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📜  JDBC-异常处理

📅  最后修改于: 2020-11-13 04:35:55             🧑  作者: Mango

异常处理使您能够以受控方式处理特殊情况，例如程序定义的错误。

发生异常情况时，将引发异常。抛出这个词意味着当前程序停止执行，并且控件被重定向到最近的适用catch子句。如果不存在适用的catch子句，则程序的执行结束。

JDBC异常处理与Java异常处理非常相似，但是对于JDBC，您要处理的最常见的异常是java.sql.SQLException。

SQLException方法

在驱动程序和数据库中都可能发生SQLException。发生此类异常时，会将SQLException类型的对象传递给catch子句。

传递的SQLException对象具有以下可用于检索有关异常的其他信息的方法-

Method Description
getErrorCode( ) Gets the error number associated with the exception.
getMessage( ) Gets the JDBC driver’s error message for an error, handled by the driver or gets the Oracle error number and message for a database error.
getSQLState( ) Gets the XOPEN SQLstate string. For a JDBC driver error, no useful information is returned from this method. For a database error, the five-digit XOPEN SQLstate code is returned. This method can return null.
getNextException( ) Gets the next Exception object in the exception chain.
printStackTrace( ) Prints the current exception, or throwable, and it’s backtrace to a standard error stream.
printStackTrace(PrintStream s) Prints this throwable and its backtrace to the print stream you specify.
printStackTrace(PrintWriter w) Prints this throwable and it’s backtrace to the print writer you specify.

通过利用Exception对象提供的信息，您可以捕获异常并适当地继续执行程序。这是try块的一般形式-

try {
   // Your risky code goes between these curly braces!!!
}
catch(Exception ex) {
   // Your exception handling code goes between these 
   // curly braces, similar to the exception clause 
   // in a PL/SQL block.
}
finally {
   // Your must-always-be-executed code goes between these 
   // curly braces. Like closing database connection.
}

例

研究以下示例代码，以了解try …. catch … finally块的用法。

//STEP 1. Import required packages
import java.sql.*;

public class JDBCExample {
   // JDBC driver name and database URL
   static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
   static final String DB_URL = "jdbc:mysql://localhost/EMP";

   //  Database credentials
   static final String USER = "username";
   static final String PASS = "password";
   
   public static void main(String[] args) {
   Connection conn = null;
   try{
      //STEP 2: Register JDBC driver
      Class.forName("com.mysql.jdbc.Driver");

      //STEP 3: Open a connection
      System.out.println("Connecting to database...");
      conn = DriverManager.getConnection(DB_URL,USER,PASS);

      //STEP 4: Execute a query
      System.out.println("Creating statement...");
      Statement stmt = conn.createStatement();
      String sql;
      sql = "SELECT id, first, last, age FROM Employees";
      ResultSet rs = stmt.executeQuery(sql);

      //STEP 5: Extract data from result set
      while(rs.next()){
         //Retrieve by column name
         int id  = rs.getInt("id");
         int age = rs.getInt("age");
         String first = rs.getString("first");
         String last = rs.getString("last");

         //Display values
         System.out.print("ID: " + id);
         System.out.print(", Age: " + age);
         System.out.print(", First: " + first);
         System.out.println(", Last: " + last);
      }
      //STEP 6: Clean-up environment
      rs.close();
      stmt.close();
      conn.close();
   }catch(SQLException se){
      //Handle errors for JDBC
      se.printStackTrace();
   }catch(Exception e){
      //Handle errors for Class.forName
      e.printStackTrace();
   }finally{
      //finally block used to close resources
      try{
         if(conn!=null)
            conn.close();
      }catch(SQLException se){
         se.printStackTrace();
      }//end finally try
   }//end try
   System.out.println("Goodbye!");
}//end main
}//end JDBCExample

现在，让我们编译以上示例，如下所示：

C:\>javac JDBCExample.java
C:\>

当您运行JDBCExample时，如果没有问题，它将产生以下结果，否则将捕获相应的错误并显示错误消息-

C:\>java JDBCExample
Connecting to database...
Creating statement...
ID: 100, Age: 18, First: Zara, Last: Ali
ID: 101, Age: 25, First: Mahnaz, Last: Fatma
ID: 102, Age: 30, First: Zaid, Last: Khan
ID: 103, Age: 28, First: Sumit, Last: Mittal
C:\>

通过传递错误的数据库名称或错误的用户名或密码来尝试上述示例，然后检查结果。