一直想了解  Android 动态加载类的原理，今天抽空分析了下DexClassLoader 源码

  DexClassLoader 是加载包含classes.dex文件的jar文件或者apk文件；   通过构造函数发现需要一个应用私有的，可写的目录去缓存优化的classes。可以用使用File dexoutputDir = context.getDir(“dex”,0);创建一个这样的目录，不要使用外部缓存，以保护你的应用被代码注入。

其源码如下：

public classDexClassLoaderextendsBaseDexClassLoader {

37    /**
38     * Creates a {@code DexClassLoader} that finds interpreted and native
39     * code.  Interpreted classes are found in a set of DEX files contained
40     * in Jar or APK files.
41     *
42     * <p>The path lists are separated using the character specified by the
43     * {@code path.separator} system property, which defaults to {@code :}.
44     *
45     * @param dexPath the list of jar/apk files containing classes and
46     *     resources, delimited by {@code File.pathSeparator}, which
47     *     defaults to {@code ":"} on Android
48     * @param optimizedDirectory directory where optimized dex files
49     *     should be written; must not be {@code null}
50     * @param libraryPath the list of directories containing native
51     *     libraries, delimited by {@code File.pathSeparator}; may be
52     *     {@code null}
53     * @param parent the parent class loader
54     */
55    public DexClassLoader(String dexPath, String optimizedDirectory,
56            String libraryPath, ClassLoader parent) {
57        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
58    }
59}
60

再解释下几个构造函数参数的意义：

dexpath为jar或apk文件目录。

optimizedDirectory为优化dex缓存目录。

libraryPath包含native lib的目录路径。

parent父类加载器。

然后执行的是父类的构造函数：

super(dexPath, new File(optimizedDirectory), libraryPath, parent);

BaseDexClassLoader 的构造函数如下：

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

第一句调用的还是父类的构造函数，也就是ClassLoader的构造函数：

protected ClassLoader(ClassLoader parentLoader) {
        this(parentLoader, false);
    }
    /*
     * constructor for the BootClassLoader which needs parent to be null.
     */
    ClassLoader(ClassLoader parentLoader, boolean nullAllowed) {
       if (parentLoader == null && !nullAllowed) {
            throw new NullPointerException(“parentLoader == null && !nullAllowed”);
      }
      parent = parentLoader;
}

该构造函数把传进来的父类加载器赋给了私有变量parent。

再来看第二句：

this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);

pathList为该类的私有成员变量，类型为DexPathList，进入到DexPathList函数：

/**
78     * Constructs an instance.
79     *
80     * @param definingContext the context in which any as-yet unresolved
81     * classes should be defined
82     * @param dexPath list of dex/resource path elements, separated by
83     * {@code File.pathSeparator}
84     * @param libraryPath list of native library directory path elements,
85     * separated by {@code File.pathSeparator}
86     * @param optimizedDirectory directory where optimized {@code .dex} files
87     * should be found and written to, or {@code null} to use the default
88     * system directory for same
89     */
90    public DexPathList(ClassLoader definingContext, String dexPath,
91            String libraryPath, File optimizedDirectory) {
92
93        if (definingContext == null) {
94            throw new NullPointerException("definingContext == null");
95        }
96
97        if (dexPath == null) {
98            throw new NullPointerException("dexPath == null");
99        }
100
101        if (optimizedDirectory != null) {
102            if (!optimizedDirectory.exists())  {
103                throw new IllegalArgumentException(
104                        "optimizedDirectory doesn't exist: "
105                        + optimizedDirectory);
106            }
107
108            if (!(optimizedDirectory.canRead()
109                            && optimizedDirectory.canWrite())) {
110                throw new IllegalArgumentException(
111                        "optimizedDirectory not readable/writable: "
112                        + optimizedDirectory);
113            }
114        }
115
116        this.definingContext = definingContext;
117
118        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();

119        // save dexPath for BaseDexClassLoader
120        this.dexElements = makePathElements(splitDexPath(dexPath), optimizedDirectory,
1        suppressedExceptions);
122
123        // Native libraries may exist in both the system and
124        // application library paths, and we use this search order:
125        //
126        //   1. This class loader's library path for application libraries (libraryPath):
127        //   1.1. Native library directories
128        //   1.2. Path to libraries in apk-files
129        //   2. The VM's library path from the system property for system libraries
130        //      also known as java.library.path
131        //
132        // This order was reversed prior to Gingerbread; see http://b/2933456.
133        this.nativeLibraryDirectories = splitPaths(libraryPath, false);
134        this.systemNativeLibraryDirectories =
135                splitPaths(System.getProperty("java.library.path"), true);
136        List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
137        allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
138
139        this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories, null,
140                                                          suppressedExceptions);
141
142        if (suppressedExceptions.size() > 0) {
143            this.dexElementsSuppressedExceptions =
144                suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
145        } else {
146            dexElementsSuppressedExceptions = null;
147        }
148    }
149

前面是一些对于传入参数的验证，然后调用了makeDexElements。

private static Element[] makeDexElements(ArrayList<File> files, File optimizedDirectory,
                                             ArrayList<IOException> suppressedExceptions) {
ArrayList<Element> elements = new ArrayList<Element>();
        for (File file : files) {
            File zip = null;
            DexFile dex = null;
            String name = file.getName();

            if (name.endsWith(DEX_SUFFIX)) {               //dex文件处理
                // Raw dex file (not inside a zip/jar).
                try {
                    dex = loadDexFile(file, optimizedDirectory);
                } catch (IOException ex) {
                    System.logE(“Unable to load dex file: ” + file, ex);
                }
            } else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
                    || name.endsWith(ZIP_SUFFIX)) {   //apk，jar，zip文件处理
                zip = file;

                try {
                    dex = loadDexFile(file, optimizedDirectory);
                } catch (IOException suppressed) {
                    suppressedExceptions.add(suppressed);
                }
            } else if (file.isDirectory()) {
                elements.add(new Element(file, true, null, null));
            } else {
                System.logW(“Unknown file type for: ” + file);
            }

            if ((zip != null) || (dex != null)) {
                elements.add(new Element(file, false, zip, dex));
            }
        }

        return elements.toArray(new Element[elements.size()]);
    }
}

不管是dex文件，还是apk文件最终加载的都是loadDexFile，跟进这个函数：

如果optimizedDirectory为null就会调用openDexFile(fileName, null, 0);加载文件。

否则调用DexFile.loadDex(file.getPath(), optimizedPath, 0);

而这个函数也只是直接调用new DexFile(sourcePathName, outputPathName, flags);

里面调用的也是openDexFile(sourceName, outputName, flags);

所以最后都是调用openDexFile，跟进这个函数：

private static DexFile loadDexFile(File file, File optimizedDirectory)
            throws IOException {
        if (optimizedDirectory == null) {
            return new DexFile(file);
        } else {
            String optimizedPath = optimizedPathFor(file, optimizedDirectory);
            return DexFile.loadDex(file.getPath(), optimizedPath, 0);
        }
}

private static int openDexFile(String sourceName, String outputName,
        int flags) throws IOException {
        return openDexFileNative(new File(sourceName).getCanonicalPath(),
                                 (outputName == null) ? null : new File(outputName).getCanonicalPath(),
                                 flags);
}

而这个函数调用的是so的openDexFileNative这个函数。打开成功则返回一个cookie。

接下来就是分析native函数的实现部分了。

———-openDexFileNative———-

static void Dalvik_dalvik_system_DexFile_openDexFileNative(const u4* args,JValue* pResult)
{
    ……………
if (hasDexExtension(sourceName)
            && dvmRawDexFileOpen(sourceName, outputName, &pRawDexFile, false) == 0) {
        ALOGV(“Opening DEX file ‘%s’ (DEX)”, sourceName);

        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = true;
        pDexOrJar->pRawDexFile = pRawDexFile;
        pDexOrJar->pDexMemory = NULL;
    } else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false) == 0) {
        ALOGV(“Opening DEX file ‘%s’ (Jar)”, sourceName);

        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = false;
        pDexOrJar->pJarFile = pJarFile;
        pDexOrJar->pDexMemory = NULL;
    } else {
        ALOGV(“Unable to open DEX file ‘%s’”, sourceName);
        dvmThrowIOException(“unable to open DEX file”);
    }
    ……………
}

这里会根据是否为dex文件或者包含classes.dex文件的jar，分别调用函数dvmRawDexFileOpen和dvmJarFileOpen来处理，最终返回一个DexOrJar的结构。

首先来看dvmRawDexFileOpen函数的处理：

最后成功的话，填充RawDexFile。

dvmJarFileOpen的代码处理也是差不多的。

最后成功的话，填充JarFile。