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上篇播客介绍了SQL注入问题。本播客将介绍PreparedStatement来解决该问题。PreparedStatement：
1 可以通过调用 Connection 对象的 preparedStatement() 方法获取 PreparedStatement 对象。
2 PreparedStatement 接口是 Statement 的子接口，它表示一条预编译过的 SQL 语句。
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3 PreparedStatement 对象所代表的 SQL 语句中的参数用问号(?)来表示，调用 PreparedStatement 对象的 setXXX() 方法来设置这些参数. setXXX() 方法有两个参数，第一个参数是要设置的 SQL 语句中的参数的索引(从 1 开始)，第二个是设置的 SQL 语句中的参数的值。
/** * 使用 PreparedStatement 将有效的解决 SQL 注入问题. */ @Test public void testSQLInjection2() { String username = "a' OR PASSWORD = "; String password = " OR '1'='1"; String sql = "SELECT * FROM users WHERE username = ? " + "AND password = ?"; Connection connection = null; PreparedStatement preparedStatement = null; ResultSet resultSet = null; try { connection = JDBCTools.getConnection(); preparedStatement = connection.prepareStatement(sql); preparedStatement.setString(1, username); preparedStatement.setString(2, password); resultSet = preparedStatement.executeQuery(); if (resultSet.next()) { System.out.println("登录成功!"); } else { System.out.println("用户名和密码不匹配或用户名不存在. "); } } catch (Exception e) { e.printStackTrace(); } finally { JDBCTools.releaseDB(resultSet, preparedStatement, connection); } }
PreparedStatement vs Statement
代码的可读性和可维护性. （防止SQL语句写错）
PreparedStatement 能最大可能提高性能：
DBServer会对预编译语句提供性能优化。因为预编译语句有可能被重复调用，所以语句在被DBServer的编译器编译后的执行代码被缓存下来，那么下次调用时只要是相同的预编译语句就不需要编译，只要将参数直接传入编译过的语句执行代码中就会得到执行。
在statement语句中,即使是相同操作但因为数据内容不一样,所以整个语句本身不能匹配,没有缓存语句的意义.事实是没有数据库会对普通语句编译后的执行代码缓存.这样每执行一次都要对传入的语句编译一次.
(语法检查，语义检查，翻译成二进制命令，缓存)
PreparedStatement 可以防止 SQL 注入
PreparedStatement更新（插入 删除 更新）操作
@Test
public void testPreparedStatement() {
Connection connection = null;
PreparedStatement preparedStatement = null;
try {
connection = JDBCTools.getConnection();
String sql = "INSERT INTO customers (name, email, birth) "
+ "VALUES(?,?,?)";
preparedStatement = connection.prepareStatement(sql);
preparedStatement.setString(1, "ATGUIGU");
preparedStatement.setString(2, "simpleit@163.com");
preparedStatement.setDate(3,
new Date(new java.util.Date().getTime()));
preparedStatement.executeUpdate();
} catch (Exception e) {
e.printStackTrace();
} finally {
JDBCTools.releaseDB(null, preparedStatement, connection);
}
}
PreparedStatement查询操作
/**
* 具体查询学生信息的. 返回一个 Student 对象. 若不存在, 则返回 null
*
* @param searchType
* : 1 或 2.
* @return
*/
private Student searchStudent(int searchType) {
String sql = "SELECT flowid, type, idcard, examcard,"
+ "studentname, location, grade " + "FROM examstudent "
+ "WHERE ";
Scanner scanner = new Scanner(System.in);
// 1. 根据输入的 searchType, 提示用户输入信息:
// 1.1 若 searchType 为 1, 提示: 请输入身份证号. 若为 2 提示: 请输入准考证号
// 2. 根据 searchType 确定 SQL
if (searchType == 1) {
System.out.print("请输入准考证号:");
String examCard = scanner.next();
sql = sql + "examcard = '" + examCard + "'";
} else {
System.out.print("请输入身份证号:");
String examCard = scanner.next();
sql = sql + "idcard = '" + examCard + "'";
}
// 3. 执行查询
Student student = getStudent(sql);
// 4. 若存在查询结果, 把查询结果封装为一个 Student 对象
return student;
}
/**
* 根据传入的 SQL 返回 Student 对象
*
* @param sql
* @return
*/
public Student getStudent(String sql, Object... args) {
Student stu = null;
Connection connection = null;
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;
try {
connection = JDBCTools.getConnection();
preparedStatement = connection.prepareStatement(sql);
for (int i = 0; i < args.length; i++) {
preparedStatement.setObject(i + 1, args[i]);
}
resultSet = preparedStatement.executeQuery();
if (resultSet.next()) {
stu = new Student();
stu.setFlowId(resultSet.getInt(1));
stu.setType(resultSet.getInt(2));
stu.setIdCard(resultSet.getString(3));
// ...
}
} catch (Exception e) {
e.printStackTrace();
} finally {
JDBCTools.releaseDB(resultSet, preparedStatement, connection);
}
return stu;
}
/**
* 从控制台读入一个整数, 确定要查询的类型
*
* @return: 1. 用身份证查询. 2. 用准考证号查询 其他的无效. 并提示请用户重新输入.
*/
private int getSearchTypeFromConsole() {
System.out.print("请输入查询类型: 1. 用身份证查询. 2. 用准考证号查询 ");
Scanner scanner = new Scanner(System.in);
int type = scanner.nextInt();
if (type != 1 && type != 2) {
System.out.println("输入有误请重新输入!");
throw new RuntimeException();
}
return type;
}
Student类：
public class Student {
// 流水号
private int flowId;
// 考试的类型
private int type;
// 身份证号
private String idCard;
// 准考证号
private String examCard;
// 学生名
private String studentName;
// 学生地址
private String location;
// 考试分数.
private int grade;
public int getFlowId() {
return flowId;
}
public void setFlowId(int flowId) {
this.flowId = flowId;
}
public int getType() {
return type;
}
public void setType(int type) {
this.type = type;
}
public String getIdCard() {
return idCard;
}
public void setIdCard(String idCard) {
this.idCard = idCard;
}
public String getExamCard() {
return examCard;
}
public void setExamCard(String examCard) {
this.examCard = examCard;
}
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
public String getLocation() {
return location;
}
public void setLocation(String location) {
this.location = location;
}
public int getGrade() {
return grade;
}
public void setGrade(int grade) {
this.grade = grade;
}
public Student(int flowId, int type, String idCard, String examCard,
String studentName, String location, int grade) {
super();
this.flowId = flowId;
this.type = type;
this.idCard = idCard;
this.examCard = examCard;
this.studentName = studentName;
this.location = location;
this.grade = grade;
}
public Student() {
// TODO Auto-generated constructor stub
}
@Override
public String toString() {
return "Student [flowId=" + flowId + ", type=" + type + ", idCard="
+ idCard + ", examCard=" + examCard + ", studentName="
+ studentName + ", location=" + location + ", grade=" + grade
+ "]";
}
}