ОШИБКА: org.springframework.jdbc.BadSqlGrammarException: StatementCallback; плохая грамматика SQL Java Spring MVC

Я пытаюсь создать веб-приложение с использованием Java Spring MVC. Эта сеть в основном выполняет функцию CRUD (Create-Read-Update-Delete).

Недавно у меня появилась такая ошибка:

HTTP Status 500 - Request processing failed; nested exception is org.springframework.jdbc.BadSqlGrammarException: StatementCallback; bad SQL grammar [select * from assignment where username=reza]; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'reza' in 'where clause'

Это мой файл DAO:

    @Override
    public List < Assignment > showAllAssignment(String username) {
        String sql = "select * from assignment where username = " + username;
        return jdbcTemplate.query(sql, new AssignmentMapper());
    }

Это мой контроллер

 @RequestMapping(value = "/showAllAssignment/{reqUserName}/show", method = RequestMethod.GET)
 public ModelAndView showAllAssignment(@PathVariable("reqUserName") String reqUserName) {
     List < Assignment > list = new ArrayList < Assignment > ();
     list = assignmentService.showAllAssignment(reqUserName);
     ModelAndView mav = new ModelAndView("show_All_Assignments");
     mav.addObject("assignment", list);
     return mav;
 }

Дальнейшая ошибка, которую я получил:

2018-05-03 01:55:08,232 [org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver]-[DEBUG] Resolving exception from handler [public org.springframework.web.servlet.ModelAndView org.assignment.controller.AssignmentController.showAllAssignment(java.lang.String)]: org.springframework.jdbc.BadSqlGrammarException: StatementCallback; bad SQL grammar [select * from assignment where username=reza]; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'reza' in 'where clause'
2018-05-03 01:55:08,234 [org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver]-[DEBUG] Resolving exception from handler [public org.springframework.web.servlet.ModelAndView org.assignment.controller.AssignmentController.showAllAssignment(java.lang.String)]: org.springframework.jdbc.BadSqlGrammarException: StatementCallback; bad SQL grammar [select * from assignment where username=reza]; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'reza' in 'where clause'
2018-05-03 01:55:08,234 [org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver]-[DEBUG] Resolving exception from handler [public org.springframework.web.servlet.ModelAndView org.assignment.controller.AssignmentController.showAllAssignment(java.lang.String)]: org.springframework.jdbc.BadSqlGrammarException: StatementCallback; bad SQL grammar [select * from assignment where username=reza]; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'reza' in 'where clause'
2018-05-03 01:55:08,235 [org.springframework.web.servlet.DispatcherServlet]-[DEBUG] Could not complete request
org.springframework.jdbc.BadSqlGrammarException: StatementCallback; bad SQL grammar [select * from assignment where username=reza]; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'reza' in 'where clause'
    at org.springframework.jdbc.support.SQLErrorCodeSQLExceptionTranslator.doTranslate(SQLErrorCodeSQLExceptionTranslator.java:235)
    at org.springframework.jdbc.support.AbstractFallbackSQLExceptionTranslator.translate(AbstractFallbackSQLExceptionTranslator.java:72)
    at org.springframework.jdbc.core.JdbcTemplate.translateException(JdbcTemplate.java:1402)
    at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:388)
    at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:446)
    at org.springframework.jdbc.core.JdbcTemplate.query(JdbcTemplate.java:456)
    at org.assignment.dao.AssignmentDaoImpl.showAllAssignment(AssignmentDaoImpl.java:67)
    at org.assignment.service.AssignmentServiceImpl.showAllAssignment(AssignmentServiceImpl.java:39)
    at org.assignment.controller.AssignmentController.showAllAssignment(AssignmentController.java:193)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:209)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:870)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:776)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:870)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:855)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:502)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1539)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1495)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'reza' in 'where clause'
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:389)
    at com.mysql.jdbc.Util.getInstance(Util.java:372)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:980)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3835)
    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3771)
    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2435)
    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2582)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2531)
    at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2489)
    at com.mysql.jdbc.StatementImpl.executeQuery(StatementImpl.java:1446)
    at org.springframework.jdbc.core.JdbcTemplate$1QueryStatementCallback.doInStatement(JdbcTemplate.java:433)
    at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:376)
    ... 42 more

Проблема в том, что я хотел бы выбрать данные в моем назначении таблицы, где определено имя пользователя.

Например, String username1 = 'reza';. Но когда я пытаюсь показать все данные, используя синтаксис выше, select * .... where username = "+username1; В результате система считала резу как столбец, а не значение в столбце.

Кто-нибудь может помочь мне решить эту проблему?