源码发现

mybatis 源码中

为了看是否支持localDate而看了下代码，最终发现是支持的，而且jpa自动生成时，LocalDate对应Mysal的date类型，LocalDateTime对应datetime类型。Mybatis是LocalDate对应DATE，LocalDateTime 对应的是TIMESTAMP。

在Mybatis 的源码中看到了org.apache.ibatis.type下的ArrayTypeHandler类里。看到了

private static final ConcurrentHashMap, String> STANDARD_MAPPING;
  static {
    STANDARD_MAPPING = new ConcurrentHashMap<>();
    STANDARD_MAPPING.put(BigDecimal.class, JdbcType.NUMERIC.name());
    STANDARD_MAPPING.put(BigInteger.class, JdbcType.BIGINT.name());
    STANDARD_MAPPING.put(boolean.class, JdbcType.BOOLEAN.name());
    STANDARD_MAPPING.put(Boolean.class, JdbcType.BOOLEAN.name());
    STANDARD_MAPPING.put(byte[].class, JdbcType.VARBINARY.name());
    STANDARD_MAPPING.put(byte.class, JdbcType.TINYINT.name());
    STANDARD_MAPPING.put(Byte.class, JdbcType.TINYINT.name());
    STANDARD_MAPPING.put(Calendar.class, JdbcType.TIMESTAMP.name());
    STANDARD_MAPPING.put(java.sql.Date.class, JdbcType.DATE.name());
    STANDARD_MAPPING.put(java.util.Date.class, JdbcType.TIMESTAMP.name());
    STANDARD_MAPPING.put(double.class, JdbcType.DOUBLE.name());
    STANDARD_MAPPING.put(Double.class, JdbcType.DOUBLE.name());
    STANDARD_MAPPING.put(float.class, JdbcType.REAL.name());
    STANDARD_MAPPING.put(Float.class, JdbcType.REAL.name());
    STANDARD_MAPPING.put(int.class, JdbcType.INTEGER.name());
    STANDARD_MAPPING.put(Integer.class, JdbcType.INTEGER.name());
    STANDARD_MAPPING.put(LocalDate.class, JdbcType.DATE.name());
    STANDARD_MAPPING.put(LocalDateTime.class, JdbcType.TIMESTAMP.name());
    STANDARD_MAPPING.put(LocalTime.class, JdbcType.TIME.name());
    STANDARD_MAPPING.put(long.class, JdbcType.BIGINT.name());
    STANDARD_MAPPING.put(Long.class, JdbcType.BIGINT.name());
    STANDARD_MAPPING.put(OffsetDateTime.class, JdbcType.TIMESTAMP_WITH_TIMEZONE.name());
    STANDARD_MAPPING.put(OffsetTime.class, JdbcType.TIME_WITH_TIMEZONE.name());
    STANDARD_MAPPING.put(Short.class, JdbcType.SMALLINT.name());
    STANDARD_MAPPING.put(String.class, JdbcType.VARCHAR.name());
    STANDARD_MAPPING.put(Time.class, JdbcType.TIME.name());
    STANDARD_MAPPING.put(Timestamp.class, JdbcType.TIMESTAMP.name());
    STANDARD_MAPPING.put(URL.class, JdbcType.DATALINK.name());
  }

在mybatis中看到的是，LocalDate对应的jdbcType是DATE，LocalDateTime 对应的是TIMESTAMP。
也就是说在xml中resultmap中就是上面这样的对应关系写法。

mybatis xml中的jdbctype 实则引用的是 org.apache.ibatis.type 下的枚举 JdbcType。
这个枚举里只有一个参数。

public final int TYPE_CODE;
  private static Map codeLookup = new HashMap<>();

  static {
    for (JdbcType type : JdbcType.values()) {
      codeLookup.put(type.TYPE_CODE, type);
    }
  }

  JdbcType(int code) {
    this.TYPE_CODE = code;
  }

  public static JdbcType forCode(int code)  {
    return codeLookup.get(code);
  }

参数TYPE_CODE 为int类型，引用的是jdk中java.sql 的Types类代码如下

package java.sql;

/**
 * 
The class that defines the constants that are used to identify generic
 * SQL types, called JDBC types.
 * 

 * This class is never instantiated.
 */
public class Types {

/**
 * 
The constant in the Java programming language, sometimes referred
 * to as a type code, that identifies the generic SQL type
 * BIT.
 */
        public final static int BIT             =  -7;

/**
 * 
The constant in the Java programming language, sometimes referred
 * to as a type code, that identifies the generic SQL type
 * TINYINT.
 */
        public final static int TINYINT         =  -6;
...
  // Prevent instantiation
    private Types() {}

代码中得到的：

1. 枚举中可以指定一个 private static Map 。在静态代码块中赋值。由此可提供一个用value值获取枚举对象的静态方法，直接由map获取。
   2.我们会遇到一些需要指定固定值的类，上面的java.sql.Type 里面的属性为final 的static 的int类型，这种对象不需要实例化，将构造方法用private修饰，并是空方法，达到一个禁止实例化的做用。