【Java/JDBC】利用ResultSetMetaData从数据库的某表中获取字段信息并存到csv文件

代码下载：https://files.cnblogs.com/files/xiandedanteng/FindNotnullColumns20191102-2.rar

这篇还不够完善，请看更完善的续篇 https://www.cnblogs.com/xiandedanteng/p/11783796.html

本例主要使用的是JDBC提供的ResultSetMetaData类去取表的相关信息，文中只取了四种，其它信息大家可以查看这个类的方法而得知。

JavaCode：

package com.hy;

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.text.MessageFormat;
import java.util.ArrayList;
import java.util.List;

import org.apache.log4j.Logger;

class Field{
    String name;    // 列名
    int size;        // 列容量
    String type;    // 列数据类型
    boolean allowNull;// 列是否允許为空
}

/**
 * 利用ResultSetMetaData从数据库的某表中获取字段信息并存到csv文件
 * @author horn1
 *
 */
public class ColumnSeeker {
    private static Logger log = Logger.getLogger(ColumnSeeker.class);

    private static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    private static final String DB_URL = "jdbc:mysql://192.168.161.130:3306/test";
    private static final String USER = "root";
    private static final String PSWD = "12345678";
    
    private List<Field> fields;
    
    /**
     * 从DB里取字段
     * @param tablename
     * @return
     */
    public List<Field> fetchAllColumns(String tablename){
        fields=new ArrayList<Field>();
        
        Connection conn = null;
        Statement stmt = null;

        try {
            Class.forName(JDBC_DRIVER).newInstance();
            conn = DriverManager.getConnection(DB_URL, USER, PSWD);
            stmt = conn.createStatement();
            
            String sql = "select * from emp order by id limit 1";
            ResultSet rs = stmt.executeQuery(sql);
            ResultSetMetaData rsMetadata = rs.getMetaData();

            while (rs.next()) {
                int count = rsMetadata.getColumnCount();
                for (int i=1; i<count+1; i++) {
                    // 列名
                    String columnName = rsMetadata.getColumnLabel(i);
                    // 列容量
                    int size =rsMetadata.getColumnDisplaySize(i);
                    // 列数据类型
                    String columnType = rsMetadata.getColumnTypeName(i);
                    // 列是否允許为空
                    boolean allowNull=(rsMetadata.isNullable(i)!=0);
                    
                    /*String raw = "columnName={0},size={1},columnType={2},allowNull={3}";
                    Object[] arr = { columnName, size, columnType,allowNull};
                    String outStr = MessageFormat.format(raw, arr);
                    log.info(outStr);*/
                    
                    Field f=new Field();
                    f.name=columnName;
                    f.size=size;
                    f.type=columnType;
                    f.allowNull=allowNull;
                    fields.add(f);
                }
            }
            
            rs.close();
        } catch (Exception e) {
            System.out.print("DB/SQL ERROR:" + e.getMessage());
        } finally {
            try {
                stmt.close();
                conn.close();
            } catch (SQLException e) {
                System.out.print("Can't close stmt/conn because of " + e.getMessage());
            }
        }
        
        return fields;
    }
    
    /**
     * 打印全部字段
     */
    public void printFields() {
        for(Field f:fields) {
            String raw = "column name={0},size={1},type={2},allowNull={3}";
            Object[] arr = { f.name, f.size, f.type,f.allowNull};
            String outStr = MessageFormat.format(raw, arr);
            //log.info(outStr);
        }
    }
    
    /**
     * 输出全部列到csv文件
     * @param csvFilename
     * @return
     */
    public boolean saveAllToCsv(String csvFilename) {
        try {
            FileWriter fileWriter = new FileWriter(new File(csvFilename), true);

            List<String> allowNulls=new ArrayList<String>();
            List<String> allowType=new ArrayList<String>();
            List<String> allowSize=new ArrayList<String>();
            List<String> allowName=new ArrayList<String>();
            
            for(Field f:fields) {
                allowNulls.add(String.valueOf(f.allowNull));
                allowType.add(f.type);
                allowSize.add(String.valueOf(f.size));
                allowName.add(f.name);
            }
            
            String line="";
            
            line =String.join(",", allowSize)+ System.getProperty("line.separator");
            fileWriter.write(line);
            
            line =String.join(",", allowType)+ System.getProperty("line.separator");
            fileWriter.write(line);
            
            line =String.join(",", allowNulls)+ System.getProperty("line.separator");
            fileWriter.write(line);
            
            line =String.join(",", allowName)+ System.getProperty("line.separator");
            fileWriter.write(line);

            fileWriter.flush();
            fileWriter.close();
            
            return true;
        } catch (IOException e) {
            e.printStackTrace();
        }
        
        return false;
    }
    
    /**
     * 仅将非空列输出到CSV
     * @param csvFilename
     * @return
     */
    public boolean saveNotnullToCsv(String csvFilename) {
        try {
            FileWriter fileWriter = new FileWriter(new File(csvFilename), true);

            List<String> allowNulls=new ArrayList<String>();
            List<String> allowType=new ArrayList<String>();
            List<String> allowSize=new ArrayList<String>();
            List<String> allowName=new ArrayList<String>();
            
            for(Field f:fields) {
                if(f.allowNull==false) {
                    allowNulls.add(String.valueOf(f.allowNull));
                    allowType.add(f.type);
                    allowSize.add(String.valueOf(f.size));
                    allowName.add(f.name);
                }
            }
            
            String line="";
            
            line =String.join(",", allowSize)+ System.getProperty("line.separator");
            fileWriter.write(line);
            
            line =String.join(",", allowType)+ System.getProperty("line.separator");
            fileWriter.write(line);
            
            line =String.join(",", allowNulls)+ System.getProperty("line.separator");
            fileWriter.write(line);
            
            line =String.join(",", allowName)+ System.getProperty("line.separator");
            fileWriter.write(line);

            fileWriter.flush();
            fileWriter.close();
            
            return true;
        } catch (IOException e) {
            e.printStackTrace();
        }
        
        return false;
    }
    
    public static void main(String[] args) {
        ColumnSeeker cs=new ColumnSeeker();
        cs.fetchAllColumns("emp");
        cs.printFields();
        cs.saveAllToCsv("d:\\all.csv");
        cs.saveNotnullToCsv("d:\\notnulls.csv");
        
        log.info("Completed");
    }
}