安卓系统对ClassLoader的设计可谓别有用心。前面分析过，赋值的地方如下：
const char* envStr = getenv("CLASSPATH"); if (envStr != NULL) { gDvm.classPathStr = strdup(envStr); } else { gDvm.classPathStr = strdup("."); } envStr = getenv("BOOTCLASSPATH"); if (envStr != NULL) { gDvm.bootClassPathStr = strdup(envStr); } else { gDvm.bootClassPathStr = strdup("."); }
分为三级：
Boot 与BOOTCLASSPATH对应
System 与CLASSPATH对应。
App 与应用程序包对应。
在应用程序里面，Context控制着一个ClassLoader，通过建立不同的ClassLoader，对外界控制着对APK包的访问权限。
主要有如下几种：
/** * Flag for use with {@link #createPackageContext}: include the application * code with the context. This means loading code into the caller's * process, so that {@link #getClassLoader()} can be used to instantiate * the application's classes. Setting this flags imposes security * restrictions on what application context you can access; if the * requested application can not be safely loaded into your process, * java.lang.SecurityException will be thrown. If this flag is not set, * there will be no restrictions on the packages that can be loaded, * but {@link #getClassLoader} will always return the default system * class loader. */ public static final int CONTEXT_INCLUDE_CODE = 0x00000001;
/** * Flag for use with {@link #createPackageContext}: ignore any security * restrictions on the Context being requested, allowing it to always * be loaded. For use with {@link #CONTEXT_INCLUDE_CODE} to allow code * to be loaded into a process even when it isn't safe to do so. Use * with extreme care! */ public static final int CONTEXT_IGNORE_SECURITY = 0x00000002;
/** * Flag for use with {@link #createPackageContext}: a restricted context may * disable specific features. For instance, a View associated with a restricted * context would ignore particular XML attributes. */ public static final int CONTEXT_RESTRICTED = 0x00000004;
1.ClassLoader如其名，就是加载class用的。
2.一开始的时候，是通过dalvik/vm/Jni.cpp中的FindClass函数来找类的。
NativeStart是一个假类，里面的main是java堆栈的root。
第一。系统启动
因为这段代码是C++的代码，那么可以肯定一定是java（或者NativeStart这个假main函数）调用过来的。
具体谁调用过来的，这里做了个判断：
如果是NativeStart.main:
这个时候要进行初始化判断，有可能vm还没有进行初始化。
如果是System.nativeload
这个时候，就用classLoaderOverride
如果是其他：
就是 thisMethod->clazz->classLoader 也就是 加载这段代码的classloader
分别进入了三个不同的分支。
static jclass FindClass(JNIEnv* env, const char* name) { ScopedJniThreadState ts(env); const Method* thisMethod = dvmGetCurrentJNIMethod(); assert(thisMethod != NULL); Object* loader; Object* trackedLoader = NULL; if (ts.self()->classLoaderOverride != NULL) { /* hack for JNI_OnLoad */ assert(strcmp(thisMethod->name, "nativeLoad") == 0); loader = ts.self()->classLoaderOverride; } else if (thisMethod == gDvm.methDalvikSystemNativeStart_main || thisMethod == gDvm.methDalvikSystemNativeStart_run) { /* start point of invocation interface */ if (!gDvm.initializing) { loader = trackedLoader = dvmGetSystemClassLoader(); } else { loader = NULL; } } else { loader = thisMethod->clazz->classLoader; } char* descriptor = dvmNameToDescriptor(name); if (descriptor == NULL) { return NULL; } ClassObject* clazz = dvmFindClassNoInit(descriptor, loader); free(descriptor); jclass jclazz = (jclass) addLocalReference(ts.self(), (Object*) clazz); dvmReleaseTrackedAlloc(trackedLoader, ts.self()); return jclazz; }
这种classLoade是BootClassLoader.
第二 app启动。
app启动 通过socket完成。通过fork来创建一个子进程。这个时候ClassLoader是与Context有关的。
不同的Context对应不同的ClassLoader。这个ClassLoader是一个PathClassLoader
@Override public ClassLoader getClassLoader() { return mPackageInfo != null ? mPackageInfo.getClassLoader() : ClassLoader.getSystemClassLoader(); }
package android.app; import dalvik.system.PathClassLoader; import java.util.HashMap; import java.util.Map; class ApplicationLoaders { public static ApplicationLoaders getDefault() { return gApplicationLoaders; } public ClassLoader getClassLoader(String zip, String libPath, ClassLoader parent) { /* * This is the parent we use if they pass "null" in. In theory * this should be the "system" class loader; in practice we * don't use that and can happily (and more efficiently) use the * bootstrap class loader. */ ClassLoader baseParent = ClassLoader.getSystemClassLoader().getParent(); synchronized (mLoaders) { if (parent == null) { parent = baseParent; } /* * If we're one step up from the base class loader, find * something in our cache. Otherwise, we create a whole * new ClassLoader for the zip archive. */ if (parent == baseParent) { ClassLoader loader = mLoaders.get(zip); if (loader != null) { return loader; } PathClassLoader pathClassloader = new PathClassLoader(zip, libPath, parent); mLoaders.put(zip, pathClassloader); return pathClassloader; } return new PathClassLoader(zip, parent); } } private final Map<String, ClassLoader> mLoaders = new HashMap<String, ClassLoader>(); private static final ApplicationLoaders gApplicationLoaders = new ApplicationLoaders(); }
具体来讲ClassLoader.getSystemClassLoader() 返回的是一个PathClassLoader，而
ClassLoader.getSystemClassLoader().getParent() 返回的是一个BootClassLoader。
如果LoadedApk这个类在构造的时候，传入了个BootClassLoader或者null，那么就会执行 PathClassLoader pathClassloader = new PathClassLoader(zip, libPath, parent); mLoaders.put(zip, pathClassloader); return pathClassloader;也就是说 把libPath进行了传入。
否则用如下构造函数执行
return new PathClassLoader(zip, parent);可以看到，少了一个参数libPath，libPath是libjni。那么这个是什么意思呢？
经过看代码，默认LoadedAPK传入的loader是个null， 因此，会使用libPath进行构造。并且它的父Loader是BootClassLoader。那么什么时候传入的loader不是nul呢。用instrument的时候传入的不是null。
