外部只能访问A的接口，不能通过A访问B

使用HttpPost进行请求，并获取InputStream流读取响应，再通过ResponseBodyEmitter进行流式响应。

import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.servlet.mvc.method.annotation.ResponseBodyEmitter;

import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import java.util.concurrent.Executors;

@RestController
@RequestMapping("sys/proxy")
public class KnowledgeBaseController {

    @RequestMapping("/stream")
    public ResponseBodyEmitter knowledgeBaseChat(@RequestBody paramJsonStringify) {

        // 这里的URL就是请求地址
        HttpPost httpPost = new HttpPost(URL);
        httpPost.addHeader("Content-Type", "application/json");
        httpPost.setEntity(new StringEntity(paramJsonStringify, StandardCharsets.UTF_8));

        byte[] bytes = new byte[4096];
        ResponseBodyEmitter emitter = new ResponseBodyEmitter();
        Executors.newSingleThreadExecutor().execute(() -> {
            try {
                CloseableHttpClient httpClient = HttpClientBuilder.create().build();
                CloseableHttpResponse execute = httpClient.execute(httpPost);
                InputStream inputStream = execute.getEntity().getContent();
                int len = 0;
                while ((len = inputStream.read(bytes)) != -1) {
                    // 只保留bytes中有效数据
                    byte[] needSend = new byte[len];
                    System.arraycopy(bytes, 0, needSend, 0, len);
                    emitter.send(needSend);
                }

            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                emitter.complete();
            }
        });
        return emitter;
    }

}