[JavaEE] Entity中Lazy Load的属性序列化JSON时报错

The server encountered an internal error that prevented it from fulfilling this request.org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: failed to lazily initialize a collection of role: com.party.dinner.entity.User.friends, could not initialize proxy - no Session (through reference chain: com.party.dinner.entity.User["friends"]); nested exception is org.codehaus.jackson.map.JsonMappingException: failed to lazily initialize a collection of role: com.party.dinner.entity.User.friends, could not initialize proxy - no Session (through reference chain: com.party.dinner.entity.User["friends"])
    org.springframework.http.converter.json.MappingJacksonHttpMessageConverter.writeInternal(MappingJacksonHttpMessageConverter.java:204)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:179)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter$ServletHandlerMethodInvoker.writeWithMessageConverters(AnnotationMethodHandlerAdapter.java:1037)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter$ServletHandlerMethodInvoker.handleResponseBody(AnnotationMethodHandlerAdapter.java:995)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter$ServletHandlerMethodInvoker.getModelAndView(AnnotationMethodHandlerAdapter.java:944)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:441)
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:428)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:925)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:856)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:936)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:827)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:812)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
   root cause org.codehaus.jackson.map.JsonMappingException: failed to lazily initialize a collection of role: com.party.dinner.entity.User.friends, could not initialize proxy - no Session (through reference chain: com.party.dinner.entity.User["friends"])
    org.codehaus.jackson.map.JsonMappingException.wrapWithPath(JsonMappingException.java:218)
    org.codehaus.jackson.map.JsonMappingException.wrapWithPath(JsonMappingException.java:183)
    org.codehaus.jackson.map.ser.std.SerializerBase.wrapAndThrow(SerializerBase.java:140)
    org.codehaus.jackson.map.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:158)
    org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:112)
    org.codehaus.jackson.map.ser.StdSerializerProvider._serializeValue(StdSerializerProvider.java:610)
    org.codehaus.jackson.map.ser.StdSerializerProvider.serializeValue(StdSerializerProvider.java:256)
    org.codehaus.jackson.map.ObjectMapper.writeValue(ObjectMapper.java:1613)

转载：http://blog.csdn.net/nomousewch/article/details/8955796

jackson在实际应用中给我们提供了一系列注解，提高了开发的灵活性，下面介绍一下最常用的一些注解

• @JsonIgnoreProperties

         此注解是类注解，作用是json序列化时将java bean中的一些属性忽略掉，序列化和反序列化都受影响。

• @JsonIgnore

         此注解用于属性或者方法上（最好是属性上），作用和上面的@JsonIgnoreProperties一样。

• @JsonFormat

        此注解用于属性或者方法上（最好是属性上），可以方便的把Date类型直接转化为我们想要的模式，比如@JsonFormat(pattern = "yyyy-MM-dd HH-mm-ss")

• @JsonSerialize

        此注解用于属性或者getter方法上，用于在序列化时嵌入我们自定义的代码，比如序列化一个double时在其后面限制两位小数点。

public class CustomDoubleSerialize extends JsonSerializer<Double> {

    private DecimalFormat df = new DecimalFormat("##.00");

    @Override
    public void serialize(Double value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {

        jgen.writeString(df.format(value));
    }
}

• @JsonDeserialize

         此注解用于属性或者setter方法上，用于在反序列化时可以嵌入我们自定义的代码，类似于上面的@JsonSerialize

public class CustomDateDeserialize extends JsonDeserializer<Date> {

    private SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");

    @Override
    public Date deserialize(JsonParser jp, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {

        Date date = null;
        try {
            date = sdf.parse(jp.getText());
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return date;
    }
}

• 完整例子

       

//表示序列化时忽略的属性
@JsonIgnoreProperties(value = { "word" })
public class Person {
    private String name;
    private int age;
    private boolean sex;
    private Date birthday;
    private String word;
    private double salary;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public boolean isSex() {
        return sex;
    }

    public void setSex(boolean sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    // 反序列化一个固定格式的Date
    @JsonDeserialize(using = CustomDateDeserialize.class)
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public String getWord() {
        return word;
    }

    public void setWord(String word) {
        this.word = word;
    }

    // 序列化指定格式的double格式
    @JsonSerialize(using = CustomDoubleSerialize.class)
    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public Person(String name, int age, boolean sex, Date birthday,
            String word, double salary) {
        super();
        this.name = name;
        this.age = age;
        this.sex = sex;
        this.birthday = birthday;
        this.word = word;
        this.salary = salary;
    }

    public Person() {
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + ", sex=" + sex
                + ", birthday=" + birthday + ", word=" + word + ", salary="
                + salary + "]";
    }

}

public class Demo {
    public static void main(String[] args) {

        writeJsonObject();

        // readJsonObject();
    }

    // 直接写入一个对象(所谓序列化)
    public static void writeJsonObject() {
        ObjectMapper mapper = new ObjectMapper();
        Person person = new Person("nomouse", 25, true, new Date(), "程序员",
                2500.0);
        try {
            mapper.writeValue(new File("c:/person.json"), person);
        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // 直接将一个json转化为对象（所谓反序列化）
    public static void readJsonObject() {
        ObjectMapper mapper = new ObjectMapper();

        try {
            Person person = mapper.readValue(new File("c:/person.json"),
                    Person.class);
            System.out.println(person.toString());
        } catch (JsonParseException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }


}

如果Entity中的延迟加载对象必须有值，可以用Hibernate.initialize把延迟加载的对象提前获取
Hibernate.initialize( Entity.getXX() );