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如何從 JSON 正確讀取此 Generics？

[英]How do I read this Generics correctly from JSON?

 原文  2020-08-18 04:48:21  8   1    java/ json/ generics/ libgdx

問題描述

我對我的第一個 generics 容器有相當的信心，但堅持如何在客戶端進行轉換。 這是我參與學習<T>之前的工作：

CommonNounContainer typeContainer = new Json().fromJson(CommonNounContainer.class, result);

我正在考慮必須為每個 class 創建一個不同的容器，這似乎不是一個好的設計。 以下是我在新的 generics 容器中讀取的更新但無效的嘗試：

JSONContainer<CommonNoun> typeContainer = new Json().fromJson(JSONContainer.class, result);

我的 IDE 不關心這個措辭，注意：

類型安全：JSONContainer 類型的表達式需要未經檢查的轉換才能符合 JSONContainer

執行時，我的錯誤日志顯示：

result = {"myObject":{"cid":{"oid":129},"name":"technology","form":1},"children":[]}
com.badlogic.gdx.utils.SerializationException: Field not found: cid (java.lang.Object)
Serialization trace:
{}.myObject.cid
myObject (semanticWeb.rep.concept.JSONContainer)
    at com.badlogic.gdx.utils.Json.readFields(Json.java:854)
    at com.badlogic.gdx.utils.Json.readValue(Json.java:1011)
    at com.badlogic.gdx.utils.Json.readFields(Json.java:863)
    at com.badlogic.gdx.utils.Json.readValue(Json.java:1011)
    at com.badlogic.gdx.utils.Json.fromJson(Json.java:789)
    at com.b2tclient.net.Communicator$2.handleHttpResponse(Communicator.java:95)
    at com.badlogic.gdx.net.NetJavaImpl$2.run(NetJavaImpl.java:224)
    at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:830)

我確定有某種方法我應該在等號右側包含對 CommonNoun 類型的引用，但我無法弄清楚。 我該怎么做？ 有很多關於generics，鑄造，JSON，剝離class信息的帖子。 我試圖遵循的其中一個不是關於上面的轉換，即在構建過程中將 T class 作為容器內的私有變量添加：

如何獲得泛型 T 的 class 實例？

但我在嘗試正確引用 class 時遇到了類似的語法問題，只是在過程中的不同位置。 我也有疑問，在告訴 JSON 如何對文件中的信息進行分類之前，我是否可以從 JSON 文件中讀取這個 class 變量。

fromJson(Class<T>, String)方法的 Javadoc ：

Type Parameters:
   <T> 
Parameters:
   type May be null if the type is unknown.
   json 
Returns:
   May be null.

我可能已經有 deduper 提交的可行答案，但是，根據要求，這里是 CommonNounContainer 和 JSONContainer 類：

import java.util.ArrayList;

public class CommonNounContainer {

   private CommonNoun myCommonNoun;
    private ArrayList<CommonNounContainer> children;
    public CommonNounContainer(CommonNoun concept) {
        myCommonNoun = concept;
        children = new ArrayList<CommonNounContainer>();
    }

    //Creates an empty shell.  This would be for categories you want to group by, but not display/select in the select box.
    public CommonNounContainer() {
        children = new ArrayList<CommonNounContainer>();        
    }
    
    public void addChildren(ArrayList<CommonNounContainer> newChildren) {
        children.addAll(newChildren);
    }

    public void addChild(CommonNoun concept) {
        children.add(new CommonNounContainer(concept));
    }
        
    public ArrayList<CommonNounContainer> getChildren() {
        return children;
    }
    
    public CommonNoun getValue() {
        return myCommonNoun;
    }
    
    public boolean hasChildren() {
        if (children.size() > 0) return true;
        else return false;
    }
    
    public String toString() {
        return myCommonNoun.toString();
    }
} 


public class JSONContainer<T> {

    private T myObject;
    private ArrayList<JSONContainer<T>> children;
//    public Class<T> typeParameterClass;
    
    public JSONContainer() {

    }
    
    public JSONContainer(T anObject) {
        myObject = anObject;
        children = new ArrayList<JSONContainer<T>>();
    }

/*  public JSONContainer(T anObject, Class<T> typeParameterClass) {
        myObject = anObject;
        children = new ArrayList<JSONContainer<T>>();
        this.typeParameterClass = typeParameterClass;
    }
*/
    
    public void addChildren(ArrayList<JSONContainer<T>> newChildren) {
        children.addAll(newChildren);
    }

    public void addChild(T concept) {
        children.add(new JSONContainer<T>(concept));
    }
        
    public ArrayList<JSONContainer<T>> getChildren() {
        return children;
    }
    
    public T getValue() {
        return myObject;
    }
    
    public boolean hasChildren() {
        if (children.size() > 0) return true;
        else return false;
    }
    
    public String toString() {
        return myObject.toString();
    }
}

要求的額外課程：

public class CommonNoun extends Concept {
        
    /**
     * 
     */
    private static final long serialVersionUID = 6444629581712454049L;

    public CommonNoun() {
        super();
    }
    
    public CommonNoun(String name, ConceptID cidIn) {
        super(name, cidIn);
        this.form = ConceptDefs.COMMON_NOUN;
    }
    
}

public class Concept implements Serializable {
    
    /**
     * 
     */
    private static final long serialVersionUID = 2561549161503772431L;
    private ConceptID cid = null;
    private final String name;
    Integer form = 0;
    
//    ArrayList<ProperRelationship> myRelationships = null;

    
/*  @Deprecated
    public Concept(String name) {
        this.name = name;
    }*/
    
    public Concept() {
        name = "";
    }

    public Concept(String name, ConceptID cidIn) {
       // this(name);
        this.name = name;
        cid = cidIn;
    }

    /*
     * This should be over-ridden by any subclasses
     */
    public Integer getForm() {
        return form;
    } 
            
    public ConceptID getID() {
        return cid;
    }
    
    public void setID(ConceptID cidIn) {
        cid = cidIn;
    }
    
    //this doesn't make any sense.  Throw exception?
    public String getName() {
        return name;
    }

    public boolean isCommon() {
        return true;
    }
    
    /**
     *
     * @return
     */
    @Override
    public String toString() {
        return getName() + "(" + cid.toString() + ")";
    }
    
    public boolean equals(Concept other) {
        return ((getID().equals(other.getID())));
    }
    
}

public class ConceptID implements Serializable {

    long oid;
    
    public ConceptID() {
        oid = -1;
    }
    public ConceptID(long oid) {
        this.oid = oid;
    }
    
    public long getValue() {
        return oid;
    }
    
    /**
     *
     * @return
     */
    @Override
    public String toString() {
        return Long.toString(oid);
    }
    
    public Long toLong() {
        return Long.valueOf(oid);
    }

    
    public boolean equals(ConceptID other) {
        return (oid == other.getValue());
    }
    
    /**
     * Factory model for generating ConceptIDs
     * 
     * This one is here as a convenience as many IDs come in as a String from web POSTs
     * @param idAsString
     * @return
     */
    static public ConceptID parseIntoID(String idAsString) {
        ConceptID returnID = null;
        try {
            returnID = new ConceptID( Long.parseLong(idAsString) );
        } catch (Exception e) {
            System.err.println("Expected the string, " + idAsString + ", to be Long parsable.");
            e.printStackTrace();
        }
        return returnID;

    }

1 個解決方案

解決方案1
 1 已采納  2020-08-18 10:54:04

TL;博士：

建議修復…

  1. System.out.println( new Json( ).toJson( new JSONContainer<>(... ) )查看JSONContainer的JSON的正確字符串格式。
  2. 確保Json.fromJson(Class<T>, String)的result輸入參數與1中打印的格式相同。
    • 例如{myObject:{class:CommonNoun,cid:{oid:139},name:Jada Pinkett Smith,form:69},children:[{myObject:{class:CommonNoun,cid:{oid:666},name:Jaden Pinkett Smith,form:-666},children:[]},{myObject:{class:CommonNoun,cid:{oid:69},name:Willow Pinkett Smith,form:69},children:[]}]}

長答案……

„我的 IDE 不關心這個措辭，注意： “

 Type safety: The expression of type JSONContainer needs unchecked conversion to conform to JSONContainer

這是編譯器警告你堆污染。 IDE 只是翻譯了這個編譯器警告（這是你在命令行上看到的）......

...Communicator.java uses unchecked or unsafe operations.
...Recompile with -Xlint:unchecked for details.

…進入更用戶友好的消息 IDE 向您展示。

這只是一個警告； 不是錯誤。 要使警告 go 消失，請將此： JSONContainer<CommonNoun> typeContainer =...更改為： JSONContainer typeContainer =...

„執行時，我的錯誤日志顯示： “

 result = {"myObject":{"cid":{"oid":129},"name":"technology","form":1},"children":[]} com.badlogic.gdx.utils.SerializationException: Field not found: cid (java.lang.Object)...

該錯誤的最可能原因是 -就像錯誤消息所說- 您的JSONContainer class 或您的CommonNoun class 沒有您嘗試解壓的JSON字符串中存在的cid字段。

我能夠用這個重現那個錯誤……

...
private static final String JADEN_AS_JSON = "{jden:{class:CommonNoun,person:Jaden,place:Hollywood,thing:HashBeen}}";

private static final String JADEN_FAILS_AS_ACTOR = "{jden:{class:CommonNoun,person:Jaden,place:Hollywood,thing:HasBeen, cid:{oid:129} }}";

static public void main( String ... args ){
    
    out.printf( "%1$22s%n", "foo");
    
    JSONContainer< CommonNoun > wtf = new JSONContainer< > ( );
    
    CommonNoun wtBrattyF = new CommonNoun( "Jaden Pinkett Smith", "Hollywood", "HasBeen" );
    
    wtf.setJden( wtBrattyF );
    
    out.printf( "%1$42s%n", wtf );
    
    Json jden = new Json();
    
    out.printf("%1$59s%n", jden.toJson( wtf ) );
            
    JSONContainer wtReifiableF = jden.fromJson(JSONContainer.class, JADEN_AS_JSON); /* This is fine */        
    
    out.printf("%1$59s%n", jden.toJson( wtReifiableF ) );
            
    JSONContainer/*< CommonNoun >*/ wtUnReifiableF = jden.fromJson( JSONContainer.class, JADEN_AS_JSON );
    
    wtUnReifiableF = jden.fromJson( JSONContainer.class, JADEN_FAILS_AS_ACTOR ); /* This causes the error you reported */
}
...

早期它成功了； 但后來它失敗了……

JSONContainer [ jden: CommonNoun [ person: Jaden Pinkett Smith, place: Hollywood, thing: HasBeen ] ]
{jden:{class:CommonNoun,person:Jaden Pinkett Smith,place:Hollywood,thing:HasBeen}}
{jden:{class:CommonNoun,person:Jaden,place:Hollywood,thing:HashBeen}}

Exception in thread "main" com.badlogic.gdx.utils.SerializationException: Field not found: cid (CommonNoun)
Serialization trace:
{}.jden.cid
jden (JSONContainer)
    at com.badlogic.gdx.utils.Json.readFields(Json.java:893)
    at com.badlogic.gdx.utils.Json.readValue(Json.java:1074)
    at com.badlogic.gdx.utils.Json.readFields(Json.java:902)
    at com.badlogic.gdx.utils.Json.readValue(Json.java:1074)
    at com.badlogic.gdx.utils.Json.fromJson(Json.java:829)
    at DeduperAnswer.main(DeduperAnswer.java:33)

我現在已經通過實驗證實了Cid類的存在……

public class Cid { 
    
    int oid;        

    /* ... getter and setter elided ... */
}

……鑒於存在一個有Cid的CommonNoun class……

public class CommonNoun { 
    
    Cid cid;
    
    String name;
    
    int form;

    /* ... getters and setters elided ... */
}

…然后嘗試從具有以下值的result中反序列JSONContainer ，將產生與您最初報告的完全相同的錯誤…

result = {"myObject":{"cid":{"oid":129},"name":"technology","form":1},"children":[]}

如果您的實際CommonNoun class 像我上面的替身一樣實現（帶有Cid字段），那么您需要重試json.fromJson(Class<?>, String)調用， result字符串格式如下......

{myObject:{class:CommonNoun,cid:{oid:139},name:Jada Pinkett Smith,form:69},children:[{myObject:{class:CommonNoun,cid:{oid:666},name:Jaden Pinkett Smith,form:-666},children:[]},{myObject:{class:CommonNoun,cid:{oid:69},name:Willow Pinkett Smith,form:69},children:[]}]}

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