1.Jpa的映射策略

在springboot使用jpa的话，会有这么一行配置：

spring.jpa.hibernate.naming.physical-strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl

这是jpa的命名策略。查阅资料后，解释是这样的：
SpringNamingStrategy 不再用作Hibernate 5.1，Hibernate 5.1已经删除了对旧的NamingStrategy 界面的支持。一个新的 SpringPhysicalNamingStrategy 现在是自动配置的，它与Hibernate的默认ImplicitNamingStrategy 结合使用 。这应该非常接近（如果不是完全相同）Spring Boot 1.3默认值，但是，您应该在升级时检查数据库模式是否正确。
如果您在升级之前已经使用Hibernate 5，则可能使用Hibernate的5默认值。如果要在升级后还原它们，请在配置中设置此属性：

spring.jpa.hibernate.naming.physical-strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl

上面的那段话的意思是说升级Hibernate到5.1的话，那么之前的hibernate.ejb.naming_strategy将不再被支持，原因是不灵活，无法正确的给定命名“规则”（the NamingStrategy contract was often not flexible enough to properly apply a given naming "rule", either because the API lacked the information to decide or because the API was honestly not well defined as it grew.）。而被替换成了下面两个属性：

hibernate.physical_naming_strategy
hibernate.implicit_naming_strategy

1.hibernate.physical_naming_strategy的解释是：

used to convert a "logical name" (either implicit or explicit) name of a table or column into a physical name (e.g. following corporate naming guidelines)，翻译后：物理命名策略，用于转换“逻辑名称”(隐式或显式)的表或列成一个物理名称。这个 physical_naming_strategy 命名策略有两个子属性，如下：

//1.PhysicalNamingStrategyStandardImpl：不做修改，直接映射
spring.jpa.hibernate.naming.physical-strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl

//2.SpringPhysicalNamingStrategy:在进行领域映射时,首字母小写，大写字母变为下划线加小写
//ex: LoginName --> login_name
spring.jpa.hibernate.naming.physical-strategy=org.springframework.boot.orm.jpa.hibernate.SpringPhysicalNamingStrategy

我们先来看看PhysicalNamingStrategyStandardImpl的源码：

/**
 * Standard implementation of the PhysicalNamingStrategy contract.
 * @author Steve Ebersole
 */
public class PhysicalNamingStrategyStandardImpl implements PhysicalNamingStrategy, Serializable {
    /**
     * Singleton access
     */
    public static final PhysicalNamingStrategyStandardImpl INSTANCE = new PhysicalNamingStrategyStandardImpl();

    @Override
    public Identifier toPhysicalCatalogName(Identifier name, JdbcEnvironment context) {
        return name;
    }

    @Override
    public Identifier toPhysicalSchemaName(Identifier name, JdbcEnvironment context) {
        return name;
    }

    @Override
    public Identifier toPhysicalTableName(Identifier name, JdbcEnvironment context) {
        return name;
    }

    @Override
    public Identifier toPhysicalSequenceName(Identifier name, JdbcEnvironment context) {
        return name;
    }

    @Override
    public Identifier toPhysicalColumnName(Identifier name, JdbcEnvironment context) {
        return name;
    }
}

从上面的源码中我们可以发现，每个方法都是直接返回name，并没有对name做修改，所以我得出结论是：这个命名策略是指直接映射。
第二个是SpringPhysicalNamingStrategy的源码：

public class SpringPhysicalNamingStrategy implements PhysicalNamingStrategy {
    public SpringPhysicalNamingStrategy() {
    }

    public Identifier toPhysicalCatalogName(Identifier name, JdbcEnvironment jdbcEnvironment) {
        return this.apply(name, jdbcEnvironment);
    }

    public Identifier toPhysicalSchemaName(Identifier name, JdbcEnvironment jdbcEnvironment) {
        return this.apply(name, jdbcEnvironment);
    }

    public Identifier toPhysicalTableName(Identifier name, JdbcEnvironment jdbcEnvironment) {
        return this.apply(name, jdbcEnvironment);
    }

    public Identifier toPhysicalSequenceName(Identifier name, JdbcEnvironment jdbcEnvironment) {
        return this.apply(name, jdbcEnvironment);
    }

    public Identifier toPhysicalColumnName(Identifier name, JdbcEnvironment jdbcEnvironment) {
        return this.apply(name, jdbcEnvironment);
    }

    private Identifier apply(Identifier name, JdbcEnvironment jdbcEnvironment) {
        if(name == null) {
            return null;
        } else {//在大写字母前加下划线：LoginName --> Login_Name
            StringBuilder builder = new StringBuilder(name.getText().replace('.', '_'));

            for(int i = 1; i < builder.length() - 1; ++i) {
                if(this.isUnderscoreRequired(builder.charAt(i - 1), builder.charAt(i), builder.charAt(i + 1))) {
                    builder.insert(i++, '_');
                }
            }

            return this.getIdentifier(builder.toString(), name.isQuoted(), jdbcEnvironment);
        }
    }

    protected Identifier getIdentifier(String name, boolean quoted, JdbcEnvironment jdbcEnvironment) {
        if(this.isCaseInsensitive(jdbcEnvironment)) {
            name = name.toLowerCase(Locale.ROOT);
        }

        return new Identifier(name, quoted);
    }

    protected boolean isCaseInsensitive(JdbcEnvironment jdbcEnvironment) {
        return true;
    }

    private boolean isUnderscoreRequired(char before, char current, char after) {
        //2.将大写字母变为小写：Login_Name--->login_name
        return Character.isLowerCase(before) && Character.isUpperCase(current) && Character.isLowerCase(after);
    }
}

从上面的源码中我们可以发现，apply方法将大写前加下划线，再调用getIdentifier方法将大写变为小写。结合网上信息，我得出的结论是：默认认情况下，Spring Boot会配置物理命名策略。此实现提供与Hibernate 4相同的表结构：所有点都被下划线替换，骆驼情况也被下划线替换。默认情况下，所有表名都以小写生成，但如果您的架构需要，则可以覆盖该标志。(在IDEA中查看源码并断点debug可检验此结论)

2.hibernate.implicit_naming_strategy解释是：

当一个实体没有显式地命名它映射到的数据库表时，我们需要隐式地确定该表的名称。或者当特定属性没有显式地命名它映射到的数据库列时，我们需要隐式地确定该列的名称。说的简单点就是表或者属性没有指定使用名称，我的理解就是没有用@TableName和@Column。（When an entity does not explicitly name the database table that it maps to, we need to implicitly determine that table name. Or when a particular attribute does not explicitly name the database column that it maps to, we need to implicitly determine that column name.）它有以下5个属性，由于篇幅有限（我太懒-.-）。并且没有得出具体结论：

1.org.hibernate.boot.model.naming.ImplicitNamingStrategyComponentPathImpl
2.org.springframework.boot.orm.jpa.hibernate.SpringImplicitNamingStrategy
3.org.hibernate.boot.model.naming.ImplicitNamingStrategyJpaCompliantImpl
4.org.hibernate.boot.model.naming.ImplicitNamingStrategyLegacyHbmImpl
5.org.hibernate.boot.model.naming.ImplicitNamingStrategyLegacyJpaImpl

但是我们依然可以自己修改命名策略。然后在配置文件中设置自己的命名策略。