<%@ page language="java" contentType="text/html; charset=gbk"?? ?pageEncoding="gbk"%><%@ page import="java.sql.*"%><%!String str = null;?? ?private void tree(Connection conn, int id, int level) {?? ??? ?Statement stmt = null;?? ??? ?ResultSet rs = null;?? ??? ?String preStr = "";?? ??? ?for (int i = 0; i < level; i++) {?? ??? ??? ?preStr += "----";?? ??? ?}?? ??? ?try {?? ??? ??? ?stmt = conn.createStatement();?? ??? ??? ?String sql = "select * from article where pid = " + id;?? ??? ??? ?rs = stmt.executeQuery(sql);?? ??? ??? ?while (rs.next()) {?? ??? ??? ??? ?str += "<tr><td>" + rs.getInt("id") + "</td><td>" + preStr + rs.getString("title") + "</td></tr>";?? ??? ??? ??? ?if (rs.getInt("isleaf") != 0) {?? ??? ??? ??? ??? ?tree(conn, rs.getInt("id"), level + 1);?? ??? ??? ??? ?}?? ??? ??? ?}?? ??? ?} catch (Exception e) {?? ??? ??? ?e.printStackTrace();?? ??? ?} finally {?? ??? ??? ?try {?? ??? ??? ??? ?if (rs != null) {?? ??? ??? ??? ??? ?rs.close();?? ??? ??? ??? ??? ?rs = null;?? ??? ??? ??? ?}?? ??? ??? ??? ?if (stmt != null) {?? ??? ??? ??? ??? ?stmt.close();?? ??? ??? ??? ??? ?stmt = null;?? ??? ??? ??? ?}?? ??? ??? ?} catch (SQLException e) {?? ??? ??? ??? ?e.printStackTrace();?? ??? ??? ?}?? ??? ?}?? ?}%><%?? ?Class.forName("com.mysql.jdbc.Driver");?? ?//System.out.println("1");?? ?String url = "jdbc:mysql://localhost/test?user=root&password=root";?? ?Connection conn = DriverManager.getConnection(url);?? ?Statement stmt = conn.createStatement();?? ?ResultSet rs = stmt.executeQuery("select * from article where pid = 0");?? ?//System.out.println("2");?? ?while (rs.next()) {?? ??? ?str += "<tr><td>" + rs.getInt("id") + "</td><td>" + rs.getString("title") + "</td></tr>";?? ??? ?System.out.println(str);?? ??? ?if (rs.getInt("isleaf") != 0) {?? ??? ??? ?tree(conn, rs.getInt("id"), 1);?? ??? ?}?? ?}?? ?rs.close();?? ?stmt.close();?? ?conn.close();%><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=gbk"><title>Insert title here</title></head><body>?? ?<table border="1"><%=str%></table></body></html>問題： String str = null;? 如果我這樣使用 瀏覽器中顯示結果為??? 在表格上方會有一個null? 但是我用String str = “”就不會出現這種問題,這是為什么呢？
關于在jsp中String = null 和String = “”的使用問題！
月亮島Superman
2017-08-04 13:32:18
