RestTemplate post請求使用map傳參 Controller 接收不到值的解決方案 postForObject方法源碼解析.md

結論

post方法中若是使用map傳參,須要使用MultiValueMap來傳遞

RestTemplate 的 postForObject 方法有四個參數

• String url => 顧名思義 這個參數是請求的url路徑

• Object request => 請求的body 這個參數須要再controller類用 @RequestBody 註解接收

• Class responseType => 接收響應體的類型

• 第四個參數 postForObject 方法多種重構

  Map<String,?> uriVariables => uri 變量 顧名思義 這是放置變量的地方

  Object... uriVariables => 可變長 Object 類型 參數

@Nullable
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Object... uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters(), this.logger);
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, (Object[])uriVariables);
}

@Nullable
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters(), this.logger);
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, (Map)uriVariables);
}

@Nullable
public <T> T postForObject(URI url, @Nullable Object request, Class<T> responseType) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters());
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor);
}

首先咱們使用最簡單的一種 可變長Object 參數 進行傳值

@Service
public class HelloService {
 
    @Autowired
    RestTemplate restTemplate;
 
    public String helloService(String name,Integer age){
        return restTemplate.postForObject("http://SERVICE-HELLO/hello?name={name}&age={age}", null, String.class, name,age);
    }
}

須要再url上拼接參數並使用{參數名}佔位符站位

而後將參數放到 第四個參數 可變長 Object 參數上 便可

Controller類代碼

@RestController
public class DemoController {
    @Value("${server.port}")
    String port;
 
    @PostMapping("hello")
    public String home(String name,Integer age){
        return "hello " + name + " you age is " + age + " ,i am from port:" + port;
    }
}

測試成功

接下來咱們使用 Map傳值 

map傳值也很簡單

public String helloService(String name,Integer age){
    Map<String,Object> map = new HashMap<>();
    map.put("name",name);
    map.put("age",age);
    return restTemplate.postForObject("http://SERVICE-HELLO/hello?name={name}&age={age}", null, String.class, map);
}

只須要將參數放入到map中便可

那有些人要問了 , 爲何不能用 第二個 request 參數傳值 , 實際上是能夠的

我試過用HashMap 和 LinkedHashMap 都是接收不到的

因此咱們來看一下源碼是怎麼寫的

首先進入到 postForObject 方法中 發現request 參數 傳入了一個 httpEntityCallBack 方法中 , 那麼接着追蹤

@Nullable
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    HttpMessageConverterExtractor<T> responseExtractor = new HttpMessageConverterExtractor(responseType, this.getMessageConverters(), this.logger);
    return this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, (Map)uriVariables);
}

進入httpEntityCallBack方法中
httpEntityCallBack方法又調用了 RestTemplate的HttpEntityRequestCallback方法

public <T> RequestCallback httpEntityCallback(@Nullable Object requestBody, Type responseType) {
    return new RestTemplate.HttpEntityRequestCallback(requestBody, responseType);
}

進入HttpEntityRequestCallback

這裏會出現一個分支 instanceof 類型斷定 requestBody 參數是不是 HttpEntity類型

public HttpEntityRequestCallback(@Nullable Object requestBody, @Nullable Type responseType) {
    super(responseType);
    if (requestBody instanceof HttpEntity) {
        this.requestEntity = (HttpEntity)requestBody;
    } else if (requestBody != null) {
        this.requestEntity = new HttpEntity(requestBody);
    } else {
        this.requestEntity = HttpEntity.EMPTY;
    }

}

若是不是則 建立一個HttpEntity類將 requestBody 參數傳入

那麼咱們來看一下 HttpEntity 是怎麼個構造

public HttpEntity(T body) {
    this(body, (MultiValueMap)null);
}

public HttpEntity(MultiValueMap<String, String> headers) {
    this((Object)null, headers);
}

這裏能夠看到 HttpEntity 有兩個構造方法 一個是 傳入 泛型的body 另外一個是傳入 MultiValueMap<String,String> headers

那麼 這個MultiValueMap 是個什麼東東

百度一下 發現

MultiValueMap 能夠讓一個key對應多個value，感受是value產生了鏈表結構，能夠很好的解決一些很差處理的字符串問題

那麼咱們來用這個奇怪的map實驗一下

首先進入 MultiValueMap 接口 找到他的實現類

實現類到idea中查找

MultiValueMap 的實現類應該是 LinkedMultiValueMap

那麼咱們走起

public String helloService(String name,Integer age){
    MultiValueMap<String, Object> paramMap = new LinkedMultiValueMap<>();
    paramMap.add("name",name);
    paramMap.add("age", age);
    return restTemplate.postForObject("http://SERVICE-HELLO/hello",paramMap,String.class);
}

controller代碼

public class DemoController {
 
    @Value("${server.port}")
    String port;
 
    @PostMapping("hello")
    public String home(String name,Integer age){
        return "MultiValueMap : hello " + name + " you age is " + age + " ,i am from port:" + port;
    }
}

測試成功

參考

原文：https://blog.csdn.net/weixin_40461281/article/details/83472648