安卓高手之路之ClassLoader(总结篇)

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。