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

<!-- 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

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

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。

...

//对应的销售单实体类

@ManyToOne

@JoinColumn(name = "order_id")

@JsonIgnore

private SalesOrder salesOrder;

...

//getters and setters

@Entity


@Table(name="sales_order")


public class SalesOrder {
...

<span class="hljs-comment">/**订单中商品列表清单**/</span>

<span class="hljs-meta">@OneToMany(mappedBy = <span class="hljs-meta-string">"salesOrder"</span>, cascade = CascadeType.ALL, orphanRemoval = true)</span>

<span class="hljs-meta">@OrderBy(value = <span class="hljs-meta-string">"order_item_id"</span>)</span>

<span class="hljs-meta">@Fetch(FetchMode.JOIN)</span>

<span class="hljs-keyword">private</span> List&lt;SalesOrderItem&gt; salesOrderItems;

...

<span class="hljs-comment">//getters and setters</span>


}
@Entity


@Table(name = "sales_order_item")


public class SalesOrderItem {
...

<span class="hljs-comment">//对应的销售单实体类</span>

<span class="hljs-meta">@ManyToOne</span>

<span class="hljs-meta">@JoinColumn(name = <span class="hljs-meta-string">"order_id"</span>)</span>

<span class="hljs-meta">@JsonIgnore</span>

<span class="hljs-keyword">private</span> SalesOrder salesOrder;

...

<span class="hljs-comment">//getters and setters</span>


}

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。

...

//对应的销售单实体类

@ManyToOne

@JoinColumn(name = "order_id")

@JsonBackReference

private SalesOrder salesOrder;

...

//getters and setters

@Entity


@Table(name="sales_order")


public class SalesOrder {
...

<span class="hljs-comment">/**订单中商品列表清单**/</span>

<span class="hljs-meta">@OneToMany(mappedBy = <span class="hljs-meta-string">"salesOrder"</span>, cascade = CascadeType.ALL, orphanRemoval = true)</span>

<span class="hljs-meta">@OrderBy(value = <span class="hljs-meta-string">"order_item_id"</span>)</span>

<span class="hljs-meta">@Fetch(FetchMode.JOIN)</span>

<span class="hljs-meta">@JsonManagedReference</span>

<span class="hljs-keyword">private</span> List&lt;SalesOrderItem&gt; salesOrderItems;

...

<span class="hljs-comment">//getters and setters</span>


}
@Entity


@Table(name = "sales_order_item")


public class SalesOrderItem {
...

<span class="hljs-comment">//对应的销售单实体类</span>

<span class="hljs-meta">@ManyToOne</span>

<span class="hljs-meta">@JoinColumn(name = <span class="hljs-meta-string">"order_id"</span>)</span>

<span class="hljs-meta">@JsonBackReference</span>

<span class="hljs-keyword">private</span> SalesOrder salesOrder;

...

<span class="hljs-comment">//getters and setters</span>


}

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

...

//对应的销售单实体类

@ManyToOne

@JoinColumn(name = "order_id")

private SalesOrder salesOrder;

...

//getters and setters

@Entity


@Table(name="sales_order")


@JsonIdentityInfo(generator=ObjectIdGenerators.IntSequenceGenerator.class, property="@id")


public class SalesOrder {
...

<span class="hljs-comment">/**订单中商品列表清单**/</span>

<span class="hljs-meta">@OneToMany(mappedBy = <span class="hljs-meta-string">"salesOrder"</span>, cascade = CascadeType.ALL, orphanRemoval = true)</span>

<span class="hljs-meta">@OrderBy(value = <span class="hljs-meta-string">"order_item_id"</span>)</span>

<span class="hljs-meta">@Fetch(FetchMode.JOIN)</span>

<span class="hljs-keyword">private</span> List&lt;SalesOrderItem&gt; salesOrderItems;

...

<span class="hljs-comment">//getters and setters</span>


}
@Entity


@Table(name = "sales_order_item")


@JsonIdentityInfo(generator=ObjectIdGenerators.IntSequenceGenerator.class, property="@id")


public class SalesOrderItem {
...

<span class="hljs-comment">//对应的销售单实体类</span>

<span class="hljs-meta">@ManyToOne</span>

<span class="hljs-meta">@JoinColumn(name = <span class="hljs-meta-string">"order_id"</span>)</span>

<span class="hljs-keyword">private</span> SalesOrder salesOrder;

...

<span class="hljs-comment">//getters and setters</span>


}

@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。

...

//对应的销售单实体类

@ManyToOne

@JoinColumn(name = "order_id")

@JsonIgnoreProperties("salesOrderItems")

private SalesOrder salesOrder;

...

//getters and setters

@Entity


@Table(name="sales_order")


public class SalesOrder {
...

<span class="hljs-comment">/**订单中商品列表清单**/</span>

<span class="hljs-meta">@OneToMany(mappedBy = <span class="hljs-meta-string">"salesOrder"</span>, cascade = CascadeType.ALL, orphanRemoval = true)</span>

<span class="hljs-meta">@OrderBy(value = <span class="hljs-meta-string">"order_item_id"</span>)</span>

<span class="hljs-meta">@Fetch(FetchMode.JOIN)</span>

<span class="hljs-meta">@JsonIgnoreProperties(<span class="hljs-meta-string">"salesOrder"</span>)</span>

<span class="hljs-keyword">private</span> List&lt;SalesOrderItem&gt; salesOrderItems;

...

<span class="hljs-comment">//getters and setters</span>


}
@Entity


@Table(name = "sales_order_item")


public class SalesOrderItem {
...

<span class="hljs-comment">//对应的销售单实体类</span>

<span class="hljs-meta">@ManyToOne</span>

<span class="hljs-meta">@JoinColumn(name = <span class="hljs-meta-string">"order_id"</span>)</span>

<span class="hljs-meta">@JsonIgnoreProperties(<span class="hljs-meta-string">"salesOrderItems"</span>)</span>

<span class="hljs-keyword">private</span> SalesOrder salesOrder;

...

<span class="hljs-comment">//getters and setters</span>


}

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-...

jackson annotation教程
http://tutorials.jenkov.com/j...

stack overflow上相关问题回答
https://stackoverflow.com/que...
https://stackoverflow.com/que...
https://stackoverflow.com/que...

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/da...

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原文地址：https://segmentfault.com/a/1190000010246362