Hibernate：概述了@GeneratedValue的工作原理 [英] Hibernate: rundown on how @GeneratedValue works

问题描述

数据库总是会被查询并返回最后一个值？如果两个不同的进程（不同的Hibernate应用程序）同时访问同一个表，会发生什么情况？特别是使用自动数字值和序列时，会发生什么？ 解决方案

我假定您正在引用JPA @GeneratedValue。

@GeneratedValue 注释告诉ORM如何计算出该字段的价值。

例如：

  @Id 
 @GeneratedValue（strategy = SEQUENCE，generator =CUST_SEQ）
 @Column（name =CUST_ID）
 public Long getId（）{return id; } 
 
例子2：
 
 @Id 
 @GeneratedValue（strategy = TABLE，generator =CUST_GEN）
 @Column（name =CUST_ID ）
长ID; 
  

要理解的关键是生成的值有一个策略，生成的值的策略决定怎么了。在上面的例子中，SEQUENCE生成策略意味着当首次保存对象时，ORM将向数据库询问序列的新值。第二个示例指定了表生成策略，这意味着ORM将查询表中的行以确定id的值。在示例2中，使用哪个表的详细信息不会显示，因为它引用了一个名为CUST_GEN的生成器。您将遇到的Typcial生成器。

• 身份 - 在插入问自动incerement列以获取物品的价值


• 序列 - 值来自数据库序列


• 表 - 该值来自数据库中的另一个表


• auto - 根据数据库类型
  
• UUID - 在执行插入操作之前生成一个UUID
  
  
  

  有可能开发定制发电机。与数据库的交互取决于生成策略。
  

  I am having troubling finding an accurate explanation of @GeneratedValue and the different strategies regarding on what happens from a database point of view.

  Will the database always be queried and the last value available returned? what happens if 2 different process (different Hibernate apps) access to the same table at the same time?specifically with auto numeric values and sequences

  解决方案

  I am assuming you are refering to the JPA @GeneratedValue.

  The @GeneratedValue annotation tells the ORM how to figure out the value of that field.

  For example:

   @Id
   @GeneratedValue(strategy=SEQUENCE, generator="CUST_SEQ")
   @Column(name="CUST_ID")
   public Long getId() { return id; }
  
   Example 2:
  
   @Id
   @GeneratedValue(strategy=TABLE, generator="CUST_GEN")
   @Column(name="CUST_ID")
   Long id;
  

  The key thing to understand is that a generated value has a strategy and the strategy of the generated value determines what happens. In the above example the SEQUENCE generation strategy means that the ORM will ask the database for a new value for the sequence when saving an object for the first time. The second example specify a table generation strategy which means that the ORM will consult a row in a table to determine the value of a id. In example example 2 the details of which table is used are not show since it reference a generator called "CUST_GEN"

  Typcial generators you will run into.

  • Identity - After an insert ask the auto incerement column for the value of the item
  • Sequence - the value comes from a db sequence
  • table - the value comes from another table in the database
  • auto - pick one of the above based on the database type
  • UUID - generate a UUID before doing an insert

  It is possible to develop custom generator. The interaction with the database will depend on generation strategy.

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