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Application design , For example, code generator , It requires database metadata .
If you know database metadata , You can have a deeper understanding of some frameworks of the database , for example jpa,Mybatis.

1.3 How to get metadata

We are JAVA Operation database , does JDBC, Whether it's MySQL still Oracle Or other databases , Basically through JDBC To deal with the database .

Use JDBC There are three main interfaces to handle databases , namely Connection,PreparedStatement and ResultSet These three interfaces .

For these three interfaces , You can also get different types of metadata .

Interface	explain
Connection	Get database metadata （DatabaseMetaData）
PreparedStatement	Get the parameter metadata we get by sending the request （ParameterMetaData）
ResultSet	Get result set metadata （ResultSetMetaData）

The following will introduce the three types of metadata objects respectively and use MYSQL Database for case description .

2、 Database metadata

Database metadata （DatabaseMetaData）： By Connection Object passing getMetaData Method , It mainly encapsulates some overall comprehensive information about the database itself , For example, the name of the database , Version number of the database , Database URL, Whether transactions are supported .

Here are some about DatabaseMetaData The common method of ：

Method	explain
getDatabaseProductName	Get the name of the database
getDatabaseProductName	Get the version number of the database
getUserName	Get the user name of the database
getURL	Get the database connection URL
getDriverName	Get the driver name of the database
driverVersion	Get the driver version number of the database
isReadOnly	Check whether the database only allows read operations
supportsTransactions	Check whether the database supports transactions

2.1 Constructing environment

I just picked up one that I used to do Demo Of project (SpringBoot project ) To write .

Introduced mySql rely on .

      <!--mysql-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>

Since it is SpringBoot project , That is, of course, direct use yml file Configure the database connection .

server:
  port: 80
spring:
  application:
    name: mp
  datasource: #  Configure data source information 
    url: jdbc:mysql://127.0.0.1:3306/mybatis_plus?characterEncoding=utf-8&useSSL=false
    username: root
    password: 1234
    driver-class-name: com.mysql.cj.jdbc.Driver

2.2 Obtain comprehensive information of the database

@SpringBootTest
class MetadataTest {

    //  data source 
    @Resource
    DataSource dataSourcee;

    @Test
    void Test01() {
        try {
            // Get database metadata 
            DatabaseMetaData dbMetaData = dataSourcee.getConnection().getMetaData();
            // Get database product name 
            String productName = dbMetaData.getDatabaseProductName();
            System.out.println(" Database product name :" + productName);

            //  Get the database version number 
            String productVersion = dbMetaData.getDatabaseProductVersion();
            System.out.println(" Database version number :"+productVersion);

            //  Get the database user name 
            String userName = dbMetaData.getUserName();
            System.out.println(" Database user name :"+userName);

        } catch (Exception throwables) {
            throwables.printStackTrace();
        }
    }
}

Here are three , It's all the same anyway , The rest of us try it by ourselves .
effect ：

2.3 Get the database list

@SpringBootTest
class MetadataTest {

    //  data source 
    @Resource
    DataSource dataSourcee;

    @Test
    void Test01() {
        try {
            // Get database metadata 
            DatabaseMetaData dbMetaData = dataSourcee.getConnection().getMetaData();
            
            // Get the database list 
            ResultSet rs = dbMetaData.getCatalogs();
            // Traverse to get all database tables 
            while (rs.next()) {
                // Print database name 
                System.out.println(" Table name ："+rs.getString(1));
            }
            // Release resources 
            rs.close();
            dataSourcee.getConnection().close();
        } catch (Exception throwables) {
            throwables.printStackTrace();
        }
    }

2.4 Get all the table information in a database

@Test
    void Test01() {
        try {
            // Get database metadata 
            DatabaseMetaData dbMetaData = dataSourcee.getConnection().getMetaData();
            //  Get the specified table information in the specified database 
            ResultSet tablers = dbMetaData.getTables(null, null, "t_user", new String[]{"TABLE"});
            // assemble table
            while (tablers.next()) {
                // Database  1 || TABLE_CAT
                System.out.println(" Database :" + tablers.getString("TABLE_CAT"));
                // Table mode  2 || TABLE_SCHEM
                System.out.println(" Table mode :" + tablers.getString("TABLE_SCHEM"));
                // Table name  3 || TABLE_NAME
                System.out.println(" Table name :" + tablers.getString("TABLE_NAME"));
                // Database table type  4 || TABLE_TYPE
                System.out.println(" type :" + tablers.getString("TABLE_TYPE"));
                // Database table remarks  5 || REMARKS
                System.out.println(" remarks :" + tablers.getString("REMARKS"));
            }
        } catch (Exception throwables) {
            throwables.printStackTrace();
        }
    }

The method of obtaining table data getTables Medium 4 The parameters are ：

Parameters	explain
catalog	Database name ,null All databases
schemaPattern	Schema name , stay mysql There is no special significance in , So fill it directly null .
tableNamePattern	Table name ,null Is all the tables
types[]	type ： TABLE： surface VIEW: View

Let's see the implementation effect ：

2.5 Get the field properties in the specified database table

@Test
    void Test01() {
        try {
            // Get database metadata 
            DatabaseMetaData dbMetaData = dataSourcee.getConnection().getMetaData();
            //  Get the field information in the specified database table 
            ResultSet columns = dbMetaData.getColumns("mybatis_plus", null, "t_user", null);
            // assemble table
            while (columns.next()) {
                // Database  1 || TABLE_CAT
                System.out.println(" Database :" + columns.getString("TABLE_CAT"));
                // Table mode  2 || TABLE_SCHEM
                System.out.println(" Table mode :" + columns.getString("TABLE_SCHEM"));
                // Table name  3 || TABLE_NAME
                System.out.println(" Table name :" + columns.getString("TABLE_NAME"));
                //  Name  4 || COLUMN_NAME
                System.out.println(" Name :" + columns.getString("COLUMN_NAME"));
                // Database table type  5 || DATA_TYPE
                System.out.println(" type :" + columns.getString("DATA_TYPE"));
                // Database table remarks  12 || REMARKS
                System.out.println(" remarks :" + columns.getString("REMARKS"));
            }
        } catch (Exception throwables) {
            throwables.printStackTrace();
        }
    }

This has not changed much , Just another way getColumns

The same parameter is that the last one becomes Name ,null It means finding all the columns .

3、 Parameter metadata

Parameter metadata （ParameterMetaData）： By PreparedStatement Object passing getParameterMetaData Method , Mainly aimed at PreparedStatement Object and its precompiled SQL Command statements provide some information ,ParameterMetaData The number of placeholder parameters that can be provided , Gets the placeholder for the specified location SQL Type, etc. .

3.1 Get precompiled SQL The number of placeholder parameters in the statement

@Test
    void Test01() {
        try {
            String sql = "select * from t_user where uid = ?";
            PreparedStatement pstmt = dataSourcee.getConnection().prepareStatement(sql);
            // obtain ParameterMetaData object 
            ParameterMetaData paramMetaData = pstmt.getParameterMetaData();
            pstmt.setString(1, "1");
            // Get the number of parameters 
            int paramCount = paramMetaData.getParameterCount();
            System.out.println(paramCount);
        } catch (Exception throwables) {
            throwables.printStackTrace();
        }
    }

Of course, there are many other ways . This pair just learned JDBC Students who connect to the database should be familiar with , Anyway, I almost forgot .

4、 ResultSet Metadata

ResultSet Metadata （ResultSetMetaData）： By ResultSet Object passing getMetaData Method , Mainly for the data Library executed SQL The result set object obtained by the script command ResultSet Some of the information provided in , For example, the number of columns in the result set 、 Specify the name of the column 、 Specifies the name of the column SQL Type, etc. , It can be said that this is a very important object for the framework .

Common methods are ：

Method	explain
getColumnCount	Get the number of column items in the result set
getColumnType	Gets the SQL The type corresponds to Java in Types Class field
getColumnTypeName	Gets the SQL type
getClassName	Get the specified column SQL The type corresponds to Java The type of ( Package name plus class name )

@Test
    void Test01() {
        try {
            String sql = "select * from t_user where uid = ?";
            PreparedStatement pstmt = dataSourcee.getConnection().prepareStatement(sql);
            // obtain ParameterMetaData object 
            ParameterMetaData paramMetaData = pstmt.getParameterMetaData();
            pstmt.setString(1, "1");
            // perform sql sentence 
            ResultSet rs = pstmt.executeQuery();
            // obtain ResultSetMetaData object 
            ResultSetMetaData metaData = rs.getMetaData();
            // Get the number of query fields 
            int columnCount = metaData.getColumnCount();
            for (int i = 1; i <= columnCount; i++) {
                // Get column name 
                String columnName = metaData.getColumnName(i);
                // obtain java type 
                String columnClassName = metaData.getColumnClassName(i);
                // obtain sql type 
                String columnTypeName = metaData.getColumnTypeName(i);
                System.out.println(" Column name "+columnName);
                System.out.println("java type "+columnClassName);
                System.out.println("sql type "+columnTypeName);
            }
            System.out.println(columnCount);
        } catch (Exception throwables) {
            throwables.printStackTrace();
        }
    }

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