ClassLoader类加载源码解析

//ClassLoader.java
public Class<?> loadClass(String name) throws ClassNotFoundException {
 //该处调用了两个参数的重载方法
  return loadClass(name, false);
 }
 
 protected Class<?> loadClass(String name, boolean resolve)
  throws ClassNotFoundException
 {
  //先查一下该类是否已经加载过了
   Class<?> c = findLoadedClass(name);
   if (c == null) {
    try {
     //双亲委托机制，先让爸爸去找
     if (parent != null) {
      c = parent.loadClass(name, false);
     } else {
      //如果parent为null，则用BootClassLoader进行加载
      c = findBootstrapClassOrNull(name);
     }
    } catch (ClassNotFoundException e) {
     // ClassNotFoundException thrown if class not found
     // from the non-null parent class loader
    }

    if (c == null) {
     //如果都找不到就自己去找,此方法在子类BaseDexClassLoader类中有重写
     c = findClass(name);
    }
   }
   return c;
 }

//BaseDexClassLoader.java
//查找class
 @Override
 protected Class<?> findClass(String name) throws ClassNotFoundException {
  ...
  //这里通过pathList变量来查找，而pathList是在BaseDexClassLoader的构造方法中初始化的
  Class c = pathList.findClass(name, suppressedExceptions);
  ...
  return c;
 }
 
private final DexPathList pathList;
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
   String librarySearchPath, ClassLoader parent, boolean isTrusted) {
  super(parent);
  //构造方法中初始化pathList变量
  this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);
 }

//DexPathList.java
//是一个Element数组，一个element中包含一个 DexFile，DexFile就代表一个Dex文件，里面的native(C/C++)函数来进行Dex的加载工作
 private Element[] dexElements;
 
public Class<?> findClass(String name, List<Throwable> suppressed) {
  for (Element element : dexElements) {
   //此处调用Element的findClass来实现，
   Class<?> clazz = element.findClass(name, definingContext, suppressed);
   if (clazz != null) {
    return clazz;
   }
  }
  return null;
 }
// Element为DexPathList的内部类
static class Element {
 private final File path;
  //一个DexFile就代表一个Dex文件
  private final DexFile dexFile;
  //有多个构造方法，但都仅是将值传过来，让Element来持有一个DexFile
  public Element(DexFile dexFile) {
 this.dexFile = dexFile;
   this.path = null;
 }
  
  public Class<?> findClass(String name, ClassLoader definingContext,
    List<Throwable> suppressed) {
    //通过DexFile来加载类
   return dexFile != null ? dexFile.loadClassBinaryName(name, definingContext, suppressed)
     : null;
  }
 }

DexPathList(ClassLoader definingContext, String dexPath,
   String librarySearchPath, File optimizedDirectory, boolean isTrusted) {
   //通过makeDexElements方法为dexElements初始化
 this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
           suppressedExceptions, definingContext, isTrusted);
}
//腾讯系的热修复，诸如微信tinker、qq空间qfix原理便是反射此方法，将修复后的类打包成dex，通过反射该方法来将文件转化为Element，并将新生成的element放到dexElements前面，这样下次系统再去寻找某个class时，会先从修复后的dex中来找class，找到后便不再继续查找，从而修复该class，此方式便为插桩
private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
   List<IOException> suppressedExceptions, ClassLoader loader, boolean isTrusted) {
  Element[] elements = new Element[files.size()];
  ...
  for (File file : files) {
 if (name.endsWith(DEX_SUFFIX)) {
    //以 .dex 结尾的
     // Raw dex file (not inside a zip/jar).
     //加载dex文件
      dex = loadDexFile(file, optimizedDirectory, loader, elements);
      if (dex != null) {
       elements[elementsPos++] = new Element(dex, null);
      }
    }
 }
   ...
  return elements;
 }

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

//DexFile.java
private DexFile(String sourceName, String outputName, int flags, ClassLoader loader,
   DexPathList.Element[] elements) throws IOException {
  ...
  //此处调用openDexFile来实现
  mCookie = openDexFile(sourceName, outputName, flags, loader, elements);
  ...
 }

private static Object openDexFile(String sourceName, String outputName, int flags,
   ClassLoader loader, DexPathList.Element[] elements) throws IOException {
  //此处通过调用 openDexFileNative来实现
  return openDexFileNative(new File(sourceName).getAbsolutePath(),
         (outputName == null)
          ? null
          : new File(outputName).getAbsolutePath(),
         flags,
         loader,
         elements);
 }
//openDexFileNative是一个native方法，是由C/C++来实现的
private static native Object openDexFileNative(String sourceName, String outputName, int flags,
   ClassLoader loader, DexPathList.Element[] elements);