关于在jsp中String = null 和String = “”的使用问题！
<%@ 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>
问题： String str = null; 如果我这样使用 浏览器中显示结果为 在表格上方会有一个null 但是我用String str = “”就不会出现这种问题,这是为什么呢？
月亮岛Superman2回答
-
拖鞋_
JSP 不要学了 现在都是前端静态化 为了 放入CDN JSP 什么的都不支持的 -
qq_逆流河_0
null标示你这个String对象不指向具体的内存数据，“”标识你这个对象指向的内存内容是空字符串
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Html/CSS
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