简介

// API 28 /libcore/dalvik/src/main/java/dalvik/system/PathClassLoader.java
public class PathClassLoader extends BaseDexClassLoader {

    // optimizedDirectory 直接设为 null
    public PathClassLoader(String dexPath, ClassLoader parent) {
        super(dexPath, null, null, parent);
    }

    // optimizedDirectory 直接设为 null
    public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
        super(dexPath, null, librarySearchPath, parent);
    }
}


// API 28 /libcore/dalvik/src/main/java/dalvik/system/DexClassLoader.java
public class DexClassLoader extends BaseDexClassLoader {
 
    public DexClassLoader(String dexPath, String optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        // 从26开始，super里面的参数就变了，看两个不同API对应的父类构造方法
        super(dexPath, null, librarySearchPath, parent);
    }
}

    // API 28 /libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java
    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String librarySearchPath, ClassLoader parent, boolean isTrusted) {
        super(parent);
        
        // DexPathList 的第四个参数是 optimizedDirectory，可以看到这儿为 null
        this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);

        if (reporter != null) {
            reportClassLoaderChain();
        }
    }

    // API 25 /libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java
    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, librarySearchPath, optimizedDirectory);
    }

DexClassLoader dexClassLoader = new DexClassLoader(dexPath,
            context.getCacheDir().getAbsolutePath(), null, context.getClassLoader());
try {
    Class<?> clazz = dexClassLoader.loadClass("com.tufusi.kkplugin.XXX");
} catch (ClassNotFoundException e) {
    e.printStackTrace();
}

// /libcore/ojluni/src/main/java/java/lang/ClassLoader.java
protected Class<?> loadClass(String name, boolean resolve)
    throws ClassNotFoundException
{
        // 首先，检测这个类是否已加载 ------------------------------ ①
        Class<?> c = findLoadedClass(name);
        if (c == null) {
            try {
                if (parent != null) {
                    // 如果parent不为null，则调用parent的loadClass进行检查加载
                    c = parent.loadClass(name, false);
                } else {
                    // 正常情况下不会命中这里，因为 BootClassLoader 重写了 loadClass 方法，结束了递归
                    c = findBootstrapClassOrNull(name);
                }
            } catch (ClassNotFoundException e) {
                // ClassNotFoundException thrown if class not found
                // from the non-null parent class loader
            }

            if (c == null) {
                // 如果仍然找不到，就调用 findClass 去查找 ------------------------------ ②
                c = findClass(name);
            }
        }
        return c;
}

// ------------------------------ ① 检测类是否已加载
protected final Class<?> findLoadedClass(String name) {
    ClassLoader loader;
    if (this == BootClassLoader.getInstance())
        loader = null;
    else
        loader = this;
    // 最后通过 native 方法实现查找
    return VMClassLoader.findLoadedClass(loader, name);
}

// ------------------------------ ② 加载器一般都会重写此方法，定义自己的加载规则
protected Class<?> findClass(String name) throws ClassNotFoundException {
    throw new ClassNotFoundException(name);
}

// /libcore/ojluni/src/main/java/java/lang/VMClassLoader.java
@FastNative
native static Class findLoadedClass(ClassLoader cl, String name);