今天使用Retrofit使用Post請求獲取Token時遇到了下面的錯誤：

java.lang.IllegalArgumentException: @Field parameters can only be used with form encoding. (parameter #1)
for method MessageAPI.getToken
at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:752)
at retrofit2.ServiceMethod$Builder.methodError(ServiceMethod.java:743)
at retrofit2.ServiceMethod$Builder.parameterError(ServiceMethod.java:761)
at retrofit2.ServiceMethod$Builder.parseParameterAnnotation(ServiceMethod.java:533)
at retrofit2.ServiceMethod$Builder.parseParameter(ServiceMethod.java:336)
at retrofit2.ServiceMethod$Builder.build(ServiceMethod.java:204)
at retrofit2.Retrofit.loadServiceMethod(Retrofit.java:170)
at retrofit2.Retrofit$1.invoke(Retrofit.java:147)
at java.lang.reflect.Proxy.invoke(Proxy.java:393)
at $Proxy6.getToken(Unknown Source)

原因

@Field parameters can only be used with form encoding. (parameter #1)

從這句話可以看出是因為對方服務(wù)器，只支持x-www-form-urlencoded不支持form-data所致，reftrofit2默認的post請求用的就是form-data

解決方法

 @FormUrlEncoded  //加上這個就可以了
    @POST("https://xxx.com/getToken")
    Observable getToken(@Field("userid") String userId);