(1) Java實現JDBC連接及事務的方式

許多數據庫的auto-commit默認是ON的，比如MySQL，PostgresSQL等。當然也有默認是OFF的，比如Oracle（Oracle里面執行DML語句是需要手動commit的）。

這里我們以MySQL為例，先寫一個基本的JDBC連接的例子：

package com.mycloud.demo.connection;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;

public class ConncetionTest1 {

    public static void main(String[] args) {

        String jdbcUrl = "jdbc:mysql://localhost:3306/test_db";
        String username = "xxx";
        String password = "xxx";

        // Basic sample
        // Connection -> Statement -> ResultSet: try with resource
        try (Connection con = DriverManager.getConnection(jdbcUrl, username, password);
                Statement stmt = con.createStatement();
                ResultSet rs = stmt.executeQuery("select id, account from test")) {
            String result = getResultSetAsString(rs);
            System.out.println(result);
        } catch (SQLException e) {
            e.printStackTrace();
            throw new RuntimeException("Error occurred!");
        }
    }

    private static String getResultSetAsString(ResultSet rs) throws SQLException {

        ResultSetMetaData rsmd = rs.getMetaData();
        int numCols = rsmd.getColumnCount();
        StringBuilder sb = new StringBuilder();

        while (rs.next()) {
            for (int i = 1; i <= numCols; i++) {
                String elem = rs.getString(i);
                if (rs.wasNull())
                    sb.append("NULL");
                else
                    sb.append(elem);

                if (i != numCols)
                    sb.append("|");
            }
            sb.append(System.lineSeparator());
        }

        return sb.toString();
    }
}

 

在auto-commit默認是ON的情況下，每一條sql都是一個獨立的事務，運行完直接commit。但是如果需要實現事務，比如我們執行一組DML，如果某一條失敗，就全部rollback。這種方式就不行了：

        try (Connection conn = DriverManager.getConnection(jdbcUrl, username, password);
                Statement stmt = conn.createStatement()) {
            stmt.execute("INSERT INTO test(id, account) VALUES (1, 100)");
            stmt.execute("INSERT INTO test(id, account) VALUES (2, 200)");
            stmt.execute("INSERT INTO test(id, account) VALUES (2, 201)"); // Exception: duplicate pk
        } catch (SQLException e) {
            e.printStackTrace();
            throw new RuntimeException("Error occurred!");
        }

我們查詢數據庫發現，成功插入了兩條數據：

id	account
1	100
2	200

 

 

 

 

在這種情況下，我們首先要通過設置connection.setAutoCommit為OFF來開啟事務，再通過connection.commit/connection.rollback來提交/回滾事務。 

        try (Connection conn = DriverManager.getConnection(jdbcUrl, username, password);
                Statement stmt = conn.createStatement()) {

            conn.setAutoCommit(false);
            try {
                stmt.execute("INSERT INTO test(id, account) VALUES (1, 100)");
                stmt.execute("INSERT INTO test(id, account) VALUES (2, 200)");
                stmt.execute("INSERT INTO test(id, account) VALUES (3, 300)");
            } catch (SQLException e) {
                e.printStackTrace();
                conn.rollback(); // rollback
                throw e;
            }
            conn.commit(); // commit

        } catch (SQLException e) {
            e.printStackTrace();
            throw new RuntimeException("Error occurred!");
        }

我們查詢數據庫發現，成功插入了3條數據：

id	account
1	100
2	200
3	300

 

 

 

 

 

當然，如果我們去掉conn.commit()這一句，數據就不會插入了。

如果這一組DML中某一條失敗，則會被SQLException捕獲，從而拋出異常並回滾。

        try (Connection conn = DriverManager.getConnection(jdbcUrl, username, password);
                Statement stmt = conn.createStatement()) {

            conn.setAutoCommit(false);
            try {
                stmt.execute("INSERT INTO test(id, account) VALUES (1, 100)");
                stmt.execute("INSERT INTO test(id, account) VALUES (2, 200)");
                stmt.execute("INSERT INTO test(id, account) VALUES (2, 201)"); // Exception: duplicate pk
                stmt.execute("INSERT INTO test(id, account) VALUES (3, 300)");
            } catch (SQLException e) {
                e.printStackTrace();
                conn.rollback(); // rollback
                throw e;
            }
            conn.commit(); // commit

        } catch (SQLException e) {
            e.printStackTrace();
            throw new RuntimeException("Error occurred!");
        }

我們再查詢數據庫發現，並沒有數據插入。