Android 热修复原理解析

前期我们已经讨论过了JVM ClassLoader机制以及理解Dalvik和ART、JVM虚拟机,今天带着大家来分析Android系统的类加载机制。

其实在Android系统中类加载器分为两种,PathClassLoader和DexClassLoader，两者都继承自BaseDexClassLoader。

Android类加载器

PathClassLoader
用来加载系统类和应用类
DexClassLoader
用来加载jar、apk、dex文件.加载jar、apk也是最终抽取里面的Dex文件进行加载

热修复就是利用dexElements的顺序来做文章，当一个补丁的patch.dex放到了dexElements的第一位，那么当加载一个bug类时，发现在patch.dex中，则直接加载这个类，原来的bug类可能就被覆盖了

首先看看PathClassLoader的源码

public class PathClassLoader extends BaseDexClassLoader {

    public PathClassLoader(String dexPath, ClassLoader parent) {
        super(dexPath, null, null, parent);
    }

    public PathClassLoader(String dexPath, String libraryPath,
            ClassLoader parent) {
        super(dexPath, null, libraryPath, parent);
    }
}

DexClassLoader的源码

public class DexClassLoader extends BaseDexClassLoader {

    public DexClassLoader(String dexPath, String optimizedDirectory, 
                          String libraryPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}

BaseDexClassLoader的源码

public class BaseDexClassLoader extends ClassLoader {
    private final DexPathList pathList;

    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, libraryPath, 
                                        optimizedDirectory);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
        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;
    }

在BaseDexClassLoader 构造函数中创建一个DexPathList类的实例,这个DexPathList的构造函数会创建一个dexElements 数组

public DexPathList(ClassLoader definingContext, String dexPath, 
                   String libraryPath, File optimizedDirectory) {
        ... 
        this.definingContext = definingContext;
        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        //创建一个数组
        this.dexElements = makeDexElements(splitDexPath(dexPath), 
                           optimizedDirectory, suppressedExceptions);
        ... 
    }

然后BaseDexClassLoader 重写了findClass方法,调用了pathList.findClass，跳到DexPathList类中.

/* package */final class DexPathList {
    ...
    public Class findClass(String name, List<Throwable> suppressed) {
            //遍历该数组
        for (Element element : dexElements) {
            //初始化DexFile
            DexFile dex = element.dexFile;

            if (dex != null) {
                //调用DexFile类的loadClassBinaryName方法返回Class实例
                Class clazz = dex.loadClassBinaryName(name, 
                                                      definingContext, suppressed);
                if (clazz != null) {
                    return clazz;
                }
            }
        }       
        return null;
    }
    ...
}

会遍历这个数组,然后初始化DexFile，如果DexFile不为空那么调用DexFile类的loadClassBinaryName方法返回Class实例.
归纳上面的话就是:ClassLoader会遍历这个数组,然后加载这个数组中的dex文件. 而ClassLoader在加载到正确的类之后,就不会再去加载有Bug的那个类了,我们把这个正确的类放在Dex文件中,让这个Dex文件排在dexElements数组前面即可.

最后编辑于：2018.06.19 21:32:53