求教json的有关问题,google库gson的bug有关问题

求教json的问题,google库gson的bug问题！

Java code

package toonyoo.ums.test;import com.google.gson.annotations.Expose;import com.google.gson.annotations.SerializedName;import com.google.gson.annotations.Since;public class User {           public User(String string, String string2, String string3, String string4) {        // TODO Auto-generated constructor stub        this.username=string;        this.password=string2;        this.gender=string3;        this.sex=string4;    }    @SerializedName("pwd")      private String password;       @Expose      @SerializedName("uname")      private String username;       @Expose      @Since(1.1)      private String gender;       @Expose      @Since(1.0)      private String sex;    public String getPassword() {        return password;    }    public void setPassword(String password) {        this.password = password;    }    public String getUsername() {        return username;    }    public void setUsername(String username) {        this.username = username;    }    public String getGender() {        return gender;    }    public void setGender(String gender) {        this.gender = gender;    }    public String getSex() {        return sex;    }    public void setSex(String sex) {        this.sex = sex;    }  }  调用：        List<User> userList = new ArrayList<User>();          User jack = new User("123456","Jack", "Male","Female");           User marry = new User("888888","Marry", "Female","Male");           userList.add(jack);           userList.add(marry);           Gson son=new Gson();        Type targetType = new TypeToken<List<User>>() {}.getType();          String str=son.toJson(userList, targetType);        /*[{"pwd":"Jack","uname":"123456","gender":"Male","sex":"Female"},{"pwd":"Marry","uname":"888888","gender":"Female","sex":"Male"}]*/        users =son.fromJson(str,targetType);///转换失败，提示下面的错误

Exception in thread "main" com.google.gson.JsonParseException: The JsonDeserializer com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter@36527f failed to deserialized json object [{"pwd":"Jack","uname":"123456","gender":"Male","sex":"Female"},{"pwd":"Marry","uname":"888888","gender":"Female","sex":"Male"}] given the type java.util.List<toonyoo.ums.test.User>
at com.google.gson.JsonDeserializerExceptionWrapper.deserialize(JsonDeserializerExceptionWrapper.java:63)
at com.google.gson.JsonDeserializationVisitor.invokeCustomDeserializer(JsonDeserializationVisitor.java:88)
at com.google.gson.JsonDeserializationVisitor.visitUsingCustomHandler(JsonDeserializationVisitor.java:76)
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:106)
at com.google.gson.JsonDeserializationContextDefault.fromJsonArray(JsonDeserializationContextDefault.java:64)
at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:49)
at com.google.gson.Gson.fromJson(Gson.java:568)
at com.google.gson.Gson.fromJson(Gson.java:515)
at com.google.gson.Gson.fromJson(Gson.java:484)
at com.google.gson.Gson.fromJson(Gson.java:434)
at toonyoo.ums.test.JSONTest.jsonToObject(JSONTest.java:85)
at com.gsontest.Startup.main(Startup.java:11)
Caused by: java.lang.RuntimeException: No-args constructor for class toonyoo.ums.test.User does not exist. Register an InstanceCreator with Gson for this type to fix this problem.
at com.google.gson.MappedObjectConstructor.constructWithNoArgConstructor(MappedObjectConstructor.java:64)

at com.google.gson.MappedObjectConstructor.construct(MappedObjectConstructor.java:53)
at com.google.gson.JsonObjectDeserializationVisitor.constructTarget(JsonObjectDeserializationVisitor.java:40)
at com.google.gson.JsonDeserializationVisitor.getTarget(JsonDeserializationVisitor.java:56)
at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:109)
at com.google.gson.JsonDeserializationContextDefault.fromJsonObject(JsonDeserializationContextDefault.java:73)
at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:51)
at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.deserialize(DefaultTypeAdapters.java:548)
at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.deserialize(DefaultTypeAdapters.java:510)
at com.google.gson.JsonDeserializerExceptionWrapper.deserialize(JsonDeserializerExceptionWrapper.java:50)
... 11 more


这难道是google的bug，还是我的写法不对，请大虾指点一下。

[解决办法]
toonyoo.ums.test.User 這個 class給一個空變數的construct

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