Common technical notes

One 、JPA Entity annotations

• @Entity - This is a tag comment , Indicates that this class is an entity , This annotation must be placed on the class name .
• @Table - When the entity class is different from its mapped database table name, you need to use @Table, A common attribute of this annotation is name, Used to indicate the table name of the database .
• @Id - Located in a specific field that holds the persistent identity attribute . This field is considered the primary key in the database .
• @Column - It is used to identify the correspondence between the attributes in the entity class and the fields in the data table
• @GeneratedValue - Generate a unique primary key for the entity class , Provides the primary key generation strategy .

It includes ：

1.  strategy： Primary key generation policy
2.  generator： key generator

Two 、Jackson

brief introduction

• Jackson It is widely used at present , Used to serialize and deserialize json Of Java Open source framework of .

• Jackson The community is relatively active , The update speed is also relatively fast , from Github According to the statistics in ,Jackson It's the most popular json One of the parsers .

• Spring MVC Default json The parser is Jackson. Jackson There are many advantages .

• Jackson The dependent jar Fewer bags , Simple and easy to use .

• And others Java Of json Framework Gson Such as compared to , Jackson Analyze the big json Documents are relatively fast ;Jackson It takes up less memory at runtime , Good performance ;

• Jackson There are flexible API, It's easy to expand and customize .

Commonly used annotations

• @JsonFormat - When obtaining data from the database and presenting it in the previous paragraph , Use this annotation
• @DataTimeFormat - When the foreground transfers data to the background , Use this annotation
• @JsonInclude - Specify the policy of entity class in serialization
  
  Common attribute values ：
    - JsonJsonInclude.Include.ALWAYS ： This is the default policy , In any case, serialize the field , It's the same effect as not writing this annotation .
  
   - JsonJsonInclude.Include.NON_NULL： This is the most commonly used , That is, if the annotated field is null, Then you won't serialize this field .
  
   - JsonJsonInclude.Include.NON_ABSENT：java There are some complex data structures in , such as AtomicReference or Optional, If a property is of this type , Although this property is not null, But the contents of the package are null Of , The purpose of this annotation is , Only this attribute is not null, And the content of its wrapper will be serialized only if it has value
  
   - JsonJsonInclude.Include.NON_EMPTY： This property contains NON_NULL,NON_ABSENT It also includes that if the field is empty, it will not be serialized . This one is also more commonly used
  
   - JsonJsonInclude.Include.NON_DEFAULT： It's easy to understand , If the field is the default value, it will not be serialized .

3、 ... and 、Mybatis

• @Param -  The basic type has parameters that must be added @Param annotation （ Only one can be ignored but it is suggested to add ）, among value The attribute value should be consistent with the configuration file parameters .

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