微信小程序序的代码中提示，使用code换取openid，但官方文档中没有给出进一步的说明。

换取openid的要点：由于安全的原因，必须由自己小程序的服务器端完成。知道了这个要点，实现起来就简单了，服务器端写一个RestController，接收code参数，使用httpclient向微信的服务端换openid就行了。

代码使用了SpringBoot，不会也不难理解。主要代码如下：

package com.wallimn.iteye.sp.asset.common.controller;

import java.util.Map;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.wallimn.iteye.sp.asset.common.util.AesUtil;
import com.wallimn.iteye.sp.asset.common.util.HttpUtil;

@RestController
@RequestMapping("/api/wx")
public class WeixinController {

	private static Logger log = LoggerFactory.getLogger(WeixinController.class);

	@Value("${wx.appId}")
	private String appId;

	@Value("${wx.appSecret}")
	private String appSecret;

	@Value("${wx.grantType}")
	private String grantType;
	// wx.grantType=authorization_code

	@Value("${wx.requestUrl}")
	private String requestUrl;
	// wx.requestUrl=https://api.weixin.qq.com/sns/jscode2session

	@RequestMapping("/session")
	public Map<String, Object> getSession(@RequestParam(required = true) String code) {
		return this.getSessionByCode(code);
	}

	@SuppressWarnings("unchecked")
	private Map<String, Object> getSessionByCode(String code) {
		String url = this.requestUrl + "?appid=" + appId + "&secret=" + appSecret + "&js_code=" + code + "&grant_type="
				+ grantType;
		// 发送请求
		String data = HttpUtil.get(url);
		log.debug("请求地址：{}", url);
		log.debug("请求结果：{}", data);
		ObjectMapper mapper = new ObjectMapper();
		Map<String, Object> json = null;
		try {
			json = mapper.readValue(data, Map.class);
		} catch (Exception e) {
			e.printStackTrace();
		}
		// 形如{"session_key":"6w7Br3JsRQzBiGZwvlZAiA==","openid":"oQO565cXXXXXEvc4Q_YChUE8PqB60Y"}的字符串
		return json;
	}
}

用到了一个httpclient封闭的工具类，代码如下：

package com.wallimn.iteye.sp.asset.common.util;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URI;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.message.BasicNameValuePair;

public class HttpUtil {

	private static final String Charset = "utf-8";


	/**
	 * 发送请求，如果失败，会返回null
	 * @param url
	 * @param map
	 * @return
	 */
	public static String post(String url, Map<String, String> map) {
		// 处理请求地址
		try {
			HttpClient client = HttpClientBuilder.create().build();
			URI uri = new URI(url);
			HttpPost post = new HttpPost(uri);

			// 添加参数
			List<NameValuePair> params = new ArrayList<NameValuePair>();
			for (String str : map.keySet()) {
				params.add(new BasicNameValuePair(str, map.get(str)));
			}
			post.setEntity(new UrlEncodedFormEntity(params, Charset));
			// 执行请求
			HttpResponse response = client.execute(post);

			if (response.getStatusLine().getStatusCode() == 200) {
				// 处理请求结果
				StringBuffer buffer = new StringBuffer();
				InputStream in = null;
				try {
					in = response.getEntity().getContent();
					BufferedReader reader = new BufferedReader(new InputStreamReader(in,Charset));
					String line = null;
					while ((line = reader.readLine()) != null) {
						buffer.append(line);
					}

				} catch (Exception e) {
					e.printStackTrace();
				} finally {
					// 关闭流
					if (in != null)
						try {
							in.close();
						} catch (Exception e) {
							e.printStackTrace();
						}
				}

				return buffer.toString();
			} else {
				return null;
			}
		} catch (Exception e1) {
			e1.printStackTrace();
		}
		return null;

	}

	/**
	 * 发送请求，如果失败会返回null
	 * @param url
	 * @param str
	 * @return
	 */
	public static String post(String url, String str) {
		// 处理请求地址
		try {
			HttpClient client = HttpClientBuilder.create().build();
			URI uri = new URI(url);
			HttpPost post = new HttpPost(uri);
			post.setEntity(new StringEntity(str, Charset));
			// 执行请求
			HttpResponse response = client.execute(post);

			if (response.getStatusLine().getStatusCode() == 200) {
				// 处理请求结果
				StringBuffer buffer = new StringBuffer();
				InputStream in = null;
				try {
					in = response.getEntity().getContent();
					BufferedReader reader = new BufferedReader(new InputStreamReader(in,"utf-8"));
					String line = null;
					while ((line = reader.readLine()) != null) {
						buffer.append(line);
					}

				} finally {
					// 关闭流
					if (in != null)
						in.close();
				}

				return buffer.toString();
			} else {
				return null;
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
		return null;

	}

	/**
	 * 发送GET方式的请求，并返回结果字符串。
	 * <br>
	 * 时间：2017年2月27日，作者：http://wallimn.iteye.com
	 * @param url
	 * @return 如果失败，返回为null
	 */
	public static String get(String url) {
		try {
			HttpClient client = HttpClientBuilder.create().build();
			URI uri = new URI(url);
			HttpGet get = new HttpGet(uri);
			HttpResponse response = client.execute(get);
			if (response.getStatusLine().getStatusCode() == 200) {
				StringBuffer buffer = new StringBuffer();
				InputStream in = null;
				try {
					in = response.getEntity().getContent();
					BufferedReader reader = new BufferedReader(new InputStreamReader(in,Charset));
					String line = null;
					while ((line = reader.readLine()) != null) {
						buffer.append(line);
					}

				} finally {
					if (in != null)
						in.close();
				}

				return buffer.toString();
			} else {
				return null;
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
		return null;

	}
}