jdk動態代理爲什麼必須是基於接口實現的？

因爲jdk動態代理生成的class文件已經繼承了Proxy，而java是單繼承的，所以是基於jdk動態代理是基於接口的。
請看代碼使用jdk動態代理生成class文件

先定義一個文件

public interface Dao {
    public void query1() ;
    public void query2() ;
}

public static void main(String[] args) {
        Class<?>[] interfaces = new Class[]{Dao.class} ;
        byte[] bytes = ProxyGenerator.generateProxyClass("Sss", interfaces);
        File file = new File("E:\\Sss.class") ;
        FileOutputStream fo = null;
        try {
            fo = new FileOutputStream(file);
            fo.write(bytes);
            fo.flush();
            fo.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    

生成的Class文件

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class Sss extends Proxy implements Dao {
    private static Method m1;
    private static Method m2;
    private static Method m4;
    private static Method m0;
    private static Method m3;

    public Sss(InvocationHandler var1) throws  {
        super(var1);
    }

    public final boolean equals(Object var1) throws  {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final String toString() throws  {
        try {
            return (String)super.h.invoke(this, m2, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void query2() throws  {
        try {
            super.h.invoke(this, m4, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void query1() throws  {
        try {
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m4 = Class.forName("com.luban.dao.Dao").getMethod("query2");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            m3 = Class.forName("com.luban.dao.Dao").getMethod("query1");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以明顯的看到class文件已經繼承了Proxy，所以不能繼承目標對象，只能實現目標對象，所以jdk動態代理是基於接口而不是繼承實現的。