背景

最近使用Spring的RestTemplate工具类请求接口，Post请求取不到参数，而Get请求是正常的。后面深入发现Map定义成以下两种类型是取不到参数的：

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Map<String, String> paramMap = new HashMap<g>();

Map<String, String> paramMap = new LinkedHashMap<>();

而当我把Map改成LinkedMultiValueMap后，后台能正常取到参数：

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MultiValueMap<String, String> paramMap = new LinkedMultiValueMap<>();

MultiValueMap

首先看LinkedMultiValueMap，实现MultiValueMap接口：

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public interface MultiValueMap<K, V> extends Map<K, List<V>> {
    @Nullable
    V getFirst(K var1);

    void add(K var1, @Nullable V var2);

    void addAll(K var1, List<? extends V> var2);

    void addAll(MultiValueMap<K, V> var1);

    void set(K var1, @Nullable V var2);

    void setAll(Map<K, V> var1);

    Map<K, V> toSingleValueMap();
}

可以看出，LinkedMultiValueMap实际就是Key-LinkedList的Map。

RestTemplate POST处理请求方式

查看RestTemplate源码，首先将请求封装成HttpEntityRequestCallback类对象，然后再处理请求。

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public <T> ResponseEntity<T> postForEntity(String url, @Nullable Object request, Class<T> responseType, Object... uriVariables) throws RestClientException {
    RequestCallback requestCallback = this.httpEntityCallback(request, responseType);
    ResponseExtractor<ResponseEntity<T>> responseExtractor = this.responseEntityExtractor(responseType);
    return (ResponseEntity)nonNull(this.execute(url, HttpMethod.POST, requestCallback, responseExtractor, uriVariables));
}

实际是把请求数据放在了一个HttpEntity中。如果requestBody是HttpEntity类型，就直接转；否则，放在HttpEntity的body中，如下：

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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;
    }

}

再看一下HttpEntity源码：

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public class HttpEntity<T> {
    public static final HttpEntity<?> EMPTY = new HttpEntity();
    private final HttpHeaders headers;
    @Nullable
    private final T body;

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

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

再看一下HttpHeaders的源码：

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public class HttpHeaders implements MultiValueMap<String, String>, Serializable {

简略了下代码，从代码已经可以看出，post请求是放在一个HttpEntity类型（header为MultiValueMap类型，body为泛型）的body中。

解决方案

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MultiValueMap<String, String> paramMap = new LinkedMultiValueMap<>();
HttpHeaders headers = new HttpHeaders();
HttpEntity<MultiValueMap<String, String>> httpEntity = new HttpEntity<MultiValueMap<String, String>>(paramMap, headers);
ResponseEntity<String> response = template.postForEntity(url, httpEntity, String.class);

改成MultiValueMap封装参数即可解决问题。

最后更新： 2019年07月27日 17:02

原始链接： http://blog.minhow.com/articles/java/restTemplate-post/

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