使用Jackson Mapper將JSON映射到Java POJO類

嘗試使用Jackson Mapper將JSON數據（請參閱下面的data.json）映射到Java POJO類。在反序列化過程中出現以下錯誤。我不確定這意味着什麼。請指教。使用Jackson Mapper將JSON映射到Java POJO類

org.springframework.http.converter.HttpMessageNotReadableException: Could not read JSON: Can not deserialize instance of org.springframework.social.test.api.impl.AccountList out of START_OBJECT token 
at [Source: [email protected]; line: 2, column: 3]; nested exception is org.codehaus.jackson.map.JsonMappingException: Can not deserialize instance of org.springframework.social.test.api.impl.AccountList out of START_OBJECT token 
at [Source: [email protected]; line: 2, column: 3] 
    at org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.readJavaType(MappingJacksonHttpMessageConverter.java:169) 
    at org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.read(MappingJacksonHttpMessageConverter.java:161) 
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:94) 
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:491) 
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:460) 
    at org.springframework.web.client.RestTemplate.getForObject(RestTemplate.java:228) 
    at org.springframework.social.test.api.impl.AccountTemplate.getAccounts(AccountTemplate.java:44) 
    at org.springframework.social.test.api.impl.AccountTemplateTest.getAccounts_current_user(AccountTemplateTest.java:24) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 
    at java.lang.reflect.Method.invoke(Method.java:601) 
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47) 
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) 
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44) 
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17) 
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26) 
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:271) 
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:70) 
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50) 
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238) 
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63) 
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236) 
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53) 
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229) 
    at org.junit.runners.ParentRunner.run(ParentRunner.java:309) 
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50) 
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390) 
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197) 
Caused by: org.codehaus.jackson.map.JsonMappingException: Can not deserialize instance of org.springframework.social.test.api.impl.AccountList out of START_OBJECT token 
at [Source: [email protected]; line: 2, column: 3] 
    at org.codehaus.jackson.map.JsonMappingException.from(JsonMappingException.java:163) 
    at org.codehaus.jackson.map.deser.StdDeserializationContext.mappingException(StdDeserializationContext.java:219) 
    at org.codehaus.jackson.map.deser.StdDeserializationContext.mappingException(StdDeserializationContext.java:212) 
    at org.codehaus.jackson.map.deser.std.CollectionDeserializer.handleNonArray(CollectionDeserializer.java:246) 
    at org.codehaus.jackson.map.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:204) 
    at org.codehaus.jackson.map.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:194) 
    at org.codehaus.jackson.map.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:30) 
    at org.codehaus.jackson.map.ObjectMapper._unwrapAndDeserialize(ObjectMapper.java:2802) 
    at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2730) 
    at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1923) 
    at org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.readJavaType(MappingJacksonHttpMessageConverter.java:166) 
    ... 31 more 


JUnit Test Class 

@Test 
    public void getAccounts() { 
     mockServer.expect(requestTo("api")) 
     .andExpect(method(GET)) 
     .andExpect(header("Authorization", "Bearer someAccessToken")) 
     .andRespond(withSuccess(jsonResource("data.json"), MediaType.APPLICATION_JSON)); 

     List<Account> accounts = tester.operations().getAccounts(); 
     assertEquals(4, accounts.size()); 

     Account account = accounts.get(0); 

     assertEquals(new Long(9880), account.getId()); 
     assertEquals("My Account", account.getAlias()); 
     assertEquals("FULL", account.getPermissions()); 
     assertEquals("USER", account.getType()); 
     assertEquals(new Character('U'),account.getStatus()); 

     Account account1 = accounts.get(1); 
     assertEquals(new Long(9887),account1.getId()); 
     assertEquals("Account",account1.getAlias()); 
     assertEquals("FULL",account1.getPermissions()); 
     assertEquals("USER",account1.getType()); 
     assertEquals(new Character('U'),account1.getStatus()); 

     Account account2 = accounts.get(2); 
     assertEquals(new Long(00008),account2.getId()); 
     assertEquals("Mom's Account",account2.getAlias()); 
     assertEquals("FULL",account2.getPermissions()); 
     assertEquals("USER",account2.getType()); 
     assertEquals(new Character('U'),account2.getStatus()); 

     Account account3 = accounts.get(3); 
     assertEquals(new Long(00009),account3.getId()); 
     assertEquals("Steph's Account",account3.getAlias()); 
     assertEquals("FULL",account3.getPermissions()); 
     assertEquals("USER",account3.getType()); 
     assertEquals(new Character('U'),account3.getStatus()); 

//The method used to configure ObjectMapper 

    @Override 
    protected MappingJacksonHttpMessageConverter getJsonMessageConverter() { 
      MappingJacksonHttpMessageConverter converter = super.getJsonMessageConverter(); 
      objectMapper = new ObjectMapper(); 
      //UnComment if root element is not required 
      objectMapper.enable(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE); 
      objectMapper.registerModule(new TestModule()); 
      converter.setObjectMapper(objectMapper); 
      return converter; 
    }

上述錯誤消失和它的工作原理如果下列3線從data.json

The lines causing problem. 

{ 
    "accounts": { 
    "account": 



Entire data json File 

{ 
    "accounts": { 
    "account": [ 
     { 
     "id": "9880, 
     "alias": "My Account", 
     "permissions": "FULL", 
     "type": "USER", 
     "status": "U" 
     }, 
     { 
     "id": "9887", 
     "alias": "Account", 
     "permissions": "FULL", 
     "type": "USER", 
     "status": "U" 
     }, 
     { 
     "id": "00008", 
     "alias": "Mom's Account", 
     "permissions": "FULL", 
     "type": "USER", 
     "status": "U" 
     }, 
     { 
     "id": "00009", 
     "alias": "Steph's Account", 
     "permissions": "FULL", 
     "type": "USER", 
     "status": "U" 
     } 
    ] 
    } 
}

來源

2013-02-22 user1377822

傑克遜試圖將JSON deserialise入爲accountList型除去。我猜這種類型映射到JSON數組，所以Jackson期待在流中有一個'[''，但是你的JSON以'{'（又名START_OBJECT標記）開頭。

基本上你的JSON與你試圖反序列化的類型不匹配。它以匹配我期望目標類型是這樣的：

class SomeClassName { 
    AccountList accounts; 
    [...] 
}

來源

2013-02-22 21:40:48

爲accountList類是子類的ArrayList例如的AccountList擴展了ArrayList 。也許這是給定的json數據結構的錯誤方法？ – user1377822 2013-02-22 22:29:21

然後它會被序列化爲一個JSON數組，但是你的JSON是一個在外層而不是數組的對象。 JSON需要被序列化爲一個擁有賬戶字段的類。我已經更新了我的答案。 – 2013-02-23 10:49:26

class AccountList {private List account;/** Accessor ** /是解決方案。 – user1377822 2013-02-23 11:04:32

的解決方案是：

@JsonRootName(value="accounts") 
public class AccountList implements Serializable{ 

    private static final long serialVersionUID = 1L; 
private ArrayList<Account> account; 

public AccountList(ArrayList<Account> account){ 
    this.account = account; 
} 
public ArrayList<Account> getAccount() { 
    return account; 
} 

public void setAccount(ArrayList<Account> account) { 
    this.account = account; 
}

來源

2013-02-23 11:06:34 user1377822

使用Jackson Mapper將JSON映射到Java POJO類

回答

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