java.sql.Date,java.sql.Time和java.sql.Timestamp

 

java.sql.Date,java.sql.Time和java.sql.Timestamp三个都是java.util.Date的子类（包装类）。他们跟java.util.Date有什么区别， 设计这三个类有什么用意呢？ 先看看一个例子： 但是为什么java.sql.Date类型的值插入到数据库中Date字段中会发生数据截取呢？ java.sql.Date是为了配合SQL DATE而设置的数据类型。“规范化”的java.sql.Date只包含年月日信息，时分秒毫秒都会清零。格式类似：YYYY-MM-DD 当我们调用ResultSet的getDate()方法来获得返回值时，java程序会参照"规范"的java.sql.Date来格式化数据库中的数值。 可以看出，java.sql.Date,java.sql.Time和java.sql.Timestamp都是为了更好的规范化java.util.Date， 让它能更好的区分日期，时分秒。 一、java.util.Date向java.sql.Date的规范化 如果数据库中存在的非规范化部分的信息将会被劫取。在sun提供的ResultSet.java中这样对getDate进行注释的： Retrieves the value of the designated column in the current row of this <code>ResultSet</code> object as a “java.sql.Date” object in the Java programming language. 同理。如果我们把一个java.sql.Date值通过PrepareStatement的setDate方法存入数据库时，java程序会对传入的java.sql.Date规范化 ，非规范化的部分将会被劫取。 然而，我们java.sql.Date一般由java.util.Date转换过来，如：java.sql.Date sqlDate=new java.sql.Date(new java.util.Date().getTime()). 显然，这样转换过来的java.sql.Date丢失了日期部分. 在 http://www.thunderguy.com/semicolon/2003/08/14/java-sql-date-is-not-a-real-date/ 文章中提到，要保存java.util.Date的精确值，  我们需要利用java.sql.Timestamp. java.sql.Date stores only date information, not times. Simply converting a java.util.Date into a java.sql.Date will silently set the time to midnight. So, to store date/times to be manipulated as java.util.Date objects, don’t do this: // BUG: loses time of day preparedStatement.setDate(1, new java.sql.Date(date.getTime())); do this instead: preparedStatement.setTimestamp(1, new java.sql.Timestamp(date.getTime())); java.sql.Timestamp is not a date                 *                        *                        * 二、java.sql.Timestamp到java.util.Date的转化 java.sql.Timestamp extends java.util.Date , but it should not be used as a Date . In JDK 1.3.1, Timestamp.getTime() (inherited from Date ) returns the time to the nearest second only, but JDK 1.4.2 and JDK 1.5 it returns the time to the nearest millisecond as expected. So in JDK 1.3, when reading a timestamp from a ResultSet , don’t do this: // Java 1.3 java.util.Date d = resultSet.getTimestamp(1); long millis = d.getTime(); // BUG: loses fractional seconds in JDK 1.3 To get the full date including milliseconds, you have to do this: java.sql.Timestamp timestamp = resultSet.getTimestamp(1); java.util.Date d = new java.util.Date(timestamp.getTime() +                                       timestamp.getNanos() / 1000000); //nanos是纳秒，1纳秒=百万分之一毫秒 In JDK 1.4.2 and JDK 1.5, you can just do this, depending on what you’re going to do with the Date: // Java 1.4+ java.util.Date d = resultSet.getTimestamp(1); But this might be safer since it avoids any other potential Timestamp problems: // Java 1.4+ java.util.Date d = new java.util.Date(resultSet.getTimestamp(1).getTime()); If your code needs to run on JDK 1.3 and later, you’ll have to do this: java.sql.Timestamp timestamp = resultSet.getTimestamp(1); long millis = (timestamp.getTime() / 1000) * 1000 + timestamp.getNanos() / 1000000; java.util.Date d = new java.util.Date(millis); 本节参考自： http://www.thunderguy.com/semicolon/2003/08/14/java-sql-date-is-not-a-real-date/

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