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The reason why JAVAWEB dbutils cannot find content when executing SQL commands and traversing the result set, and the solution is as follows:
When traversing the result set, only bean objects are traversed to output the first line content (the first line is the output of an instance of the UserEntity class), so re.getRepoTableName() is needed here to call the corresponding content through the object
In this way, you can get the value.
PS: Detailed introduction to DBUtils in JavaWeb as follows:
First, what is DBUtils and its role
DBUtils is written by Apache company. DBUtils is a practical tool for database operations in Java programming, which is small, simple and practical.
DBUtils encapsulates the operations of JDBC, simplifying JDBC operations. It can write less code.
1.For read operations on data tables, it can convert the results into List, Array, Set and other Java collections, which is convenient for programmers to operate;
2.For write operations on data tables, it becomes very simple (just write sql statements)
3.It can use data source, JNDI, database connection pool and other technologies to optimize performance--Reuse the already built database connection object
Second, the three core objects of DBUtils
2}1、QueryRunner class
The QueryRunner provides API for sql statement operations. It mainly has three methods: query() for executing select, update() for executing insert update delete, and batch() for batch processing. Below, I will introduce the usage of these methods in detail.
2}2and ResultSetHandler interface
It is used to define how to encapsulate the result set after the select operation. It has a total of9common implementation classes, and I will introduce in detail how to use them for everyone.
2}3、DbUtils class
It is a utility class that defines methods for closing resources and transaction processing
Third, how to use the DBUtils framework
3}1、Using steps
Import the corresponding jar packages
Create a QueryRunner object
Use the query method to execute the select statement
Use ResultSetHandler to encapsulate the result set
Use the DbUtils class to release resources
3}2instance
Note: I use C3P0 connection pool
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.ResultSetHandler;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import org.junit.Test;
import com.jxlg.domain.User;
public class TestSelect {
@Test
public void testSelect(){
//Create a QueryRunner object
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
// The new ResultSetHandler<List<User>> tells us how to encapsulate the result set
List<User> list = qr.query("select * from user", new ResultSetHandler<List<User>>(){
@Override
//After the query statement executes the select statement, the result is passed back as a return value
public List<User> handle(ResultSet rs) throws SQLException {
List<User> list = new ArrayList<User>();
while(rs.next()){
User u = new User();
u.setId(rs.getInt(1));
u.setUsername(rs.getString(2));
u.setPassword(rs.getString(3));
u.setEmail(rs.getString(4));
u.setBirthday(rs.getDate(5));
list.add(u);
}
return list;
}
});
for (User user : list) {
System.out.println(user);
}
}
e.printStackTrace();
}
}
@Test
public void testSelect2(){
//Create a QueryRunner object
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
//Execute SQL statement, return results
List<User> list = qr.query("select * from user where id=? and username=?", new BeanListHandler<User>(User.class),1,"tom");
for (User user : list) {
System.out.println(user);
}
}
e.printStackTrace();
}
}
}
Four, Detailed Explanation of Three Core Objects of DBUtils
4}1and QueryRunner object
4}1}1and constructor
new QueryRunner(); Its transaction can be manually controlled.
That is, the methods called by this object (such as: query, update, batch) must have a Connection object in the parameters.
new QueryRunner(DataSource ds); Its transaction is automatically controlled. One SQL statement, one transaction.
The methods called by this object (such as: query, update, batch) do not require a Connection object.
4}1}2and common methods
4}2and ResultSetHandler interface
4}2}1and it has9a result processor
ArrayHandler: suitable for fetching1record. Each column value of the record is encapsulated into an array of Object[]
ArrayListHandler: suitable for fetching multiple records. Each record's column values are encapsulated into an array of Object[], and the arrays are encapsulated into a List
ColumnListHandler: Retrieve data of a specific column. Encapsulated into a List.
KeyedHandler: Retrieve multiple records, each record is encapsulated into a Map, and then this Map is encapsulated into another Map, with the key being the specified field value.
MapHandler: Suitable for fetching1records. Place the column name and column value of the current record into a Map
MapListHandler: Suitable for fetching multiple records. Wrap each record into a Map and then wrap the Map into a List
ScalarHandler: Suitable for fetching single row, single column data
BeanHandler
BeanListHandler
4}2}2instance
import static org.junit.Assert.*;
import java.sql.SQLException;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.ArrayHandler;
import org.apache.commons.dbutils.handlers.ArrayListHandler;
import org.apache.commons.dbutils.handlers.BeanHandler;
import org.apache.commons.dbutils.handlers.ColumnListHandler;
import org.apache.commons.dbutils.handlers.KeyedHandler;
import org.apache.commons.dbutils.handlers.MapHandler;
import org.apache.commons.dbutils.handlers.MapListHandler;
import org.apache.commons.dbutils.handlers.ScalarHandler;
import org.junit.Test;
import com.jxlg.domain.User;
public class TestResultSetHandler {
@Test
public void test1() {
//ArrayHandler: suitable for fetching1record. Each column value of the record is encapsulated into an array of Object[]
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
Object[] o = qr.query("select * from user where id=?", new ArrayHandler(),5);
for (Object object : o) {
System.out.println(object);
}
}
e.printStackTrace();
}
}
@Test
public void test2() throws SQLException {
//ArrayListHandler: suitable for fetching multiple records. Each record's column values are encapsulated into an array of Object[], and the arrays are encapsulated into a List
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
List<Object[]> list = qr.query("select * from user", new ArrayListHandler());
for (Object[] objects : list) {
for (Object object : objects) {
System.out.println(object);
}
System.out.println("----------------------");
}
}
@Test
public void test3() throws SQLException {
//ColumnListHandler: Fetch data of a specific column. Wrap it into a List
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
List<Object> list = qr.query("select username,password from user ", new ColumnListHandler(1));
for (Object object : list) {
System.out.println(object);
}
}
@Test
public void test4() throws SQLException {
//KeyedHandler: Fetch multiple records, each record wrapped into a Map,
//Then wrap this Map into another Map, with the key as the specified field value.
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
//The key of the large Map is a column data in the table, and the key of the small Map is the column name, so the large Map's key uses Object type, and the small one uses String.
Map<Object, Map<String, Object>> map = qr.query("select * from user", new KeyedHandler(1));
for (Map.Entry<Object, Map<String, Object>> m : map.entrySet()) {
System.out.println(m);//It is the id because it is set to "1".
for (Map.Entry<String, Object> mm : m.getValue().entrySet()) {
System.out.println(mm);//Extract the key and value from the small map
}
System.out.println("--------------------");
}
}
@Test
public void test5() throws SQLException {
//MapHandler: Suitable for fetching1records. Place the column name and column value of the current record into a Map
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
Map<String, Object> map = qr.query("select * from user", new MapHandler());
for (Map.Entry<String, Object> m : map.entrySet()) {
System.out.println(m.getKey())+"\t"+m.getValue());
//The default is to take the first row of data, and you need to add conditions with 'where' to access other rows
}
}
@Test
public void test6() throws SQLException {
//MapListHandler: Suitable for fetching multiple records. Wrap each record into a Map and then wrap the Map into a List
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
List<Map<String, Object>> list = qr.query("select * from user", new MapListHandler());
for (Map<String, Object> map : list) {
for (Map.Entry<String, Object> m : map.entrySet()) {
System.out.println(m);
}
System.out.println("-----------");
}
}
@Test
public void test7() throws SQLException {
//ScalarHandler: Suitable for fetching single row, single column data
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
Object o = qr.query("select * from user", new ScalarHandler(2));
System.out.println(o);
}
@Test
public void test8() throws SQLException {
//BeanHandler: Suitable for fetching single row, single column data
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
User user = qr.query("select * from user", new BeanHandler<User>(User.class));
System.out.println(user);
}
}
Five, use DBUtils to create an example of CRUD operations
import static org.junit.Assert.*;
import java.sql.SQLException;
import java.util.Date;
import javax.crypto.spec.OAEPParameterSpec;
import org.apache.commons.dbutils.QueryRunner;
import org.junit.Test;
public class TestInCURD {
@Test
public void testInsert() {
//Create a QueryRunner object
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
qr.update("insert into user (username,password,email,birthday)values(\"63;,?,?,?)", "guapi","4646","[email protected]",new Date());
}
e.printStackTrace();
}
}
@Test
public void testUpdate() {
//Create a QueryRunner object
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
qr.update("update user set username=\"63;,password=\"63; where id=4 ", "meizimeizi","520520");
}
e.printStackTrace();
}
}
@Test
public void testDelete() {
//Create a QueryRunner object
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
qr.update("delete from user where id=\"63; ","}}4);
}
e.printStackTrace();
}
}
@Test
public void testBatch() {
//Create a QueryRunner object
QueryRunner qr = new QueryRunner(C3P0Util.getDataSource());
try {
Object[][] params = new Object[10]]; //High-dimensional represents how many times the sql statement is executed
for(int i =0;i<params.length;i++){
params[i] =new Object[]{"guapi"+i,"4646","[email protected]",new Date()});
}
qr.batch("insert into user (username,password,email,birthday)values(?,?,?,?)", params );
}
e.printStackTrace();
}
}
}
Summary
The above-mentioned is the analysis of the reasons why the JavaWeb dbutils cannot find the content when executing sql commands and traversing the result set, I hope it will be helpful to everyone. If you have any questions, please leave a message, and I will reply to everyone in time. I also want to express my sincere gratitude to everyone for their support of the yanan tutorial!
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