1)DatabaseMetaData
/**
* 了解即可:DatabaseMetaData是描述数据库的元数据对象
* 可以由Connection得到
*/@Test
2 public void testDatabaseMetaData(){
3 Connection connection=null;
4 ResultSet resultSet=null;
5 try {
6 connection=JDBCTools.getConnection();
7 DatabaseMetaData data=connection.getMetaData();
8 //1.可以得到数据库本身的一些基本的信息
9 //得到数据库的版本号
10 int version=data.getDatabaseMajorVersion();
11 System.out.println(version);
12 //2.得到连接数据库的用户名
13 String user=data.getUserName();
14 System.out.println(user);
15 //3.得到MySQl中有哪些数据库
16 resultSet=data.getCatalogs();
17 while(resultSet.next()){
18 System.out.println(resultSet.getString(1));
19 }
20 } catch (Exception e) {
21 e.printStackTrace();
22 }finally{
23 JDBCTools.release(resultSet, null, connection);
24 }
25 }2)ResultSetMetaData
/**
* ResultSetMetaData：描述结果集的元数据
* 可以得到结果集中的基本信息：结果集中有哪些列，列名，列的别名等等;
*/ 1 @Test
2 public void testResultSetMetaData(){
3 Connection connection=null;
4 PreparedStatement preparedStatement=null;
5 ResultSet resultSet=null;
6 try {
7 connection=JDBCTools.getConnection();
8 String sql="select id,name,email,birth"+
9 " from customers";
10 preparedStatement=connection.prepareStatement(sql);
11 resultSet=preparedStatement.executeQuery();
12 ResultSetMetaData rsmd=resultSet.getMetaData();
13 //得到列数
14 int columnCount=rsmd.getColumnCount();
15 System.out.println(columnCount);
16 for(int i=0;i<columnCount;i++){
17 //得到列名
18 String columnName=rsmd.getColumnName(i+1);
19 //得到列的别名
20 String columnLabel=rsmd.getColumnLabel(i+1);
21 System.out.println(columnName+":"+columnLabel);
22 }
23
24 } catch (Exception e) {
25 e.printStackTrace();
26 }finally{
27 JDBCTools.release(resultSet, preparedStatement, connection);
28 }
29 }我们的查询操作，对于不同的数据表examstudent和customers,会有不同的代码编写过程，利用反射和JDBC元数据可以编写通用的方法进行对不同数据表的查询。
在此之前我们是这样做的：
查询customers表中的字段以及字段值：
1 public Customer getCustomer(String sql, Object... args) {
2 Customer customer = null;
3 Connection connection = null;
4 PreparedStatement preparedStatement = null;
5 ResultSet resultSet = null;
6 try {
7 connection = JDBCTools.getConnection();
8 preparedStatement = connection.prepareStatement(sql);
9 for (int i = 0; i < args.length; i++) {
10 preparedStatement.setObject(i + 1, args[i]);
11 }
12 resultSet = preparedStatement.executeQuery();
13 if (resultSet.next()) {
14 // student = new Student(resultSet.getInt(1),
15 // resultSet.getInt(2),
16 // resultSet.getString(3), resultSet.getString(4),
17 // resultSet.getString(5), resultSet.getString(6),
18 // resultSet.getInt(7));
19 customer = new Customer();
20 customer.setId(resultSet.getInt(1));
21 customer.setName(resultSet.getString(2));
22 customer.setEmail(resultSet.getString(3));
23 customer.setBirth(resultSet.getDate(4));
24 }
25 } catch (Exception e) {
26 e.printStackTrace();
27 } finally {
28 JDBCTools.release(resultSet, preparedStatement, connection);
29 }
30 return customer;
31
32 }查询examstudent表中的字段以及字段值：
1 public Student getStudent(String sql, Object... args) {
2 Student student = null;
3 Connection connection = null;
4 PreparedStatement preparedStatement = null;
5 ResultSet resultSet = null;
6 try {
7 connection = JDBCTools.getConnection();
8 preparedStatement = connection.prepareStatement(sql);
9 for (int i = 0; i < args.length; i++) {
10 preparedStatement.setObject(i + 1, args[i]);
11 }
12 resultSet = preparedStatement.executeQuery();
13 if (resultSet.next()) {
14 student = new Student(resultSet.getInt(1), resultSet.getInt(2),
15 resultSet.getString(3), resultSet.getString(4),
16 resultSet.getString(5), resultSet.getString(6),
17 resultSet.getInt(7));
18 }
19 } catch (Exception e) {
20 e.printStackTrace();
21 } finally {
22 JDBCTools.release(resultSet, preparedStatement, connection);
23 }
24 return student;
25
26 }元数据：元数据就是描述数据的数据
可以看到两个操作有共同的步骤，不同的地方代码相似度也很高，于是我们就可以在此基础上编写一个通用的方法实现我们查询数据表的操作。
1 public <T> T get(Class<T> classz, String sql, Object... args) {
2 T entity = null;
3 Connection connection = null;
4 PreparedStatement preparedStatement = null;
5 ResultSet resultSet = null;
6 try {
7 connection = JDBCTools.getConnection();
8 preparedStatement = connection.prepareStatement(sql);
9 for (int i = 0; i < args.length; i++) {
10 preparedStatement.setObject(i + 1, args[i]);
11 }
12 resultSet = preparedStatement.executeQuery();
13
14 ResultSetMetaData rsmd = resultSet.getMetaData();
15 Map<String, Object> values = new HashMap<String, Object>();
16 if (resultSet.next()) {
17 for (int i = 0; i < rsmd.getColumnCount(); i++) {
18 String columnLabel = rsmd.getColumnLabel(i + 1);
19 Object columnValue = resultSet.getObject(columnLabel);
20 values.put(columnLabel, columnValue);
21 }
22 }
23 if (values.size() > 0) {
24 entity = classz.newInstance();
25 for (Map.Entry<String, Object> entry : values.entrySet()) {
26 String fieldName = entry.getKey();
27 Object fieldValue = entry.getValue();
28 ReflectionUtils
29 .setFieldValue(entity, fieldName, fieldValue);
30 }
31 }
32 } catch (Exception e) {
33 e.printStackTrace();
34 } finally {
35 JDBCTools.release(resultSet, preparedStatement, connection);
36 }
37 // 返回包含查询信息的实体类对象
38 return entity;
39 } 1 @Test 测试
2 public void testGet() {
3 String sql = "select id,name,email,birth "
4 + " from customers where id=?";
5 System.out.println(sql);
6 Customer customer = get(Customer.class, sql, 3);
7 System.out.println(customer);
8 sql = "select flow_id flowId,type, idcard idCard, exam_card examCard,"
9 + "student_name studentName,location,grade from examstudent where "
10 + " flow_id=?";
11 System.out.println(sql);
12 Student student = get(Student.class, sql, 3);
13 System.out.println(student);
14 }select id,name,email,birth from customers where id=?
Customer [id=3, name=ABCD, [email protected], birth=1992-06-07]
select flow_id flowId,type, idcard idCard, exam_card examCard,student_name studentName,location,grade from examstudent where flow_id=?
Student [flowId=3, type=6, idCard=371522199206078411, examCard=2015534083, studentName=li, location=dalian, grade=87]