* loader,
bool doInit)
{
ClassObject* clazz = NULL;
char* name = NULL;
char* descriptor = NULL;
......
// 初始化的参数在这里体现
if (doInit)
clazz = dvmFindClass(descriptor, loader);
else
clazz = dvmFindClassNoInit(descriptor, loader);
.......
}
由于dvmFindClass会先调用dvmFindClassNoInit，下面优先分析dvmFindClassNoInit
由于代码比较多，下面只列出了调用函数关系：
dvmFindClassNoInit-->findClassFromLoaderNoInit-->dvmLookupClass-->dvmHashTableLookup
关键看dvmHashTableLookup这个函数：
void* dvmHashTableLookup(HashTable* pHashTable, u4 itemHash, void* item,
HashCompareFunc cmpFunc, bool doAdd)
{
pEntry = &pHashTable->pEntries[itemHash & (pHashTable->tableSize-1)];
pEnd = &pHashTable->pEntries[pHashTable->tableSize];
while (pEntry->data != NULL) {
if (pEntry->data != HASH_TOMBSTONE &&
pEntry->hashValue == itemHash &&
(*cmpFunc)(pEntry->data, item) == 0) // 这里是需要ClassObject的关键点
}
return result;
}
此函数寻找ClassObject的逻辑是：
通过classname生成hash值，在HashTable中寻找此类的ClassObject，而这个HashTable保存在全局变量DvmGlobals中，ClassObject也就是C++中的对象，在虚拟机的堆中，classname和classloader两者对应唯一的一个ClassObject。看一下关键的比较函数cmpFunc如何判定此ClassObject是classname要寻找的Object
match = (strcmp(clazz->descriptor, pCrit->descriptor) == 0 &&
(clazz->classLoader == pCrit->loader ||
(pCrit->loader != NULL &&
dvmLoaderInInitiatingList(clazz, pCrit->loader)) ));
这里通过clazz->descriptor,classLoader 决定是否是要寻找的类对象。
所以有关java中ClassLoader内容，不同的类在不同的ClassLoader中加载，对应的不同的ClassObject，有此函数可以得出此结论。
回到刚才的函数，如果通过dvmLookupClass寻找不到对应的ClassObject，怎么处理？
会调用Invoke loadClass()进行class加载和初始化。
const Method* loadClass =
loader->clazz->vtable[gDvm.voffJavaLangClassLoader_loadClass];
JValue result;
dvmCallMethod(self, loadClass, loader, &result, nameObj);
clazz = (ClassObject*) result.l;
这段函数还是比较重要的，通过ClassLoader，进行重新加载，dvmCallMethod是java调用native的入口。
2. ClassLoader Class<?> loadClass(String className, boolean resolve)
这个函数可能会被表面定义混淆，这个函数的逻辑也是先在HashTable中需要className对应的ClassObjet，如果寻找不到，在进行加载和初始化。
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;
}
一、首先看Class<?> clazz = findLoadedClass(className);这句
对应的native源码在java_lang_VMClassLoader.cpp中。
static void Dalvik_java_lang_VMClassLoader_loadClass(const u4* args,
JValue* pResult)
{
StringObject* nameObj = (StringObject*) args[0];
bool resolve = (args[1] != 0);
ClassObject* clazz;
clazz = dvmFindClassByName(nameObj, NULL, resolve);
assert(clazz == NULL || dvmIsClassLinked(clazz));
RETURN_PTR(clazz);
}
重点看一下dvmFindClassByName
ClassObject* dvmFindClassByName(StringObject* nameObj, Object* loader,
bool doInit)
{
ClassObject* clazz = NULL;
char* name = NULL;
char* descriptor = NULL;
if (doInit) // 这里是这个函数的重点逻辑
clazz = dvmFindClass(descriptor, loader);
else
clazz = dvmFindClassNoInit(descriptor, loader);
}
这里如果查找不到，便会返回null。
二、clazz = parent.loadClass(className, false);
这里是调用classloader的父类进行加载，其加载逻辑相同。
三、 clazz = findClass(className);
如果一和二都不能找到className对应的ClassObject，那么就要通过findClass(className)进行寻找，这个函数的定义在classLoader，而android平台下定义在BaseDexClassLoader，看一下他的函数：
protected Class<?> findClass(String name) throws ClassNotFoundException {
Class clazz = pathList.findClass(name);
if (clazz == null) {
throw new ClassNotFoundException("Didn't find class \"" + name + "\" on path: " + originalPath);
}
return clazz;
}
这里调用的是pathList.findClass(name);
继续跟踪函数：
public Class findClass(String name) {
for (Element element : dexElements) {
DexFile dex = element.dexFile;
if (dex != null) {
Class clazz = dex.loadClassBinaryName(name, definingContext);
if (clazz != null) {
return clazz;
}
}
}
return null;
}
这里调用的是dex.loadClassBinaryName(name, definingContext);，对应的native函数为：
dalvik_system_DexFile.cpp
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];
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);
}
clazz = dvmDefineClass(pDvmDex, descriptor, loader);这句是核心，
直接进入Class.cpp中的static ClassObject* findClassNoInit(const char* descriptor, Object* loader,
DvmDex* pDvmDex)函数，这个函数的简单逻辑是dvmLookupClass寻找class，如果找不到，会创建对应的ClassObject，并且添加到Globals中。
clazz = dvmLookupClass(descriptor, loader, true); // 寻找对应的ClassObject
if (clazz == NULL) {
const DexClassDef* pClassDef;
dvmMethodTraceClassPrepBegin();
profilerNotified = true;
#if LOG_CLASS_LOADING
u8 startTime = dvmGetThreadCpuTimeNsec();
#endif
// 通过dex加载对应的类
if (pDvmDex == NULL) {
assert(loader == NULL); /* shouldn't be here otherwise */
pDvmDex = searchBootPathForClass(descriptor, &pClassDef);
} else {
pClassDef = dexFindClass(pDvmDex->pDexFile, descriptor);
}
if (pDvmDex == NULL || pClassDef == NULL) {
if (gDvm.noClassDefFoundErrorObj != NULL) {
/* usual case -- use prefabricated object */
dvmSetException(self, gDvm.noClassDefFoundErrorObj);
} else {
/* dexopt case -- can't guarantee prefab (core.jar) */
dvmThrowNoClassDefFoundError(descriptor);
}
goto bail;
}
/* found a match, try to load it */
clazz = loadClassFromDex(pDvmDex, pClassDef, loader);
if (dvmCheckException(self)) {
/* class was found but had issues */
if (clazz != NULL) {
dvmFreeClassInnards(clazz);
dvmReleaseTrackedAlloc((Object*) clazz, NULL);
}
goto bail;
}
/*
* Lock the class while we link it so other threads must wait for us
* to finish. Set the "initThreadId" so we can identify recursive
* invocation. (Note all accesses to initThreadId here are
* guarded by the class object's lock.)
*/
dvmLockObject(self, (Object*) clazz);
clazz->initThreadId = self->threadId;
/*
* Add to hash table so lookups succeed.
*
* [Are circular references possible when linking a class?]
*/
assert(clazz->classLoader == loader);
if (!dvmAddClassToHash(clazz)) { 添加创建的类到HashTable
/*
* Another thread must have loaded the class after we
* started but before we finished. Discard what we've
* done and leave some hints for the GC.
*
* (Yes, this happens.)
*/
//ALOGW("WOW: somebody loaded %s simultaneously", descriptor);
clazz->initThreadId = 0;
dvmUnlockObject(self, (Object*) clazz);
/* Let the GC free the class.
*/
dvmFreeClassInnards(clazz);
dvmReleaseTrackedAlloc((Object*) clazz, NULL);
/* Grab the winning class.
*/
clazz = dvmLookupClass(descriptor, loader, true);
assert(clazz != NULL);
goto got_class;
}
dvmReleaseTrackedAlloc((Object*) clazz, NULL);
......
}
到此整个流程已经介绍完毕。
由此看出我们直接调用ClassLoader.loadClass要比调用Class.forName更高效。