try-catch中导致全局变量无法变化的bug

最新推荐文章于 2024-07-16 16:13:52 发布

转载最新推荐文章于 2024-07-16 16:13:52 发布 · 619 阅读

0 ·

CC 4.0 BY-SA版权

原文链接：https://my.oschina.net/iioschina/blog/847725

文章标签：

#python #scala #java

本文探讨了Java程序中try-catch结构对外部全局变量的影响，特别是在count++操作的位置变化如何影响变量count的值。同时，通过一个具体的例子展示了数据库连接和SQL查询的异常处理过程。

2019独角兽企业重金招聘Python工程师标准>>>

当count++出现在try-catch之外或者exception代码之前时，全局变量加成功，在exception代码之后全局count都为1；

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

public class TestTryCatch {
private static int count = 1;
private static int sum=0;

   public static void main(String[] args) {
       while(true){
           sum++;
           if(sum>10){
               break;
           }
           reduce();
           System.out.println(count);
       }

}

   private static void reduce() {
       count++;
       try {


           Class.forName("com.mysql.jdbc.Driver");
           String url="jdbc:mysql://localhost:3306/mysql";
           String username="root";
           String password="root";
           String sql="select * * from help_topic";
           Connection conn=DriverManager.getConnection(url,username,password);
           java.sql.PreparedStatement pstmt=conn.prepareStatement(sql);
           ResultSet rs=pstmt.executeQuery();
           List<Integer> list=new ArrayList<Integer>();
           while(rs.next()){
               int helpTopicId=rs.getInt(1);
               list.add(helpTopicId);
           }
             count++;
       } catch (ClassNotFoundException | SQLException e) {
           e.printStackTrace();
       }

}

结果

1
   at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
   at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
   at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
   at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
   at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
   at com.mysql.jdbc.Util.getInstance(Util.java:386)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1052)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3597)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3529)
   at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1990)
   at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2151)
   at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2625)
   at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2119)
   at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:2281)


com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '* from help_topic' at line 1
   at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
   at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
   at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
   at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
   at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
   at com.mysql.jdbc.Util.getInstance(Util.java:386)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1052)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3597)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3529)
   at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1990)
   at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2151)
   at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2625)
   at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2119)
   at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:2281)

1
1
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '* from help_topic' at line 1
   at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
   at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
   at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
   at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
   at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
   at com.mysql.jdbc.Util.getInstance(Util.java:386)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1052)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3597)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3529)
   at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1990)
   at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2151)
   at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2625)
   at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2119)
   at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:2281)

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '* from help_topic' at line 1
   at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
   at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
   at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
   at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
   at com.mysql.jdbc.Util.handleNewInstance(Util.java:411)
   at com.mysql.jdbc.Util.getInstance(Util.java:386)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1052)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3597)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3529)
   at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1990)
   at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2151)
   at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2625)
   at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2119)
   at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:2281)

1