​ 本文将研究如何使用Hibernate/JPA进行批量插入或更新实体。批量处理使我们可以在单个网络调用中向数据库发送一组SQL语句。这样，可以优化应用程序的网络和内存使用率。

1、创建实体

​ 首先，创建一个 School 实体：

@Entity
@Data
public class School {
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private long id;
    private String name;
    @OneToMany(mappedBy = "school")
    private List<Student> students;
每所school至少有零个student：
@Entity
@Data
public class Student {
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    private long id;
    private String name;
    @ManyToOne
    private School school;
2、跟踪SQL查询
​       在运行示例时，我们需要验证插入/更新语句确实是批量发送的。无奈的是，我们无法从Hibernate日志语句中了解SQL语句是否已批处理。因此，我们将使用数据源代理来跟踪Hibernate / JPA SQL语句：
private static class ProxyDataSourceInterceptor implements MethodInterceptor {
    private final DataSource dataSource;
    public ProxyDataSourceInterceptor(final DataSource dataSource) {
        this.dataSource = ProxyDataSourceBuilder.create(dataSource)
            .name("Batch-Insert-Logger")
            .asJson().countQuery().logQueryToSysOut().build();
    // Other methods...
3、默认行为
​       Hibernate默认情况下不启用批处理。这意味着它将为每个插入/更新操作发送单独的SQL语句：
@Transactional
@Test
public void whenNotConfigured_ThenSendsInsertsSeparately() {
    for (int i = 0; i < 10; i++) {
        School school = createSchool(i);
        entityManager.persist(school);
    entityManager.flush();
​       在这里，persist了10个School实体。如果查看查询日志，可以看到Hibernate分别发送每个插入语句：
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School1","1"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School2","2"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School3","3"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School4","4"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School5","5"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School6","6"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School7","7"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School8","8"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School9","9"]]
"querySize":1, "batchSize":0, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School10","10"]]
​       因此，我们应该配置Hibernate以启用批处理。为此，我们应该将hibernate.jdbc.batch_size属性设置为大于0的数字。如果我们手动创建EntityManager，则应将hibernate.jdbc.batch_size添加到Hibernate属性中：
public Properties hibernateProperties() {
    Properties properties = new Properties();
    properties.put("hibernate.jdbc.batch_size", "5");
    // Other properties...
    return properties;
​       如果使用的是Spring Boot，则可以将其定义为应用程序属性：
spring.jpa.properties.hibernate.jdbc.batch_size=5
4、批量插入单个表
4.1、批量插入，无显式刷新
​       首先，看一下在仅处理一种实体类型时如何使用批处理插入。使用先前的代码示例，但是这次启用了批处理：
@Transactional
@Test
public void whenInsertingSingleTypeOfEntity_thenCreatesSingleBatch() {
    for (int i = 0; i < 10; i++) {
        School school = createSchool(i);
        entityManager.persist(school);
​       在这里，persist了10个School实体。当查看日志时，我们可以验证Hibernate是否批量发送insert语句：
"batch":true, "querySize":1, "batchSize":5, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School1","1"],["School2","2"],["School3","3"],["School4","4"],["School5","5"]]
"batch":true, "querySize":1, "batchSize":5, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School6","6"],["School7","7"],["School8","8"],["School9","9"],["School10","10"]]
​       这里要提到的重要一件事是内存消耗。当我们持久化一个实体时，Hibernate将其存储在持久化上下文中。例如，如果我们在一个事务中保留100,000个实体，则最终将在内存中拥有100,000个实体实例，可能会导致OutOfMemoryException。
4.2、批量插入与显式刷新
​       现在，我们将研究如何在批处理操作期间优化内存使用。让我们深入研究持久性上下文的作用。
​       首先，持久性上下文将新创建的实体以及修改后的实体存储在内存中。同步事务后，Hibernate将这些更改发送到数据库。这通常发生在交易结束时。但是，调用EntityManager.flush()也会触发事务同步。
​       其次，持久性上下文用作实体缓存，因此也称为第一级缓存。要在持久性上下文中清除实体，我们可以调用EntityManager.clear()。
​       因此，为了减少批处理期间的内存负载，只要达到批处理大小，我们就可以在应用程序代码上调用EntityManager.flush（）和EntityManager.clear（）：
@Transactional
@Test
public void whenFlushingAfterBatch_ThenClearsMemory() {
    for (int i = 0; i < 10; i++) {
        if (i > 0 && i % BATCH_SIZE == 0) {
            entityManager.flush();
            entityManager.clear();
        School school = createSchool(i);
        entityManager.persist(school);
​       在这里，我们在持久性上下文中刷新实体，从而使Hibernate将查询发送到数据库。此外，通过清除持久性上下文，我们从内存中删除了School实体。批处理行为将保持不变
5、批量插入多个表
​       现在让我们看看在一个事务中处理多种实体类型时如何配置批处理插入。
​       当我们要保留几种类型的实体时，Hibernate为每种实体类型创建一个不同的批处理。这是因为在同一批中只能有一种类型的实体。
​       此外，由于Hibernate收集插入语句，因此每当遇到与当前批处理中不同的实体类型时，它将创建一个新批处理。即使已经有该实体类型的批次，也是如此：
@Transactional
@Test
public void whenThereAreMultipleEntities_ThenCreatesNewBatch() {
    for (int i = 0; i < 10; i++) {
        if (i > 0 && i % BATCH_SIZE == 0) {
            entityManager.flush();
            entityManager.clear();
        School school = createSchool(i);
        entityManager.persist(school);
        Student firstStudent = createStudent(school);
        Student secondStudent = createStudent(school);
        entityManager.persist(firstStudent);
        entityManager.persist(secondStudent);
​       在这里，我们要插入School并将其分配给两个Student，然后重复此过程10次。
​       在日志中，我们看到Hibernate 以几批大小为1的方式发送School插入语句，而我们原本只希望收到2批大小为5的数据。此外，Student插入语句也以几批大小为2的方式发送，而不是4批大小为5的方式发送。 ：
"batch":true, "querySize":1, "batchSize":1, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School1","1"]]
"batch":true, "querySize":1, "batchSize":2, "query":["insert into student (name, school_id, id) 
  values (?, ?, ?)"], "params":[["Student-School1","1","2"],["Student-School1","1","3"]]
"batch":true, "querySize":1, "batchSize":1, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School2","4"]]
"batch":true, "querySize":1, "batchSize":2, "query":["insert into student (name, school_id, id) 
  values (?, ?, ?)"], "params":[["Student-School2","4","5"],["Student-School2","4","6"]]
"batch":true, "querySize":1, "batchSize":1, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School3","7"]]
"batch":true, "querySize":1, "batchSize":2, "query":["insert into student (name, school_id, id) 
  values (?, ?, ?)"], "params":[["Student-School3","7","8"],["Student-School3","7","9"]]
Other log lines...
​       要批处理具有相同实体类型的所有插入语句，我们应该配置hibernate.order_inserts属性。
我们可以使用EntityManagerFactory手动配置Hibernate属性：
public Properties hibernateProperties() {
    Properties properties = new Properties();
    properties.put("hibernate.order_inserts", "true");
    // Other properties...
    return properties;
​       如果使用的是Spring Boot，则可以在application.properties中配置属性：
spring.jpa.properties.hibernate.order_inserts=true
​       添加此属性后，我们将有1批用于School插入和2批针对Student插入：
"batch":true, "querySize":1, "batchSize":5, "query":["insert into school (name, id) values (?, ?)"], 
  "params":[["School6","16"],["School7","19"],["School8","22"],["School9","25"],["School10","28"]]
"batch":true, "querySize":1, "batchSize":5, "query":["insert into student (name, school_id, id) 
  values (?, ?, ?)"], "params":[["Student-School6","16","17"],["Student-School6","16","18"],
  ["Student-School7","19","20"],["Student-School7","19","21"],["Student-School8","22","23"]]
"batch":true, "querySize":1, "batchSize":5, "query":["insert into student (name, school_id, id) 
  values (?, ?, ?)"], "params":[["Student-School8","22","24"],["Student-School9","25","26"],
  ["Student-School9","25","27"],["Student-School10","28","29"],["Student-School10","28","30"]]
6、批量更新
​       现在，让我们继续进行批处理更新。与批处理插入类似，我们可以对多个更新语句进行分组，然后一次性将它们发送到数据库。
​       为此，我们将配置hibernate.order_updates和hibernate.jdbc.batch_versioned_data属性。如果我们手动创建EntityManagerFactory，则可以通过编程方式设置属性：
public Properties hibernateProperties() {
    Properties properties = new Properties();
    properties.put("hibernate.order_updates", "true");
    properties.put("hibernate.batch_versioned_data", "true");
    // Other properties...
    return properties;
​       如果使用Spring Boot，则将它们添加到application.properties中：
spring.jpa.properties.hibernate.order_updates=true
spring.jpa.properties.hibernate.batch_versioned_data=true
​       配置完这些属性后，Hibernate应该将更新语句分批分组：
@Transactional
@Test
public void whenUpdatingEntities_thenCreatesBatch() {
    TypedQuery<School> schoolQuery = 
      entityManager.createQuery("SELECT s from School s", School.class);
    List<School> allSchools = schoolQuery.getResultList();
    for (School school : allSchools) {
        school.setName("Updated_" + school.getName());
​       在这里，我们更新了学校实体，并且Hibernate分2批发送了大小为5的SQL语句：
"batch"``:``true``, ``"querySize"``:1, ``"batchSize"``:5, ``"query"``:[``"update school set name=? where id=?"``], 
 ``"params"``:[[``"Updated_School1"``,``"1"``],[``"Updated_School2"``,``"2"``],[``"Updated_School3"``,``"3"``],
 ``[``"Updated_School4"``,``"4"``],[``"Updated_School5"``,``"5"``]]
"batch"``:``true``, ``"querySize"``:1, ``"batchSize"``:5, ``"query"``:[``"update school set name=? where id=?"``], 
 ``"params"``:[[``"Updated_School6"``,``"6"``],[``"Updated_School7"``,``"7"``],[``"Updated_School8"``,``"8"``],
 ``[``"Updated_School9"``,``"9"``],[``"Updated_School10"``,``"10"``]]
7、@Id生成策略
当我们想使用批处理进行插入/更新时，我们应该了解主键生成策略。如果我们的实体使用GenerationType.IDENTITY标识符生成器，则Hibernate将静默禁用批处理插入/更新。