虚拟机.png

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dexopt

在dalvik虚拟机加载dex文件时，对dex文件进行验证和优化操作，对dex文件的优化结果变成了odex（Optimized dex）文件，这个文件和dex很像，使用了一些优化【操作码】

dex2oat

Art预先编译机制：在安装时对dex文件执行AOT 提前编译操作，编译为OAT（ELF文件）可执行文件（机器码）。

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ClassLoader

https://cs.android.com/

8.0之前的实现方式

DexClassLoader

public class DexclassLoader extends BaseDexclassLoader{
  public DexClassLoader(String dexPath,String optimizedDirectory,
  String librarySearchPath,ClassLoader parent){
    super(dexPath,new File(optimizedDirectory),librarySearchPath,parent);
  }
}

PathClassLoader

public class PathclassLoader extends BaseDexclassLoader{
  public PathclassLoader(String dexPath,classLoader parent){
    super(dexPath,null,null,parent);
  }

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

两者唯一的区别：DexClassLoader需要传递一个optimizedDirectory参数，而PathClassLoader则直接给null。
optimizedDirectory：dexopt的产出目录odex
optimizedDirectory为null时的默认路径为：/data/dalvik-cache
8.0之后

双亲委托机制

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    c = findClass(name);
                }
            }
            return c;
    }

ClassLoader整理图.png