java8 通过反射执行接口的default方法

Java基础

浏览数:240

2019-1-25

最近更新light-dao遇到的。
需要在一个接口中增加default方法,同时需要对非default方法重写。
大概类似这样:

public interface BaseDao {
    default String getName() {
        return "name";
    }

    @Select("select * from ad")
    List<Object> getAll(); 
}

我们重写的Handler类似这样:

public class DaoInvocationHandler implements InvocationHandler {
    private Map<Method, SqlExecutor> sqlExecutorMap = new ConcurrentHashMap<Method, SqlExecutor>();
    private DataSourceHolder dataSourceHolder;

    public DaoInvocationHandler(DataSourceHolder dataSourceHolder) {
        this.dataSourceHolder = dataSourceHolder;
    }

    //对应于Dao接口里的函数
    //将每一个函数映射为一个SqlExecutor
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
     if (!sqlExecutorMap.containsKey(method)) {
            synchronized (method) {
                if (!sqlExecutorMap.containsKey(method)) {
                    SqlExecutor sqlExecutor = new SqlExecutor.Builder(method, dataSourceHolder.getLightTemplate()).build();

                    sqlExecutorMap.put(method, sqlExecutor);
                }
            }
        }

        return sqlExecutorMap.get(method).execute(args);
    }
}

我们需要对default方法做特殊处理,搜索了一下stackoverflow

http://stackoverflow.com/questions/22614746/how-do-i-invoke-java-8-default-methods-refletively

里面提供了一种解决方式:

Object result = MethodHandles.lookup()
    .in(method.getDeclaringClass())
    .unreflectSpecial(method,method.getDeclaringClass())
    .bindTo(target)
    .invokeWithArguments();

但是在使用时,报错如下:

Caused by: java.lang.IllegalAccessException: no private access for invokespecial

又尝试了几次,发现这种方式的改进版:

Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
            .getDeclaredConstructor(Class.class, int.class);
    constructor.setAccessible(true);

    Class<?> declaringClass = method.getDeclaringClass();
    int allModes = MethodHandles.Lookup.PUBLIC | MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED | MethodHandles.Lookup.PACKAGE;

    return constructor.newInstance(declaringClass, allModes)
            .unreflectSpecial(method, declaringClass)
            .bindTo(proxy)
            .invokeWithArguments(args);

通过设置constructor.setAccessible(true);来解决access权限问题。目前来看,问题基本解决。最终的invocationHandler代码如下:

public class DaoInvocationHandler implements InvocationHandler {
    private Map<Method, SqlExecutor> sqlExecutorMap = new ConcurrentHashMap<Method, SqlExecutor>();
    private DataSourceHolder dataSourceHolder;

    public DaoInvocationHandler(DataSourceHolder dataSourceHolder) {
        this.dataSourceHolder = dataSourceHolder;
    }

    //对应于Dao接口里的函数
    //将每一个函数映射为一个SqlExecutor
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        //default方法不重写
        if (method.isDefault()) {
            Constructor<MethodHandles.Lookup> constructor = MethodHandles.Lookup.class
                    .getDeclaredConstructor(Class.class, int.class);
            constructor.setAccessible(true);

            Class<?> declaringClass = method.getDeclaringClass();
            int allModes = MethodHandles.Lookup.PUBLIC | MethodHandles.Lookup.PRIVATE | MethodHandles.Lookup.PROTECTED | MethodHandles.Lookup.PACKAGE;

            return constructor.newInstance(declaringClass, allModes)
                    .unreflectSpecial(method, declaringClass)
                    .bindTo(proxy)
                    .invokeWithArguments(args);
        } else if (!sqlExecutorMap.containsKey(method)) {
            synchronized (method) {
                if (!sqlExecutorMap.containsKey(method)) {
                    SqlExecutor sqlExecutor = new SqlExecutor.Builder(method, dataSourceHolder.getLightTemplate()).build();

                    sqlExecutorMap.put(method, sqlExecutor);
                }
            }
        }

        return sqlExecutorMap.get(method).execute(args);
    }
}

虽然解决了问题,但还是觉得java 8应该在method里添加针对interface default方法的直接调用方式,这样绕一大圈的解决方式显然不够优雅。
只能期待reflect api下个版本的修改了。

原文地址:https://www.jianshu.com/p/63691220f81f