java如何检查字符串是否只有数字

Question

我有一段代码，它向列中添加了一个默认值。如果我的字符串只由数字组成，但如果我有其他不是数字的字符（例如：字母，！），它就完全可以工作SQL返回错误，因为必须使用量化标记

如何检查字符串是否只有数字

if(defaultValue!=null)
{
   sqlStatement+=" DEFAULT "+this.defaultValue;
}

我想我还需要避开引号，对吗？我怎么做

if (this.getSuffix() != null) {
                while (rs.next()) {
                    String tableName = rs.getString(3);
                    if (tableName.endsWith(this.getSuffix())) {
                        tablesFound = true;
                        if (!checkColumnsExists(s, tableName)) {
                            String sql = sqlStatement.replaceAll("name", tableName);
                            System.out.println("SQL:"+sql);
                            s.execute(sql);
                            columnsAdded = true;
                        }
                    }
                }
            }

带PreparedStatement的代码

public void execute(Database database) throws CustomChangeException {

        try {
            //check if there are any errors in the changelog file
            checkArgumentsNumber();

            boolean tablesFound = false;
            boolean columnsAdded = false;
            String sqlStatement = "ALTER TABLE NAME ADD COLUMN " +this.getColumnName()+" "+this.getColumnType();

            if(notNull){
                sqlStatement+=" NOT NULL";
            }

            if(defaultValue!=null){
                sqlStatement+=" DEFAULT ?";
            }

            if (this.after != null) {
                sqlStatement += " AFTER " +this.after;
            }

            System.out.println("The statement is: "+sqlStatement);

            //get tables in the database
            JdbcConnection connection = (JdbcConnection) database.getConnection();
            DatabaseMetaData metadata;
            metadata = connection.getMetaData();
            String[] types = {"TABLE"};
            ResultSet rs = metadata.getTables(connection.getCatalog(), null, "%", types);



            //if the user chose to use a suffix
            if (this.getSuffix() != null) {
                while (rs.next()) {
                    String tableName = rs.getString(3);
                    if (tableName.endsWith(this.getSuffix())) {
                        tablesFound = true;
                        String addStatement=sqlStatement.replaceAll("NAME",tableName);
                        PreparedStatement s = connection.prepareStatement(addStatement);
                        if (!checkColumnsExists(s, tableName)) {
                            if(this.defaultValue!=null){
                                System.out.println("ENTERED DEFAULT");
                                s.setString(1,this.defaultValue);
                            }
                            s.executeUpdate();
                            columnsAdded = true;
                        }
                    }
                }
            }

           

            checkInvalidInfo(tablesFound, columnsAdded, "All the matching tables already had that column");
        } catch (InvalidArgumentsNumberException | SQLException | InvalidInfoException | DatabaseException e) {
            throw new CustomChangeException("Error enabling trigger: " + e);
        }
    }

Answer 1

你也可以用try-catch

try {
    // try to convert to number if it's integer
    Long number = Long.valueOf("number");
         
    // without question
    sqlStatement+=" DEFAULT "+ number; 

} catch (Exception ignored){
    // default value or string with question
    sqlStatement+=" DEFAULT "+this.defaultValue;
}

更新

也可以是这样

Long number = null;
try {
    Long number = Long.valueOf("string number");
} catch (Exception ignored){}

// #1 if you have just number field
sqlStatement+= number != null ? number : this.defaultValue;

// #2 Or if want to use question
if(number != null)
    sqlStatement+= number       // without question
else
    sqlStatement+= " DEFAULT "  // without question

Answer 2

String s = "Example String";
if (s.matches("[0-9]+")) {
  System.out.println("Only numbers");
}else{
  System.out.println("Contains spaces or letters");
}

Answer 3

可以使用正则表达式

String str=" your string 123";
if(str.matches("[0-9]+")){
//Do something
}

Answer 4

这不是您应该如何构造SQL的，因为它使您对SQL injection attacks敞开大门

您应该改用JDBCPreparedStatement类。这使您可以在需要参数的地方使用?字符编写SQL，然后以编程方式设置值。它处理所有适当的转义以防止SQL注入

这是Oracle Java Tutorial for Prepared Statements