Android中插件开发篇之----类加载器

前言

关于插件,已经在各大平台上出现过很多,eclipse插件、chrome插件、3dmax插件,所有这些插件大概都为了在一个主程序中实现比较通用的功能,把业务相关或者让可以让用户自定义扩展的功能不附加在主程序中,主程序可在运行时安装和卸载。在android如何实现插件也已经被广泛传播,实现的原理都是实现一套插件接口,把插件实现编成apk或者dex,然后在运行时使用DexClassLoader动态加载进来,不过在这个开发过程中会遇到很多的问题,所以这一片就先不介绍如何开发插件,而是先解决一下开发过程中会遇到的问题,这里主要就是介绍DexClassLoader这个类使用的过程中出现的错误


导读

Java中的类加载器:http://blog.csdn.net/jiangwei0910410003/article/details/17733153

Android中的动态加载机制:http://blog.csdn.net/jiangwei0910410003/article/details/17679823

System.loadLibrary的执行过程:http://blog.csdn.net/jiangwei0910410003/article/details/41490133


一、预备知识

Android中的各种加载器介绍

插件开发的过程中DexClassLoader和PathClassLoader这两个类加载器了是很重要的,但是他们也是有区别的,而且我们也知道PathClassLoader是Android应用中的默认加载器。他们的区别是:

DexClassLoader可以加载任何路径的apk/dex/jar

PathClassLoader只能加载/data/app中的apk,也就是已经安装到手机中的apk。这个也是PathClassLoader作为默认的类加载器的原因,因为一般程序都是安装了,在打开,这时候PathClassLoader就去加载指定的apk(解压成dex,然后在优化成odex)就可以了。


我们可以看一下他们的源码:

DexClassLoader.java

/*
 * Copyright (C) 2008 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package dalvik.system;

import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.zip.ZipFile;

/**
 * Provides a simple {@link ClassLoader} implementation that operates on a
 * list of jar/apk files with classes.dex entries.  The directory that
 * holds the optimized form of the files is specified explicitly.  This
 * can be used to execute code not installed as part of an application.
 *
 * The best place to put the optimized DEX files is in app-specific
 * storage, so that removal of the app will automatically remove the
 * optimized DEX files.  If other storage is used (e.g. /sdcard), the
 * app may not have an opportunity to remove them.
 */
public class DexClassLoader extends ClassLoader {

    private static final boolean VERBOSE_DEBUG = false;

    /* constructor args, held for init */
    private final String mRawDexPath;
    private final String mRawLibPath;
    private final String mDexOutputPath;

    /*
     * Parallel arrays for jar/apk files.
     *
     * (could stuff these into an object and have a single array;
     * improves clarity but adds overhead)
     */
    private final File[] mFiles;         // source file Files, for rsrc URLs
    private final ZipFile[] mZips;       // source zip files, with resources
    private final DexFile[] mDexs;       // opened, prepped DEX files

    /**
     * Native library path.
     */
    private final String[] mLibPaths;

    /**
     * Creates a {@code DexClassLoader} that finds interpreted and native
     * code.  Interpreted classes are found in a set of DEX files contained
     * in Jar or APK files.
     *
     * The path lists are separated using the character specified by
     * the "path.separator" system property, which defaults to ":".
     *
     * @param dexPath
     *  the list of jar/apk files containing classes and resources
     * @param dexOutputDir
     *  directory where optimized DEX files should be written
     * @param libPath
     *  the list of directories containing native libraries; may be null
     * @param parent
     *  the parent class loader
     */
    public DexClassLoader(String dexPath, String dexOutputDir, String libPath,
        ClassLoader parent) {

        super(parent);
......
我们看到,他是继承了ClassLoader类的,ClassLoader是类加载器的鼻祖类。同时我们也会发现DexClassLoader只有一个构造函数,而且这个构造函数是:dexPath、dexOutDir、libPath、parent

dexPath:是加载apk/dex/jar的路径

dexOutDir:是dex的输出路径(因为加载apk/jar的时候会解压除dex文件,这个路径就是保存dex文件的)

libPath:是加载的时候需要用到的lib库,这个一般不用

parent:给DexClassLoader指定父加载器


我们在来看一下PathClassLoader的源码

PathClassLoader.java

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package dalvik.system;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;

/**
 * Provides a simple {@link ClassLoader} implementation that operates on a list
 * of files and directories in the local file system, but does not attempt to
 * load classes from the network. Android uses this class for its system class
 * loader and for its application class loader(s).
 */
public class PathClassLoader extends ClassLoader {

    private final String path;
    private final String libPath;

    /*
     * Parallel arrays for jar/apk files.
     *
     * (could stuff these into an object and have a single array;
     * improves clarity but adds overhead)
     */
    private final String[] mPaths;
    private final File[] mFiles;
    private final ZipFile[] mZips;
    private final DexFile[] mDexs;

    /**
     * Native library path.
     */
    private final List<String> libraryPathElements;

    /**
     * Creates a {@code PathClassLoader} that operates on a given list of files
     * and directories. This method is equivalent to calling
     * {@link #PathClassLoader(String, String, ClassLoader)} with a
     * {@code null} value for the second argument (see description there).
     *
     * @param path
     *            the list of files and directories
     *
     * @param parent
     *            the parent class loader
     */
    public PathClassLoader(String path, ClassLoader parent) {
        this(path, null, parent);
    }

    /**
     * Creates a {@code PathClassLoader} that operates on two given
     * lists of files and directories. The entries of the first list
     * should be one of the following:
     *
     * <ul>
     * <li>Directories containing classes or resources.
     * <li>JAR/ZIP/APK files, possibly containing a "classes.dex" file.
     * <li>"classes.dex" files.
     * </ul>
     *
     * The entries of the second list should be directories containing
     * native library files. Both lists are separated using the
     * character specified by the "path.separator" system property,
     * which, on Android, defaults to ":".
     *
     * @param path
     *            the list of files and directories containing classes and
     *            resources
     *
     * @param libPath
     *            the list of directories containing native libraries
     *
     * @param parent
     *            the parent class loader
     */
    public PathClassLoader(String path, String libPath, ClassLoader parent) {
        super(parent);
....
看到了PathClassLoader类也是继承了ClassLoader的,但是他的构造函数和DexClassLoader有点区别就是,少了一个dexOutDir,这个原因也是很简单,因为PathClassLoader是加载/data/app中的apk,而这部分的apk都会解压释放dex到指定的目录:

/data/dalvik-cache

Android中插件开发篇之----类加载器_第1张图片

这个释放解压操作是系统做的。所以PathClassLoader可以不需要这个参数的。


上面看了他们两的区别,下面在来看一下Android中的各种类加载器分别加载哪些类:

package com.example.androiddemo;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListView;

public class MainActivity extends Activity {

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		Log.i("DEMO", "Context的类加载加载器:"+Context.class.getClassLoader());
		Log.i("DEMO", "ListView的类加载器:"+ListView.class.getClassLoader());
		Log.i("DEMO", "应用程序默认加载器:"+getClassLoader());
		Log.i("DEMO", "系统类加载器:"+ClassLoader.getSystemClassLoader());
		Log.i("DEMO", "系统类加载器和Context的类加载器是否相等:"+(Context.class.getClassLoader()==ClassLoader.getSystemClassLoader()));
		Log.i("DEMO", "系统类加载器和应用程序默认加载器是否相等:"+(getClassLoader()==ClassLoader.getSystemClassLoader()));
		
		Log.i("DEMO","打印应用程序默认加载器的委派机制:");
		ClassLoader classLoader = getClassLoader();
		while(classLoader != null){
			Log.i("DEMO", "类加载器:"+classLoader);
			classLoader = classLoader.getParent();
		}
		
		Log.i("DEMO","打印系统加载器的委派机制:");
		classLoader = ClassLoader.getSystemClassLoader();
		while(classLoader != null){
			Log.i("DEMO", "类加载器:"+classLoader);
			classLoader = classLoader.getParent();
		}
		
	}

}

运行结果:

Android中插件开发篇之----类加载器_第2张图片

依次来看一下

1) 系统类的加载器

Log.i("DEMO", "Context的类加载加载器:"+Context.class.getClassLoader());
Log.i("DEMO", "ListView的类加载器:"+ListView.class.getClassLoader());
从结果看到他们的加载器是:BootClassLoader,关于他源码我没有找到,只找到了class文件(用jd-gui查看):


看到他也是继承了ClassLoader类。


2) 应用程序的默认加载器

Log.i("DEMO", "应用程序默认加载器:"+getClassLoader());
运行结果:


默认类加载器是PathClassLoader,同时可以看到加载的apk路径,libPath(一般包括/vendor/lib和/system/lib)


3) 系统类加载器

Log.i("DEMO", "系统类加载器:"+ClassLoader.getSystemClassLoader());
运行结果:


系统类加载器其实还是PathClassLoader,只是加载的apk路径不是/data/app/xxx.apk了,而是系统apk的路径:/system/app/xxx.apk


4) 默认加载器的委派机制关系

Log.i("DEMO","打印应用程序默认加载器的委派机制:");
ClassLoader classLoader = getClassLoader();
while(classLoader != null){
	Log.i("DEMO", "类加载器:"+classLoader);
	classLoader = classLoader.getParent();
}
打印结果:


默认加载器PathClassLoader的父亲是BootClassLoader

5) 系统加载器的委派机制关系

Log.i("DEMO","打印系统加载器的委派机制:");
classLoader = ClassLoader.getSystemClassLoader();
while(classLoader != null){
	Log.i("DEMO", "类加载器:"+classLoader);
	classLoader = classLoader.getParent();
}
运行结果:


可以看到系统加载器的父亲也是BootClassLoader


二、分析遇到的问题的原因和解决办法

DexClassLoader加载原理和分析在实现插件时不同操作造成错误的原因分析

这里主要用了三个工程:

PluginImpl:插件接口工程(只是接口的定义)

PluginSDK:插件工程(实现插件接口,定义具体的功能)

HostProject:宿主工程(需要引用插件接口工程,然后动态的加载插件工程)(例子项目中名字是PluginDemos)

Android中插件开发篇之----类加载器_第3张图片


第一、项目介绍

下面来看一下源代码:

1、PluginImpl工程:

1) IBean.java

package com.pluginsdk.interfaces;

public abstract interface IBean{
  public abstract String getName();
  public abstract void setName(String paramString);
}


2) IDynamic.java

package com.pluginsdk.interfaces;

import android.content.Context;

public abstract interface IDynamic{
  public abstract void methodWithCallBack(YKCallBack paramYKCallBack);
  public abstract void showPluginWindow(Context paramContext);
  public abstract void startPluginActivity(Context context,Class<?> cls);
  public abstract String getStringForResId(Context context);
}
其他的就不列举了。


2、PluginSDK工程:

1) Dynamic.java

/**
 * Dynamic1.java
 * com.youku.pluginsdk.imp
 *
 * Function: TODO 
 *
 *   ver     date      		author
 * ──────────────────────────────────
 *   		 2014-10-20 		Administrator
 *
 * Copyright (c) 2014, TNT All Rights Reserved.
*/

package com.pluginsdk.imp;

import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.app.Dialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;

import com.pluginsdk.bean.Bean;
import com.pluginsdk.interfaces.IDynamic;
import com.pluginsdk.interfaces.YKCallBack;
import com.youku.pluginsdk.R;

/**
 * ClassName:Dynamic1
 *
 * @author   jiangwei
 * @version  
 * @since    Ver 1.1
 * @Date	 2014-10-20		下午5:57:10
 */
public class Dynamic implements IDynamic{
	/**

	 */
	public void methodWithCallBack(YKCallBack callback) {
		Bean bean = new Bean();
		bean.setName("PLUGIN_SDK_USER");
		callback.callback(bean);
	}
	
	public void showPluginWindow(Context context) {
		 AlertDialog.Builder builder = new Builder(context);
		  builder.setMessage("对话框");
		  builder.setTitle(R.string.hello_world);
		  builder.setNegativeButton("取消", new Dialog.OnClickListener() {
			   @Override
			   public void onClick(DialogInterface dialog, int which) {
				   dialog.dismiss();
			   }
			  });
		  Dialog dialog = builder.create();//.show();
		  dialog.show();
	}
	
	public void startPluginActivity(Context context,Class<?> cls){
		/**
		*这里要注意几点:
		*1、如果单纯的写一个MainActivity的话,在主工程中也有一个MainActivity,开启的Activity还是主工程中的MainActivity
		*2、如果这里将MainActivity写成全名的话,还是有问题,会报找不到这个Activity的错误
		*/
		Intent intent = new Intent(context,cls);
		context.startActivity(intent);
	}
	
	public String getStringForResId(Context context){
		return context.getResources().getString(R.string.hello_world);
	}

}


2) Bean.java

/**
 * User.java
 * com.youku.pluginsdk.bean
 *
 * Function: TODO 
 *
 *   ver     date      		author
 * ──────────────────────────────────
 *   		 2014-10-20 		Administrator
 *
 * Copyright (c) 2014, TNT All Rights Reserved.
*/

package com.pluginsdk.bean;


/**
 * ClassName:User
 *
 * @author   jiangwei
 * @version  
 * @since    Ver 1.1
 * @Date	 2014-10-20		下午1:35:16
 */
public class Bean implements com.pluginsdk.interfaces.IBean{

	/**
	 *
	 */
	private String name = "这是来自于插件工程中设置的初始化的名字";

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

}

3、宿主工程HostProject

1) MainActivity.java

package com.plugindemo;
import java.io.File;
import java.lang.reflect.Method;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.content.res.AssetManager;
import android.content.res.Resources;
import android.content.res.Resources.Theme;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;

import com.pluginsdk.interfaces.IBean;
import com.pluginsdk.interfaces.IDynamic;
import com.pluginsdk.interfaces.YKCallBack;
import com.youku.plugindemo.R;

import dalvik.system.DexClassLoader;

public class MainActivity extends Activity {
    private AssetManager mAssetManager;//资源管理器
    private Resources mResources;//资源
    private Theme mTheme;//主题
    private String apkFileName = "PluginSDKs.apk";
    private String dexpath = null;//apk文件地址
    private File fileRelease = null; //释放目录
    private DexClassLoader classLoader = null;
	@SuppressLint("NewApi")
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		dexpath =  Environment.getExternalStorageDirectory() + File.separator+apkFileName;
		fileRelease = getDir("dex", 0);
		
		/*初始化classloader
		 * dexpath dex文件地址
		 * fileRelease 文件释放地址 
		 *  父classLoader
		 */
		
		Log.d("DEMO", (getClassLoader()==ListView.class.getClassLoader())+"");
		Log.d("DEMO",ListView.class.getClassLoader()+"");
		Log.d("DEMO", Context.class.getClassLoader()+"");
		Log.d("DEMO", Context.class.getClassLoader().getSystemClassLoader()+"");
		Log.d("DEMO",Activity.class.getClassLoader()+"");
		Log.d("DEMO", (Context.class.getClassLoader().getSystemClassLoader() == ClassLoader.getSystemClassLoader())+"");
		Log.d("DEMO",ClassLoader.getSystemClassLoader()+"");
		
		classLoader = new DexClassLoader(dexpath, fileRelease.getAbsolutePath(),null,getClassLoader());
		
		Button btn_1 = (Button)findViewById(R.id.btn_1);
		Button btn_2 = (Button)findViewById(R.id.btn_2);
		Button btn_3 = (Button)findViewById(R.id.btn_3);
		Button btn_4 = (Button)findViewById(R.id.btn_4);
		Button btn_5 = (Button)findViewById(R.id.btn_5);
		Button btn_6 = (Button)findViewById(R.id.btn_6);
		
		btn_1.setOnClickListener(new View.OnClickListener() {//普通调用  反射的方式
			@Override
			public void onClick(View arg0) {
				Class mLoadClassBean;
				try {
					mLoadClassBean = classLoader.loadClass("com.pluginsdk.bean.Bean");
					Object beanObject = mLoadClassBean.newInstance();
					Log.d("DEMO", "ClassLoader:"+mLoadClassBean.getClassLoader());
					Log.d("DEMO", "ClassLoader:"+mLoadClassBean.getClassLoader().getParent());
					Method getNameMethod = mLoadClassBean.getMethod("getName");
					getNameMethod.setAccessible(true);
					String name = (String) getNameMethod.invoke(beanObject);
					Toast.makeText(MainActivity.this, name, Toast.LENGTH_SHORT).show();
				} catch (Exception e) {
					Log.e("DEMO", "msg:"+e.getMessage());
				} 
			}
		});
		btn_2.setOnClickListener(new View.OnClickListener() {//带参数调用
			@Override
			public void onClick(View arg0) {
				Class mLoadClassBean;
				try {
					mLoadClassBean = classLoader.loadClass("com.pluginsdk.bean.Bean");
					Object beanObject = mLoadClassBean.newInstance();
					//接口形式调用
					Log.d("DEMO", beanObject.getClass().getClassLoader()+"");
					Log.d("DEMO",IBean.class.getClassLoader()+"");
					Log.d("DEMO",ClassLoader.getSystemClassLoader()+"");
			        IBean bean = (IBean)beanObject;
			        bean.setName("宿主程序设置的新名字");
			        Toast.makeText(MainActivity.this, bean.getName(), Toast.LENGTH_SHORT).show();
				}catch (Exception e) {
					Log.e("DEMO", "msg:"+e.getMessage());
				}
	           
			}
		});
		btn_3.setOnClickListener(new View.OnClickListener() {//带回调函数的调用
			@Override
			public void onClick(View arg0) {
				Class mLoadClassDynamic;
				try {
					mLoadClassDynamic = classLoader.loadClass("com.pluginsdk.imp.Dynamic");
					 Object dynamicObject = mLoadClassDynamic.newInstance();
					  //接口形式调用
		            IDynamic dynamic = (IDynamic)dynamicObject;
		            //回调函数调用
		            YKCallBack callback = new YKCallBack() {//回调接口的定义
		            	public void callback(IBean arg0) {
		            		Toast.makeText(MainActivity.this, arg0.getName(), Toast.LENGTH_SHORT).show();
		            	};
					};
		            dynamic.methodWithCallBack(callback);
				} catch (Exception e) {
					Log.e("DEMO", "msg:"+e.getMessage());
				}
	           
			}
		});
		btn_4.setOnClickListener(new View.OnClickListener() {//带资源文件的调用
			@Override
			public void onClick(View arg0) {
				loadResources();
				Class mLoadClassDynamic;
				try {
					mLoadClassDynamic = classLoader.loadClass("com.pluginsdk.imp.Dynamic");
					Object dynamicObject = mLoadClassDynamic.newInstance();
					//接口形式调用
		            IDynamic dynamic = (IDynamic)dynamicObject;
		            dynamic.showPluginWindow(MainActivity.this);
				} catch (Exception e) {
					Log.e("DEMO", "msg:"+e.getMessage());
				}
			}
		});
		btn_5.setOnClickListener(new View.OnClickListener() {//带资源文件的调用
			@Override
			public void onClick(View arg0) {
				loadResources();
				Class mLoadClassDynamic;
				try {
					mLoadClassDynamic = classLoader.loadClass("com.pluginsdk.imp.Dynamic");
					Object dynamicObject = mLoadClassDynamic.newInstance();
					//接口形式调用
		            IDynamic dynamic = (IDynamic)dynamicObject;
		            dynamic.startPluginActivity(MainActivity.this,
		            		classLoader.loadClass("com.plugindemo.MainActivity"));
				} catch (Exception e) {
					Log.e("DEMO", "msg:"+e.getMessage());
				}
			}
		});
		btn_6.setOnClickListener(new View.OnClickListener() {//带资源文件的调用
			@Override
			public void onClick(View arg0) {
				loadResources();
				Class mLoadClassDynamic;
				try {
					mLoadClassDynamic = classLoader.loadClass("com.pluginsdk.imp.Dynamic");
					Object dynamicObject = mLoadClassDynamic.newInstance();
					//接口形式调用
		            IDynamic dynamic = (IDynamic)dynamicObject;
		            String content = dynamic.getStringForResId(MainActivity.this);
		            Toast.makeText(getApplicationContext(), content+"", Toast.LENGTH_LONG).show();
				} catch (Exception e) {
					Log.e("DEMO", "msg:"+e.getMessage());
				}
			}
		});
		
	}

	 protected void loadResources() {
	        try {
	            AssetManager assetManager = AssetManager.class.newInstance();
	            Method addAssetPath = assetManager.getClass().getMethod("addAssetPath", String.class);
	            addAssetPath.invoke(assetManager, dexpath);
	            mAssetManager = assetManager;
	        } catch (Exception e) {
	            e.printStackTrace();
	        }
	        Resources superRes = super.getResources();
	        superRes.getDisplayMetrics();
	        superRes.getConfiguration();
	        mResources = new Resources(mAssetManager, superRes.getDisplayMetrics(),superRes.getConfiguration());
	        mTheme = mResources.newTheme();
	        mTheme.setTo(super.getTheme());
	    }
	
	@Override
    public AssetManager getAssets() {
        return mAssetManager == null ? super.getAssets() : mAssetManager;
    }

    @Override
    public Resources getResources() {
        return mResources == null ? super.getResources() : mResources;
    }

    @Override
    public Theme getTheme() {
        return mTheme == null ? super.getTheme() : mTheme;
    }
}
三个工程的下载地址:http://download.csdn.net/detail/jiangwei0910410003/8188011

第二、项目引用关系

工程文件现在大致看完了,我们看一下他们的引用关系吧:

1、将接口工程PluginImpl设置成一个Library

Android中插件开发篇之----类加载器_第4张图片

2、插件工程PluginSDKs引用插件的jar

注意是lib文件夹,不是libs,这个是有区别的,后面会说道

Android中插件开发篇之----类加载器_第5张图片


3、HostProject项目引用PluginImpl这个library

Android中插件开发篇之----类加载器_第6张图片

项目引用完成之后,我们编译PluginSDKs项目,生成PluginSDKs.apk放到手机的sdcard的根目录(因为我代码中是从这个目录进行加载apk的,当然这个目录是可以修改的),然后运行HostProject

Android中插件开发篇之----类加载器_第7张图片

看到效果了吧。运行成功,其实这个对话框是在插件中定义的,但是我们知道定义对话框是需要context变量的,所以这个变量就是通过参数从宿主工程中传递到插件工程即可,成功了就不能这么了事,因为我还没有说道我遇到的问题,下面就来看一下遇到的几个问题


三、问题分析

问题一:Could not find class...(找不到指定的类)


这个问题产生的操作:

插件工程PluginSDKs的引用方式不变,宿主工程PluginDemos的引用方式改变

Android中插件开发篇之----类加载器_第8张图片

在说这个原因之前先来了解一下Eclipse中引用工程的不同方式和区别

第一种:最常用的将引用工程打成jar放到需要引用工程的libs下面(这里是将PluginImpl打成jar,放到HostProject工程的libs中)

这种方式是Eclipse推荐使用的,当我们在建立一个项目的时候也会自动产生这个文件夹,当我们将我们需要引用的工程打成jar,然后放到这个文件夹之后,Eclipse就自动导入了(这个功能是Eclipse3.7之后有的)。

第二种:和第一种的区别是,我们可以从新新建一个文件夹比如是lib,然后将引用的jar放到这个文件夹中,但是此时Eclipse是不会自动导入的,需要我们手动的导入(add build path...),但是这个是一个区别,还有一个区别,也是到这个这个报错原因的区别,就是libs文件夹中的jar,在运行的时候是会将这个jar集成到程序中的,而我们新建的文件夹(名字非libs即可),及时我们手动的导入,编译是没有问题的,但是运行的时候,是不会将jar集成到程序中。

第三种:和前两种的区别是不需要将引用工程打成jar,直接引用这个工程

Android中插件开发篇之----类加载器_第9张图片

这种方式其实效果和第一种差不多,唯一的区别就是不需要打成jar,但是运行的时候是不会将引用工程集成到程序中的。

第四种:和第三种的方式是一样的,也是不需要将引用工程打成jar,直接引用工程:

Android中插件开发篇之----类加载器_第10张图片

这个前提是需要设置PluginImpl项目为Library,同时引用的项目和被引用的项目必须在一个工作空间中,不然会报错,这种的效果和第二种是一样的,在运行的时候是会将引用工程集成到程序中的。

第五种:和第一种、第二种差不多,导入jar:

Android中插件开发篇之----类加载器_第11张图片

这里有很多种方式选择jar的位置,但是这些操作的效果和第一种是一样的,运行的时候是不会将引用的jar集成到程序中的。


总结上面的五种方式,我们可以看到,第二种和第四种的效果是一样的,也是最普遍的导入引用工程的方式,因为其他三种方式的话,其实在编译的时候是不会有问题的,但是在运行的时候会报错(找不到指定的类,可以依次尝试一下),不过这三种方式只要一步就可以和那两种方式实现的效果一样了


只要设置导出的时候勾选上这个jar就可以了。那么其实这五种方式都是可以的,性质和效果是一样的。


说完了Eclipse中引用工程的各种方式以及区别之后,我们在回过头来看一下,上面遇到的问题:Could not find class...

其实这个问题就简单了,原因是:插件工程PluginSDKs使用的是lib文件夹导入的jar(这个jar是不会集成到程序中的),而宿主工程PluginDemos的引用工程的方式也变成了lib文件夹(jar也是不会集成到程序中的)。那么程序运行的时候就会出现错误:

Could not find class 'com.pluginsdk.interfaces.IBean' 


问题二:Class ref in pre-verified class resolved to unexpected implementation(相同的类加载了两次)

Android中插件开发篇之----类加载器_第12张图片

这个问题产生的操作:

插件工程PluginSDKs和宿主工程PluginDemos引用工程的方式都变成library(或者是都用libs文件夹导入jar)

Android中插件开发篇之----类加载器_第13张图片

这个错误的原因也是很多做插件的开发者第一次都会遇到的问题,其实这个问题的本质是PluginImpl中的接口被加载了两次,因为插件工程和宿主工程在运行的时候都会把PluginImpl集成到程序中。对于这个问题,我们来分析一下,首先对于宿主apk,他的类加载器是PathClassLoader(这个对于每个应用来说是默认的加载器,原因很简单,PathClassLoader只能加载/data/app目录下的apk,就是已经安装的apk,一般我们的apk都是安装之后在运行,所以用这个加载器也是理所当然的)。这个加载器开始加载插件接口工程(宿主工程中引入的PluginImpl)中的IBean。当使用DexClassLoader加载PluginSDKs.apk的时候,首先会让宿主apkPathClassLoader加载器去加载,这个好多人有点迷糊了,为什么会先让PathClassLoader加载器去加载呢?

这个就是Java中的类加载机制的双亲委派机制:http://blog.csdn.net/jiangwei0910410003/article/details/17733153

Android中的加载机制也是类似的,我们这里的代码设置了DexClassLoader的父加载器为当前类加载器(宿主apk的PathClassLoader),不行的话,可以打印一下getClassLoader()方法的返回结果看一下。

classLoader = new DexClassLoader(dexpath, fileRelease.getAbsolutePath(),null,getClassLoader());
那么加载器就是一样的了(宿主apk的PathClassLoader),那么就奇怪了,都是一个为什么还有错误呢?查看系统源码可以了解:

Resolve.c源码(这个是在虚拟机dalvik中的):源码下载地址为:http://blog.csdn.net/jiangwei0910410003/article/details/37988637
我们来看一下他的一个主要函数:

/*
 * Find the class corresponding to "classIdx", which maps to a class name
 * string.  It might be in the same DEX file as "referrer", in a different
 * DEX file, generated by a class loader, or generated by the VM (e.g.
 * array classes).
 *
 * Because the DexTypeId is associated with the referring class' DEX file,
 * we may have to resolve the same class more than once if it's referred
 * to from classes in multiple DEX files.  This is a necessary property for
 * DEX files associated with different class loaders.
 *
 * We cache a copy of the lookup in the DexFile's "resolved class" table,
 * so future references to "classIdx" are faster.
 *
 * Note that "referrer" may be in the process of being linked.
 *
 * Traditional VMs might do access checks here, but in Dalvik the class
 * "constant pool" is shared between all classes in the DEX file.  We rely
 * on the verifier to do the checks for us.
 *
 * Does not initialize the class.
 *
 * "fromUnverifiedConstant" should only be set if this call is the direct
 * result of executing a "const-class" or "instance-of" instruction, which
 * use class constants not resolved by the bytecode verifier.
 *
 * Returns NULL with an exception raised on failure.
 */
ClassObject* dvmResolveClass(const ClassObject* referrer, u4 classIdx,
    bool fromUnverifiedConstant)
{
    DvmDex* pDvmDex = referrer->pDvmDex;
    ClassObject* resClass;
    const char* className;

    /*
     * Check the table first -- this gets called from the other "resolve"
     * methods.
     */
    resClass = dvmDexGetResolvedClass(pDvmDex, classIdx);
    if (resClass != NULL)
        return resClass;

    LOGVV("--- resolving class %u (referrer=%s cl=%p)\n",
        classIdx, referrer->descriptor, referrer->classLoader);

    /*
     * Class hasn't been loaded yet, or is in the process of being loaded
     * and initialized now.  Try to get a copy.  If we find one, put the
     * pointer in the DexTypeId.  There isn't a race condition here --
     * 32-bit writes are guaranteed atomic on all target platforms.  Worst
     * case we have two threads storing the same value.
     *
     * If this is an array class, we'll generate it here.
     */
    className = dexStringByTypeIdx(pDvmDex->pDexFile, classIdx);
    if (className[0] != '\0' && className[1] == '\0') {
        /* primitive type */
        resClass = dvmFindPrimitiveClass(className[0]);
    } else {
        resClass = dvmFindClassNoInit(className, referrer->classLoader);
    }

    if (resClass != NULL) {
        /*
         * If the referrer was pre-verified, the resolved class must come
         * from the same DEX or from a bootstrap class.  The pre-verifier
         * makes assumptions that could be invalidated by a wacky class
         * loader.  (See the notes at the top of oo/Class.c.)
         *
         * The verifier does *not* fail a class for using a const-class
         * or instance-of instruction referring to an unresolveable class,
         * because the result of the instruction is simply a Class object
         * or boolean -- there's no need to resolve the class object during
         * verification.  Instance field and virtual method accesses can
         * break dangerously if we get the wrong class, but const-class and
         * instance-of are only interesting at execution time.  So, if we
         * we got here as part of executing one of the "unverified class"
         * instructions, we skip the additional check.
         *
         * Ditto for class references from annotations and exception
         * handler lists.
         */
        if (!fromUnverifiedConstant &&
            IS_CLASS_FLAG_SET(referrer, CLASS_ISPREVERIFIED))
        {
            ClassObject* resClassCheck = resClass;
            if (dvmIsArrayClass(resClassCheck))
                resClassCheck = resClassCheck->elementClass;

            if (referrer->pDvmDex != resClassCheck->pDvmDex &&
                resClassCheck->classLoader != NULL)
            {
                LOGW("Class resolved by unexpected DEX:"
                     " %s(%p):%p ref [%s] %s(%p):%p\n",
                    referrer->descriptor, referrer->classLoader,
                    referrer->pDvmDex,
                    resClass->descriptor, resClassCheck->descriptor,
                    resClassCheck->classLoader, resClassCheck->pDvmDex);
                LOGW("(%s had used a different %s during pre-verification)\n",
                    referrer->descriptor, resClass->descriptor);
                dvmThrowException("Ljava/lang/IllegalAccessError;",
                    "Class ref in pre-verified class resolved to unexpected "
                    "implementation");
                return NULL;
            }
        }

        LOGVV("##### +ResolveClass(%s): referrer=%s dex=%p ldr=%p ref=%d\n",
            resClass->descriptor, referrer->descriptor, referrer->pDvmDex,
            referrer->classLoader, classIdx);

        /*
         * Add what we found to the list so we can skip the class search
         * next time through.
         *
         * TODO: should we be doing this when fromUnverifiedConstant==true?
         * (see comments at top of oo/Class.c)
         */
        dvmDexSetResolvedClass(pDvmDex, classIdx, resClass);
    } else {
        /* not found, exception should be raised */
        LOGVV("Class not found: %s\n",
            dexStringByTypeIdx(pDvmDex->pDexFile, classIdx));
        assert(dvmCheckException(dvmThreadSelf()));
    }

    return resClass;
}
我们看下面的判断可以得到,就是在这里抛出的异常,代码逻辑我们就不看了,因为太多的头文件相互引用,看起来很费劲,直接看一下函数的说明:

Android中插件开发篇之----类加载器_第14张图片
红色部分内容,他的意思是我们需要解决从不同的dex文件中加载相同的class,需要使用不同的类加载器。

说白了就是,同一个类加载器从不同的dex文件中加载相同的class。所以上面是同一个类加载器PathClassLoader去加载(宿主apk和插件apk)来自不同的dex中的相同的类IBean。所以我们在做动态加载的时候都说过:不要把接口的jar一起打包成jar/dex/apk


问题三:Connot be cast to....(类型转化异常)


这个问题产生的操作:

插件工程PluginSDKs和宿主工程都是用Library方式引用工程(或者是libs),同时将上面的一行代码

classLoader = new DexClassLoader(dexpath, fileRelease.getAbsolutePath(),null,getClassLoader());

修改成:

classLoader = new DexClassLoader(dexpath, fileRelease.getAbsolutePath(),null,ClassLoader.getSystemClassLoader());
就是将DexClassLoader的父加载器修改了一下:我们知道getClassLoader()获取到的是应用的默认加载器PathClassLoader,而ClassLoader.getSystemClassLoader()是获取系统类加载器,这样修改之后会出现这样的错误的原因是:插件工程和宿主工程都集成了PluginImpl,所以DexClassLoader在加载Bean的时候,首先会让ClassLoader.getSystemClassLoader()类加载器(DexClassLoader的父加载器)去查找,因为Bean是实现了IBean接口,这时候ClassLoader.getSystemClassLoader就会从插件工程的apk中查找这个接口,结果没找到,没找到的话就让DexClassLoader去找,结果在PluginSDKs.apk中找到了,就加载进来,同时宿主工程中也集成了插件接口PluginImpl,他使用PathClassLoader去宿主工程中去查找,结果也是查找到了,也加载进来了,但是在进行类型转化的时候出现了错误:

IBean bean = (IBean)beanObject;
原因说白了就是:同一个类,用不同的类加载器进行加载产生出来的对象是不同的,不能进行相互赋值,负责就会出现转化异常。


总结

上面就说到了一些开发插件的过程中会遇到的一些问题,当我们知道这些问题之后,解决方法自然就会有了,

1) 为了避免Could not find class...,我们必须要集成PluginImpl,方式是使用Library或者是libs文件夹导入jar

(这里要注意,因为我们运行的其实是宿主工程apk,所以宿主工程一定要集成PluginImpl,如果他不集成的话,即使插件工程apk集成了也还是没有效果的)

2) 为了避免Class ref in pre-verified class resolved to unexpected implementation,我们在宿主工程和插件工程中只能集成一份PluginImpl,在结合上面的错误避免方式,可以得到正确的方式:

一定是宿主工程集成PluginImpl,插件工程一定不能集成PluginImpl。

(以后再制作插件的时候记住一句话就可以了,插件工程打包不能集成接口jar,宿主工程打包一定要集成接口jar)

关于第三个问题,其实在开发的过程中一般不会碰到,这里说一下主要是为了马上介绍Android中的类加载器的相关只是来做铺垫的


(PS:问题都解决了,后续就要介绍插件的制作了~~)

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