奇怪的jackson ObjectMapper行为

Question

我的Java应用程序接收Spotify API身份验证令牌并用Jackson将其映射到POJO时遇到了麻烦。

每次我的应用程序从Spotify API请求数据时，我都会从以下链接获取一个新的访问令牌：https://accounts.Spotify.com/API/token?grant_type=client_credentials

答案是JSON，如下所示：

{
    "access_token":"BQAJmzZOdh2egvWEOEwy4wv-VKdhTUc4eZYJrIfAibjWLR4MPfrbV6KBNIiomPwJKsQN-3vmrGmG7lOXFaI",
    "token_type":"Bearer",
    "expires_in":3600,
    "scope":""
}

Exception in thread "main" com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.slowedandthrowed.darkjazzbot.mapping.spotify.TokenRequest` (although at least 
one Creator exists): no String-argument constructor/factory method to deserialize from String value ('BQCcDMOKUiVPscDrrBH77b2QbN9FuqjAJHuM3_1QD39MO9L20XzXneZUlJeIyukBVhPpaCWnKWRjUdggaCM') at [Source: (String)"{"access_token":"BQCcDMOKUiVPscDrrBH77b2QbN9FuqjAJHuM3_1QD39MO9L20XzXneZUlJeIyukBVhPpaCWnKWRjUdggaCM","token_type":"Bearer","expires_in":3600,"scope":""}"; line: 1, column: 17]

每次需要将访问令牌JSON映射到POJO时，我都会创建一个新的ObjectMapper来解决这个问题，但如果它是一个生产应用程序，就会损害性能，所以我需要找出一直使用一个ObjectMapper实例的问题是什么。

我还尝试将这个JSON映射到map 而不是映射到 tokenrequest.class,结果是一样的,所以我不认为这是映射失败的原因。

private String requestAccessToken() throws IOException {
        TokenRequest tokenRequest = null;
        
        URL accessTokenUrl = new URL(SPOTIFY_TOKEN_LINK);
        
        HttpURLConnection tokenConnection = (HttpURLConnection) accessTokenUrl.openConnection();
        tokenConnection.setRequestProperty("Authorization", authString);
        tokenConnection.setDoOutput(true);
        tokenConnection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        
        tokenConnection.setRequestMethod("POST");
        
        OutputStreamWriter wr = new OutputStreamWriter(tokenConnection.getOutputStream());
        wr.write(TOKEN_REQUEST_PARAMETERS);
        wr.flush();
        
        System.out.println("Wow! Posted!");
        
        InputStream inputStream = tokenConnection.getInputStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"));

        
        StringBuilder inputBuilder = new StringBuilder();
        String input = null;
        
        while ((input = reader.readLine()) != null) inputBuilder.append(input);
        
        
        System.out.println("================================= TOKEN INPUT ======================================");
        System.out.println(inputBuilder.toString());
        System.out.println("================================= TOKEN INPUT ======================================");

        
        tokenRequest = spotifyObjectMapper.readValue(inputBuilder.toString(), TokenRequest.class);
        

        inputStream.close();
        reader.close();
        tokenConnection.disconnect();
        
        return tokenRequest.getAccessToken();
    }

tokenRequest.java：

package com.slowedandthrowed.darkjazzbot.mapping.spotify;

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyOrder;
import com.fasterxml.jackson.annotation.JsonRootName;

@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonPropertyOrder({
        "access_token",
        "token_type",
        "expires_in",
        "scope"
})
@JsonRootName("access_token")
public class TokenRequest {

    @JsonProperty("access_token")
    private String accessToken;
    @JsonProperty("token_type")
    private String tokenType;
    @JsonProperty("expires_in")
    private Long expiresIn;
    @JsonProperty("scope")
    private String scope;

    @JsonProperty("access_token")
    public String getAccessToken() {
        return accessToken;
    }

    @JsonProperty("access_token")
    public void setAccessToken(String accessToken) {
        this.accessToken = accessToken;
    }

    @JsonProperty("token_type")
    public String getTokenType() {
        return tokenType;
    }

    @JsonProperty("token_type")
    public void setTokenType(String tokenType) {
        this.tokenType = tokenType;
    }

    @JsonProperty("expires_in")
    public Long getExpiresIn() {
        return expiresIn;
    }

    @JsonProperty("expires_in")
    public void setExpiresIn(Long expiresIn) {
        this.expiresIn = expiresIn;
    }

    @JsonProperty("scope")
    public String getScope() {
        return scope;
    }

    @JsonProperty("scope")
    public void setScope(String scope) {
        this.scope = scope;
    }
}

Answer 1

我修复了我的程序，因为我给自己制造了问题。我通过不同的API接收不同的数据，并通过Jackson ObjectMapper的单个实例将其映射到POJOs。某些JSON对象本身包含数据：

{
"property1":value1,
"property2":value2
}

而另一些则只有一个嵌套对象,其中包含数据：

{
"object":{"property1":value1,
"property2":value2
}}

所以我决定将UNWRAP_ROOT_VALUE转换为TRUE，然后我的对象映射器改变了它的状态，因此它的行为在我第一次和第二次尝试接收访问令牌时变得不同。