写个完整全面的，是服务端，主动给客户端发

在Java中，如果你想要服务端主动向客户端发送数据，通常是在一个长连接或定时任务场景下。比如WebSocket服务器就是常见的主动推送的例子。下面是一个简单的基于`Netty`框架实现的WebSocket服务端，它可以主动向客户端发送消息：

首先，你需要在项目中引入Netty库，可以通过Maven或Gradle添加依赖。

<!-- Maven -->
<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-all</artifactId>
    <version>4.x.y</version> <!-- 使用最新的稳定版本 -->
</dependency>

// Gradle
implementation 'io.netty:netty-all:x.y.z'

然后，创建一个WebSocket服务器：

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.http.websocketx.TextWebSocketFrame;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;

public class WebSocketServer {
    private static final int PORT = 8080;

    public static void main(String[] args) {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .handler(new LoggingHandler(LogLevel.INFO))
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline()
                                    .addLast(new TextWebSocketFrameDecoder())
                                    .addLast(new TextWebSocketFrameEncoder())
                                    .addLast(new WebSocketServerHandler());
                        }
                    });

            ChannelFuture f = b.bind(PORT).sync();

            // 主动向所有在线连接发送消息
            for (Object channel : b.childGroup().allChannels()) {
                WebSocketServerHandler handler = (WebSocketServerHandler) ((Channel) channel).pipeline().get(WebSocketServerHandler.class);
                handler.sendMessageToAll("Server message to all clients");
            }

            // 等待关闭
            f.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            workerGroup.shutdownGracefully();
            bossGroup.shutdownGracefully();
        }
    }
}

class WebSocketServerHandler extends ChannelInboundHandlerAdapter {
    //...
    public void sendMessageToAll(String message) {
        for (WebSocketSession session : sessions) {
            session.textMessage(message);
        }
    }
    //...
}

在这个例子中，`WebSocketServerHandler`负责接收和处理WebSocket连接，`sendMessageToAll`方法会遍历所有在线的`WebSocketSession`，并向每个客户端发送一条消息。