hibernate中因双向依赖而造成的json怪相--springmvc项目

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简单说一下Jackson

如果想要详细了解一下Jackson，可以去其github上的项目主页查看其版本情况以及各项功能。除此以外，需要格外提一下Jackson的版本问题。Jackson目前主流版本有两种，1.x和2.x，而这二者的核心包是不同的。在2.0以后的Jackson版本中，groupId从原来的org.codehaus.jackson.core转而变成了com.fasterxml.jackson.core。所以如果希望用到新性能的Jackson，请将原来的maven依赖改为以下的依赖。

下文中的jackson-2-version是指特定的版本，当前的稳定版本为2.8.9。想要了解最新的版本情况还请去项目主页或是maven官网上查看。但是如果是要和spring mvc配合使用的话，还要注意一下他们之间的兼容问题。目前我采用的是4.2.6版本的springmvc和2.6.6版本的jackson

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<!-- the core, which includes Streaming API, shared low-level abstractions (but NOT data-binding) -->
 <dependency>
   <groupId>com.fasterxml.jackson.core</groupId>
   <artifactId>jackson-core</artifactId>
   <version>${jackson-2-version}</version>
 </dependency>

 <!-- Just the annotations; use this dependency if you want to 
    attach annotations to classes without connecting them to the code. -->
 <dependency>
   <groupId>com.fasterxml.jackson.core</groupId>
   <artifactId>jackson-annotations</artifactId>
   <version>${jackson-2-version}</version>
</dependency>

<!-- databinding; ObjectMapper, JsonNode and related classes are here -->
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>${jackson-2-version}</version>
</dependency>

<!-- smile (binary JSON). Other artifacts in this group do other formats. -->
<dependency>
  <groupId>com.fasterxml.jackson.dataformat</groupId>
  <artifactId>jackson-dataformat-smile</artifactId>
  <version>${jackson-2-version}</version>
</dependency>
<!-- JAX-RS provider -->
<dependency>
   <groupId>com.fasterxml.jackson.jaxrs</groupId>
   <artifactId>jackson-jaxrs-json-provider</artifactId>
   <version>${jackson-2-version}</version>
</dependency>
<!-- Support for JAX-B annotations as additional configuration -->
<dependency>
  <groupId>com.fasterxml.jackson.module</groupId>
  <artifactId>jackson-module-jaxb-annotations</artifactId>
  <version>${jackson-2-version}</version>
</dependency>

如果是在springmvc中配置maven依赖，则需要的依赖包为core，annotation和databind

从一个bug说起

刚开始上手springmvc的时候并没有详细去了解更多的JSON操作，只是简单的了解了一下如何将对象转化为JSON数据传送回前端。但是在这时出现了一个问题，就是当两个entity之间存在双向依赖时，传回的JSON数据会出现无限的读取情况。也就是说，因为两个实体中都存在着指向对方的指针，导致序列化的时候会出现二者之间不断相互访问的情况。hibernate这种实体设计方式一直让我有些困惑，毕竟在一般代码的设计模式中是应当尽量避免出现双向依赖的。

这里举一个具体的例子说明这个情况。
假设我有一个订单，订单中有多个商品。也就是订单和商品之间是一对多的关系。订单和商品的实体类如下：

订单实体类

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import org.hibernate.annotations.Fetch;
import org.hibernate.annotations.FetchMode;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

/**
 * Created by rale on 7/15/17.
 * 销售单
 */
@Entity
@Table(name="sales_order")
public class SalesOrder {

    @Id
    @Column(name = "sales_order_id")
    private Long salesOrderId;

    /**订单创建人员**/
    @Column(name = "salesman_id", nullable = false)
    private Long userId;
    
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "sales_order_created_at")
    private Date createAt;


    /**订单中商品列表清单**/
    @OneToMany(mappedBy = "salesOrder", cascade = CascadeType.ALL, orphanRemoval = true)
    @OrderBy(value = "order_item_id")
    @Fetch(FetchMode.JOIN)
    private List<SalesOrderItem> salesOrderItems;


    public Long getSalesOrderId() {
        return salesOrderId;
    }

    public void setSalesSource(SalesSource salesSource) {
        this.salesSource = salesSource;
    }

    public Long getUserId() {
        return userId;
    }

    public void setUserId(Long userId) {
        this.userId = userId;
    }

    public Date getCreateAt() {
        return createAt;
    }

    public void setCreateAt(Date createAt) {
        this.createAt = createAt;
    }


    public List<SalesOrderItem> getSalesOrderItems() {
        return salesOrderItems==null ? new ArrayList<SalesOrderItem>() : salesOrderItems;
    }

    public void setSalesOrderItems(List<SalesOrderItem> salesOrderItems) {
        this.salesOrderItems = salesOrderItems;
    }

    public void addSalesOrderItem(SalesOrderItem salesOrderItem){
        if (this.salesOrderItems == null) this.salesOrderItems = new ArrayList<SalesOrderItem>();
        salesOrderItem.setSalesOrder(this);
        this.salesOrderItems.add(salesOrderItem);
    }


}

订单商品实体类

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import javax.persistence.*;

/**
 * Created by rale on 7/15/17.
 * 销售清单中的单品和数量
 */
@Entity
@Table(name = "sales_order_item")
public class SalesOrderItem {
    @Id
    @GeneratedValue
    @Column(name = "order_item_id")
    private Long salesOrderItemId;

    @Column(name = "order_item_quantity")
    private int quantity;

    @Column(name = "order_item_price")
    private double salesPrice;

    //对应的销售单实体类
    @ManyToOne
    @JoinColumn(name = "order_id")
    private SalesOrder salesOrder;

    public SalesOrderItem() {
    }

   
    public SalesOrderItem(Long salesOrderItemId){
        this.salesOrderItemId = salesOrderItemId;
    }

    public Long getSalesOrderItemId() {
        return salesOrderItemId;
    }

    public void setSalesOrderItemId(Long salesOrderItemId) {
        this.salesOrderItemId = salesOrderItemId;
    }

    
    public int getQuantity() {
        return quantity;
    }

    public void setQuantity(int quantity) {
        this.quantity = quantity;
    }

    public double getSalesPrice() {
        return salesPrice;
    }

    public void setSalesPrice(double salesPrice) {
        this.salesPrice = salesPrice;
    }

    public SalesOrder getSalesOrder() {
        return salesOrder;
    }

    public void setSalesOrder(SalesOrder salesOrder) {
        this.salesOrder = salesOrder;
    }
}

解决双向依赖的方法如下：

@JsonIgnore

在不希望被序列化的field或property上使用@JsonIgnore标记，即可使该属性在序列化和解序列化的过程中不被访问。

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@Entity
@Table(name="sales_order")
public class SalesOrder {

    ...
    
    /**订单中商品列表清单**/
    @OneToMany(mappedBy = "salesOrder", cascade = CascadeType.ALL, orphanRemoval = true)
    @OrderBy(value = "order_item_id")
    @Fetch(FetchMode.JOIN)
    private List<SalesOrderItem> salesOrderItems;
    
    ...
    //getters and setters
}

@Entity
@Table(name = "sales_order_item")
public class SalesOrderItem {

    ...

    //对应的销售单实体类
    @ManyToOne
    @JoinColumn(name = "order_id")
    @JsonIgnore
    private SalesOrder salesOrder;
    
    ...
    //getters and setters
}

这里可能会出现不适用的场景，比如说，当我希望从SalesOrderItem的方向获取SalesOrder的数据，将会出现无法被序列化的情况。

这里需要特别强调一下 不要使用transient标记属性 会报错

@JsonManagedReference and @JsonBackReference.

将@JsonManagedReference标记在父类对子类的引用变量上，并将@JsonBackReference标记在子类对父类的引用变量上。

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@Entity
@Table(name="sales_order")
public class SalesOrder {

    ...
    
    /**订单中商品列表清单**/
    @OneToMany(mappedBy = "salesOrder", cascade = CascadeType.ALL, orphanRemoval = true)
    @OrderBy(value = "order_item_id")
    @Fetch(FetchMode.JOIN)
    @JsonManagedReference
    private List<SalesOrderItem> salesOrderItems;
    
    ...
    //getters and setters
}

@Entity
@Table(name = "sales_order_item")
public class SalesOrderItem {

    ...

    //对应的销售单实体类
    @ManyToOne
    @JoinColumn(name = "order_id")
    @JsonBackReference
    private SalesOrder salesOrder;
    
    ...
    //getters and setters
}

通过这种方式确保在双向关系中只有单个反向的实例被序列化

@JsonIdentityInfo

该annotation用于标注在entity上。当entity被标注后，jackson在每一次序列化的时候都会为该实例生成专门的ID（也可以是实例自带的属性），通过这种方式辨别实例。这种方式适用于存在一个实体关联链的场景。比如Order -> OrderLine -> User -> Order

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@Entity
@Table(name="sales_order")
@JsonIdentityInfo(generator=ObjectIdGenerators.IntSequenceGenerator.class, property="@id")
public class SalesOrder {

    ...
    
    /**订单中商品列表清单**/
    @OneToMany(mappedBy = "salesOrder", cascade = CascadeType.ALL, orphanRemoval = true)
    @OrderBy(value = "order_item_id")
    @Fetch(FetchMode.JOIN)
    private List<SalesOrderItem> salesOrderItems;
    
    ...
    //getters and setters
}

@Entity
@Table(name = "sales_order_item")
@JsonIdentityInfo(generator=ObjectIdGenerators.IntSequenceGenerator.class, property="@id")
public class SalesOrderItem {

    ...

    //对应的销售单实体类
    @ManyToOne
    @JoinColumn(name = "order_id")
    private SalesOrder salesOrder;
    
    ...
    //getters and setters
}

@JsonIgnoreProperties 个人心中的全场最佳

@JsonIgnoreProperties不同于@JsonIgnore在于，你可以注明该变量中的哪个属相不被序列化。从而允许在双向访问上都不存在环或是缺失。

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@Entity
@Table(name="sales_order")
public class SalesOrder {

    ...
    
    /**订单中商品列表清单**/
    @OneToMany(mappedBy = "salesOrder", cascade = CascadeType.ALL, orphanRemoval = true)
    @OrderBy(value = "order_item_id")
    @Fetch(FetchMode.JOIN)
    @JsonIgnoreProperties("salesOrder")
    private List<SalesOrderItem> salesOrderItems;
    
    ...
    //getters and setters
}

@Entity
@Table(name = "sales_order_item")
public class SalesOrderItem {

    ...

    //对应的销售单实体类
    @ManyToOne
    @JoinColumn(name = "order_id")
    @JsonIgnoreProperties("salesOrderItems")
    private SalesOrder salesOrder;
    
    ...
    //getters and setters
}

半场小结

其实jackson中还有很多很实用的功能，例如如何将Date类序列化成界面展示的格式等，将在下一次更新中说明。有兴趣的可以收藏加关注哦。

其它教程传送门

springmvc + ajax 实现
http://www.mkyong.com/spring-mvc/spring-4-mvc-ajax-hello-world-example/

jackson annotation教程
http://tutorials.jenkov.com/java-json/jackson-annotations.html

stack overflow上相关问题回答
https://stackoverflow.com/questions/12505141/only-using-jsonignore-during-serialization-but-not-deserialization
https://stackoverflow.com/questions/3325387/infinite-recursion-with-jackson-json-and-hibernate-jpa-issue
https://stackoverflow.com/questions/30589257/json-jpa-manytoone-and-onetomany-recursive

这里需要指出的是，虽然某些回答说，要将annotation标注在私有变量的get方法上，但是po主发现标注在私有变量上还是可以实现功能的。

data hiding using jsonignore and spring data jpa
https://dzone.com/articles/data-hiding-using-jsonignore-and-spring-data-jpa