Web 开发问题汇总(三)

1.How to send email in java?

import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;
import javax.activation.*;

public class SendEmail {

   public static void main(String [] args) {
      // Recipient's email ID needs to be mentioned.
      String to = "abcd@gmail.com";

      // Sender's email ID needs to be mentioned
      String from = "web@gmail.com";

      // Assuming you are sending email from localhost
      String host = "localhost";

      // Get system properties
      Properties properties = System.getProperties();

      // Setup mail server
      properties.setProperty("mail.smtp.host", host);

      // Get the default Session object.
      Session session = Session.getDefaultInstance(properties);

      try {
         // Create a default MimeMessage object.
         MimeMessage message = new MimeMessage(session);

         // Set From: header field of the header.
         message.setFrom(new InternetAddress(from));

         // Set To: header field of the header.
         message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));

         // Set Subject: header field
         message.setSubject("This is the Subject Line!");

         // Now set the actual message
         message.setText("This is actual message");

         // Send message
         Transport.send(message);
         System.out.println("Sent message successfully....");
      } catch (MessagingException mex) {
         mex.printStackTrace();
      }
   }
}

Reference:Java - Sending Email
Send email using java

2.Eclipse error: ‘Failed to create the Java Virtual Machine’

A:安装了 Eclipse 提示的升级后再重新打开时，出现上面的报错。Google 一圈后显示可能是 eclipse.ini 的锅，正好我安装了 cpp, java 和 jee 三个版本，尝试打开 java 版本的，正常打开，于是对比它们的 eclipse.ini 文件:

$ cp ~/eclipse/jee-oxygen/Eclipse.app/Contents/Eclipse/eclipse.ini  ~/eclipse-workspace/jee-eclipse.ini
cp ~/eclipse/java-oxygen/Eclipse.app/Contents/Eclipse/eclipse.ini ~/eclipse-workspace/java-eclipse.ini

$ diff eclipse-workspace/jee-eclipse.ini eclipse-workspace/java-eclipse.ini
3,4d2
< --launcher.library
< /Users/dongmeiliang/.p2/pool/plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.551.v20171108-1834
6a5,6
> --launcher.library
> /Users/dongmeiliang/.p2/pool/plugins/org.eclipse.equinox.launcher.cocoa.macosx.x86_64_1.1.551.v20171108-1834
8c8
< org.eclipse.epp.package.jee.product
---
> org.eclipse.epp.package.java.product
18,23d17
< -javaagent:/Users/dongmeiliang/.p2/pool/plugins/com.zeroturnaround.eclipse.optimizer.plugin_1.0.11/agent/eclipse-optimizer-agent.jar
< -server
< -XX:PermSize=256m
< -XX:MaxPermSize=256m
< -XX:+UseParallelGC
< -Xverify:none
32c26
< -Xms512m
---
> -Xms256m

总共有五处不同，细看之后发现只有两处可能会产生影响，先尝试将 -Xms512m 改成 -Xms256m，问题仍然存在，将文件恢复原样，再尝试将

< -javaagent:/Users/dongmeiliang/.p2/pool/plugins/com.zeroturnaround.eclipse.optimizer.plugin_1.0.11/agent/eclipse-optimizer-agent.jar
< -server
< -XX:PermSize=256m
< -XX:MaxPermSize=256m
< -XX:+UseParallelGC
< -Xverify:none

删除，问题顺利解决。

Reference:Eclipse error: ‘Failed to create the Java Virtual Machine’

3.`Warning: mysqli::__construct(): (HY000/2002): No such file or directory in /code/index.php on line 8 Connection failed: No such file or directory`

A:问题出现的原因：当主机填写为localhost时MySQL会采用 unix domain socket连接，当主机填写为127.0.0.1时MySQL会采用TCP/IP的方式连接。使用Unix socket的连接比TCP/IP的连接更加快速与安全。这是MySQL连接的特性，可以参考官方文档的说明4.2.2. Connecting to the MySQL Server:

On Unix, MySQL programs treat the host name localhost specially, in a way that is likely different from what you expect compared to other network-based programs. For connections to localhost, MySQL programs attempt to connect to the local server by using a Unix socket file. This occurs even if a –port or -P option is given to specify a port number. To ensure that the client makes a TCP/IP connection to the local server, use –host or -h to specify a host name value of 127.0.0.1, or the IP address or name of the local server. You can also specify the connection protocol explicitly, even for localhost, by using the –protocol=TCP option.

这个问题有以下几种解决方法：

使用TCP/IP代替Unix socket。即在连接的时候将localhost换成127.0.0.1。
修改MySQL的配置文件my.cnf，指定mysql.socket的位置： /var/lib/mysql/mysql.sock (你的mysql.socket路径)。  
直接在php建立连接的时候指定my.socket的位置（官方文档：mysqli_connect）。比如： $db = new MySQLi('localhost', 'root', 'root', 'my_db', '3306', '/var/run/mysqld/mysqld.sock')

Reference:mysqli不能使用localhost，请问这是怎么回事？

4.超链接元素的 onclick 方法直接使用表单元素的名字调用其 submit 方法。

<form name="bm_form" action="/delete_bms" method="POST">
  <tr>
      <td><a href="https://www.google.com">https://www.google.com</a></td>
      <td><input type="checkbox" name="del_me" value="https://www.google.com" /></td>
  </tr>
  
  <tr>
      <td><a href="https://www.twitter.com">https://www.twitter.com</a></td>
      <td><input type="checkbox" name="del_me" value="https://www.twitter.com" /></td>
  </tr>
</form>

<a href="#" onclick="bm_form.submit();">Delete BM</a>
  

这里展示了可以直接使用表单元素的名字直接来提交的技巧。

5.Ionic - Cannot find module “.”

A:Just remove all imports that have /umd at the final. In my case, I changed: import { IonicPageModule } from 'ionic-angular/umd'; To: import { IonicPageModule } from 'ionic-angular';

Reference:Ionic 2 - Runtime error Cannot find module “.”

6.Google maps report google is not define in ionic app.

go to your project and do “ionic plugin add cordova-plugin-whitelist”
add CSP meta

<meta http-equiv="Content-Security-Policy" content="script-src 'self' https://maps.googleapis.com/ https://maps.gstatic.com/ https://mts0.googleapis.com/ 'unsafe-inline' 'unsafe-eval'">

Live reload unfortunately seems not work, I get this error:

Refused to load the script 'http://localhost:35729/livereload.js?snipver=1' because it violates the following Content Security Policy directive: "script-src 'self' https://maps.googleapis.com/ https://maps.gstatic.com/ https://mts0.googleapis.com/ 'unsafe-inline' 'unsafe-eval'".

After some google, update CSP meta works, new CSP meta as follow:

  <meta http-equiv="Content-Security-Policy" content="script-src localhost:35729 'self' https://maps.googleapis.com/ https://maps.gstatic.com/ https://mts0.googleapis.com/ 'unsafe-inline' 'unsafe-eval'">

Reference:Ionic + Google Maps: ReferenceError: google is not defined
Solution for livereload problems with new CSP rules

7.Error: No provider for Navbar!

A:Normally navbar don’t provide with injector, we should access like follow:

// Template
<ion-navbar #navbar color="primary">
    <ion-title>Whatever</ion-title>
    <ion-buttons right>
      <button icon-only ion-button>
        <ion-icon name='pause'></ion-icon>
      </button>
    </ion-buttons>
</ion-navbar>

// Typescript
export class Page {

@ViewChild('navbar') navBar: Navbar;

}

Reference:Error in ./HomePage class HomePage - caused by: No provider for Navbar!

8.How to custom back button of ionic app?

<ion-header>
  <ion-navbar color='danger' hideBackButton>
    <ion-title>product page</ion-title>
      <ion-buttons left>
      <button ion-button navPop icon-only>
              <ion-icon ios="ios-arrow-forward" md="md-arrow-forward"></ion-icon>
      </button>
  </ion-buttons>
  </ion-navbar>
</ion-header>

Reference:Change default ion-navbar “back” button ios

9.How to make a div take the remaining height?

Absolute Positioning
Tables
CSS3 calc

Absolute Positioning

// html
<div id="inner_fixed">
    I have a fixed height
</div>
 
<div id="inner_remaining">
    I take up the remaining height
</div>

css

html, body {
      height: 100%;
      width: 100%;
      margin: 0;
  }
   
  #inner_fixed {
      height: 100px;
      background-color: grey;
  }
   
  #inner_remaining {
      background-color: #DDDDDD;    
   
      position: absolute;
      top: 100px;
      bottom: 0;
      width: 100%; 
  }

pros

easy to implement
intuitive

cons

tedious to maintain (hard-coded positions)

Tables

// html
<div id="outer">
    <div id="inner_fixed">
        I have a fixed height
    </div>
 
    <div id="inner_remaining">
        I take up the remaining height
    </div>
</div>

css

html, body, #outer {
      height: 100%;
      width: 100%;
      margin: 0;
  }
   
  #outer {
      display: table;
  }
   
  #inner_fixed {
      height: 100px;
      background-color: grey;
   
      display: table-row;
  }
   
  #inner_remaining {
      background-color: #DDDDDD;
   
      display: table-row;    
  }

pros

rather “clean” solution
no hard-coded values, other elements can change their height

cons

might cause some side-effects with the layout

CSS3 calc

// html
<div id="inner_fixed">
    I have a fixed height
</div>
 
<div id="inner_remaining">
    I take up the remaining height
</div>

css

html, body {
      height: 100%;
      width: 100%;
      margin: 0;
  }
   
  #inner_fixed {
      height: 100px;
      background-color: grey;
  }
   
  #inner_remaining {
      background-color: #DDDDDD;
   
      height: calc(100% - 100px);    
  }

pros

easy to implement
less code than the other solutions

cons

the calc function is rather new (no support for older browsers)
tedious to maintain (hard-coded height)

Reference:How to make a div take the remaining height

10.Property ‘of’ does not exist on type ‘typeof Observable

A: for Angular >= 6.0.0 uses RxJS 6.0.0

import { of } from 'rxjs';

And its usage has been changed, you no longer call it off of Observable:

of('token');

for Angular <= 5.x.xx

import 'rxjs/add/observable/of';

Observable.of('token');

Reference:Property ‘of’ does not exist on type ‘typeof Observable

11.class path resource [keystore.jks ] cannot be resolved to URL because it does not exist

A:Reason is that a space appended to end. Remove end space in application.properties works. I confirm keystore.jks is in target classes, but runtime complain can’t find it. why? After some google, still not found right solution, suddenly I notice there a space at end, may be that is the reason. Fact show my doubt is right.

12.How to pass program argument start with two hyphen(dash) to java in eclipse?

A:Run > Debug Configurations… > Arguments > Right click in the text field, deselect “Substitutions” > “Smart Dashes”

Reference:Double dash in “Program argument” of “Run configurations” becomes “\u2014”

13.How to find out what auto-configuration is currently being applied by Spring Boot?

A:If you need to find out what auto-configuration is currently being applied, and why, start your application with the –debug switch. For example:

$ java -jar myproject-0.0.1-SNAPSHOT.jar --debug

I also find we can enable debug property archive this goal.

#application.properites
debug=true

14.How to search files inside source jars in eclipse?

A:Recently discovered the following plugin has beta support for searching into linked source jars: eclipse-instasearch

You have to enable searching source jars in the preferences as it is turned off by default.To enable searching in jars, go to Window > Preferences > General > Search > InstaSearch and select the option that says “Index JAR source attachments (beta)” .

Depending on how much source you have, the indexing process is very slow, but then search is very fast.

Reference:How to make eclipse “File Search” to also search inside source jars containing some text?

15.How to get Pseudo-Element properties with JavaScript?

A:Assume your CSS looks like:

.element:before {
  content: 'NEW';
  color: rgb(255, 0, 0);
}

To retrieve the color property of the .element:before, you could use the following JavaScript:

var color = window.getComputedStyle(
  document.querySelector('.element'), ':before'
).getPropertyValue('color')

// Access with chrome provide variable $0
var beforePesudoElem = $0;
var display = window.getComputedStyle(beforePesudoElem).getPropertyValue('display');

Reference:Get Pseudo-Element Properties with JavaScript

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