• A variable holds a value, while an object reference variable points to the memory that holds the object.
• The short-circuit logical operators && and || operate only on boolean operands. For example, the expression
  9&&7 will not compile. Understand the difference between short-circuit logical operators and bitwise
  operators.
• If the first expression of a && operator is false, the second expression is not evaluated, and if the first
  expression of a || operator is true, the second expression is not evaluated.

Look Out

• Be on guard for words that are only slightly different from the Java keywords, such as synchronize instead of synchronized, implement instead of implements, and protect instead of protected.
• Whether an array stores primitive variables or object references, the array itself is always an object.
• It is illegal to include the array size in the array declaration.
• Division by zero generates a runtime ArithmeticException only if the operands are integers. In case of float and double, the result of division by zero is infinity.
• The result of the modulo operator always carries the sign of the first operand (i.e. the one before the operator);
  you can ignore the sign of the second operand.
• You cannot instantiate an enum by using the new operator.

Memorize

• Know the Java language keywords.
• Know the range of values for all primitive data types.
• All primitive data types except boolean and char are signed.
• The first character of an identifier must be a letter, a dollar sign ($), or an underscore (_). Characters other than the first character in an identifier may be a letter, a dollar sign, an underscore, or a digit.

• The method name and return type are mandatory in a method declaration. Even though you are not required to specify a modifier in a method declaration, the default modifier is assigned to the method, if you don’t declare one.
• A static variable belongs to the class and not to a particular instance of the class, and therefore is initialized when the class is loaded, and before the class is instantiated.
• Because a static method belongs to a class and not to a particular instance of the class, it cannot access the nonstatic methods and variables of the class in which it is defined.
• An instance of an inner class can only exist in an instance of the outer class, and has direct access to all the instance variables and methods of the outer instance.
• If you make a super call or a this call, it must be in the beginning of a constructor. That means you can make either a super call or a this call, but not both.

Look Out

• The variable-length parameters list must appear last in the parentheses of a method and it consists of a data type, three dots, and a name, in that order.
• A Java class cannot inherit from more than one class, but it can inherit from one class and one or more interfaces.
• The class that inherits from an interface must provide implementation for all the methods that are declared in the interface if the class is not abstract.
• An interface can extend another interface but it cannot implement another interface or a class.

Memorize

• If you do not provide any constructor for a class you write, the compiler provides the default constructor for that class. If you write at least one constructor for the class, the compiler provides no constructor.
• If you don not make a this or a super call in the beginning of a constructor, the compiler places a super() call there.
• You use the keyword extends to write a derived class that inherits from a parent class, and use the keyword implements to write a class that inherits from an interface.
• The methods in an interface are inherently public and abstract, and the variables in the interface are inherently public, final, and static.

呃，好吧，我只写过三个JavaMail的程序。

都是批量邮件守护程序。决定总结一下，希望非常幸运找到这篇文章的人不会再对这个困惑。

必须明白的基础知识

1. STMP协议是如何工作的
   协议的标准在这里 http://www.ietf.org/rfc/rfc2821.txt?number=2821
   下面是扼要说明（http://www.freesoft.org/CIE/Topics/94.htm ）：

   Simple Mail Transfer Protocol (SMTP), documented in RFC 821, is Internet’s standard host-to-host mail transport protocol and traditionally operates over TCP, port 25. In other words, a UNIX user can type telnet hostname 25 and connect with an SMTP server, if one is present.

   SMTP uses a style of asymmetric request-response protocol popular in the early 1980s, and still seen occasionally, most often in mail protocols. The protocol is designed to be equally useful to either a computer or a human, though not too forgiving of the human. From the server’s viewpoint, a clear set of commands is provided and well-documented in the RFC. For the human, all the commands are clearly terminated by newlines and a HELP command lists all of them. From the sender’s viewpoint, the command replies always take the form of text lines, each starting with a three-digit code identifying the result of the operation, a continuation character to indicate another lines following, and then arbitrary text information designed to be informative to a human.

   事实上，你可以像使用DOS命令一样发送电子邮件。http://bbs.stcore.com/archiver/tid-8024.htm 当然因为各种原因，你的尝试不可能成功。事实上SMTP工作的时候就是简单的发送命令。取得认证，发送数据。得到反馈。确认退出这么简单。

2. SMTP中用于发送的数据
   SMTP中发送的数据，遵从Multipurpose Internet Mail Extensions (MIME)标准，呃，我不得不说，这是这个星球上最重要的标准之一。所有的互联网通信基本都是基于这个标准的演化。除了电子邮件，常见的应用还包括HTTP报文等（也就是所有网页了），另外即使在20年后发展的XML，其2进制数据发送仍然实用的MIME中的编码方式。
   恩，这里就涉及到邮件附件如何处理的问题。恩，简单地说就是BASE64编码

Table 1: The Base64 Alphabet

Value Encoding  Value Encoding  Value Encoding  Value Encoding

0 A            17 R            34 i            51 z

1 B            18 S            35 j            52 0

2 C            19 T            36 k            53 1

3 D            20 U            37 l            54 2

4 E            21 V            38 m            55 3

5 F            22 W            39 n            56 4

6 G            23 X            40 o            57 5

7 H            24 Y            41 p            58 6

8 I            25 Z            42 q            59 7

9 J            26 a            43 r            60 8

10 K            27 b            44 s            61 9

11 L            28 c            45 t            62 +

12 M            29 d            46 u            63 /

13 N            30 e            47 v

14 O            31 f            48 w         (pad) =

15 P            32 g            49 x

16 Q            33 h            50 y

在这种编码中，我们将字符或者二进制编码以6个比特位为一组，替换成相应的字符形式。比如

100110111010001011101001

转换结果就是

100110 -> 38

111010 -> 58

001011 -> 11

101001 -> 41

38 -> m

58 -> 6

11 -> L

41 -> p

m6Lp

于是，我们就可以以文本的方式编码二进制流以及扩展ASCII字符，比如中文字符。

基础知识完毕，下面是FAQ

Java发送电子邮件需要哪些软件包

mail.jar 通常还会需要 activation.jar

下载地址
http://java.sun.com/products/javabeans/jaf/downloads/index.html
https://maven-repository.dev.java.net/nonav/repository/javax.mail/

如何发送邮件

关于：

• 如何发送邮件
• 如何发送带有附件的邮件
• 如何发送中文邮件
• 邮件中文标题乱码怎么办
• 邮件附件乱码怎么办等等问题

请查看以下代码

	public static synchronized void sendMail(Properties settings)
 
 		throws Exception {
 
 	Properties props = new Properties();
 
 	props.put("mail.smtp.host", settings.get(StartCore.MAIL_SERVER));
 
 	props.put("mail.smtp.user", settings.get(StartCore.USER_NAME));
 
 	props.put("mail.smtp.auth", "true");
 
//SMTP服务器用户验证
 
 	Authenticator auth = new SMTPAuthenticator((String) settings
 
 		.get(StartCore.USER_NAME), (String) settings
 
 		.get(StartCore.PASSWORD));
 
 	Session session = Session.getDefaultInstance(props, auth);
 
if ("true".compareToIgnoreCase((String) settings.get("DEBUG")) == 0) {
 
 		session.setDebug(true);
 
 	}
 
//创建消息体
 
 	MimeMessage msg = new MimeMessage(session);
 
//设置发送人邮件
 
 	msg.setFrom(new InternetAddress((String) settings
 
 		.get(StartCore.USER_MAIL)));
 
//设置接收人邮件
 
 	address = new InternetAddress[] { new InternetAddress(rs
 
 		.getString("GRE_mail")) };
 
 	msg.setRecipients(Message.RecipientType.TO, address);
 
//设置主题，中文编码
 
 	msg.setSubject(subject, "gbk");
 
 	msg.setSentDate(new Date());
 
String content = "邮件正文";
 
 	MimeBodyPart mbp1 = new MimeBodyPart();
 
 	mbp1.setText(content, "gbk");
 
//邮件附件
 
MimeBodyPart attachFilePart = new MimeBodyPart();
 
File file = new File("中文附件.txt");
 
FileDataSource fds = new FileDataSource(file.getName());
 
attachFilePart.setDataHandler(new DataHandler(fds));
 
 	sun.misc.BASE64Encoder enc = new sun.misc.BASE64Encoder();
 
 	//解决中文附件名称
 
 	attachFilePart.setFileName("=?gbk?B?"
 
 		+ enc.encode(file.getName().getBytes("gbk")) + "?=");
 
Multipart mp = new MimeMultipart();
 
 	mp.addBodyPart(mbp1);
 
 	mp.addBodyPart(attachFilePart);
 
msg.setContent(mp);
 
// send the message
 
 	msg.saveChanges();
 
 	Transport.send(msg);
 
}

这是上面用户验证用到的类

class SMTPAuthenticator extends javax.mail.Authenticator {

	private String username;
 
	private String password;
 
	/**
 
  	* @param username
 
 	* @param password
 
  	*/
 
 	public SMTPAuthenticator(String username, String password) {
 
 		this.username = username;
 
 		this.password = password;
 
	}
 
	public PasswordAuthentication getPasswordAuthentication() {
 
 		return new PasswordAuthentication(username, password);
 
 	}
 
}

邮件发送出错

What causes an “javax.activation.UnsupportedDataTypeException: no object DCH for MIME type xxx/xxxx javax.mail.MessagingException: IOException while sending message;” to be sent and how do I fix this? [This happens for known MIME types like text/htm

事实上这个是邮件发送时验证组件设置不当引起的，这个组件配置方法如下

(http://java.sun.com/j2ee/1.4/docs/api/javax/activation/MailcapCommandMap.html)

The MailcapCommandMap looks in various places in the user’s system for mailcap file entries. When requests are made to search for commands in the MailcapCommandMap, it searches mailcap files in the following order:

1) Programatically added entries to the MailcapCommandMap instance.
2) The file .mailcap in the user’s home directory.
3) The file /lib/mailcap.
4) The file or resources named META-INF/mailcap.
5) The file or resource named META-INF/mailcap.default (usually found only in the activation.jar file).

我选用了第四种方法，在生成的Jar文件中加入了 META-INF/mailcap.

#
# This is a very simple 'mailcap' file
#
image/gif;;		x-java-view=com.sun.activation.viewers.ImageViewer
image/jpeg;;		x-java-view=com.sun.activation.viewers.ImageViewer
text/*;;		x-java-view=com.sun.activation.viewers.TextViewer
text/*;;		x-java-edit=com.sun.activation.viewers.TextEditor
text/html;; 	x-java-content-handler=com.sun.mail.handlers.text_html
text/xml;; 		x-java-content-handler=com.sun.mail.handlers.text_xml
text/plain;; 	x-java-content-handler=com.sun.mail.handlers.text_plain
multipart/*;; 	x-java-content-handler=com.sun.mail.handlers.multipart_mixed
message/rfc822;; x-java-content-handler=com.sun.mail.handlers.message_rfc822

For instance, according to the Java class library documentation, the read( ) method of java.io.InputStream returns "the next byte of data, or -1 if the end of the stream is reached." Upon reflection, this sounds suspicious. How is a -1 that appears as part of the stream data to be distinguished from a -1 indicating end of stream? In point of fact, the read( ) method does not return a byte; its signature shows that it returns an int:

1 public   abstract   int  read( )  throws  IOException

This int is not a Java byte with a value between -128 and 127 but a more general unsigned byte with a value between 0 and 255. Hence, -1 can easily be distinguished from valid data values read from the stream.

JVM and byte, byte array

In fact, a single byte still takes up four bytes of space inside the Java virtual machine, but a byte array occupies only the amount of space it actually needs. The virtual machine includes special instructions for operating on byte arrays but does not include any instructions for operating on single bytes. They're just promoted to ints.

Input, Output, Reader and Writer

For the most part, these classes have methods that are extremely similar to the equivalent stream classes. Often the only difference is that a byte in the signature of a stream method becomes a char in the signature of the matching reader or writer method. For example, the java.io.OutputStream class declares these three write( ) methods:

The java.io.Writer class, therefore, declares these three write( ) methods:

As you can see, the signatures match except that in the latter two methods the byte array data has changed to a char array. There's also a less obvious difference not reflected in the signature. While the int passed to the OutputStream write( ) method is reduced modulo 256 before being output, the int passed to the Writer write( ) method is reduced modulo 65,536. This reflects the different ranges of chars and bytes.

java.io.Writer also has two more write( ) methods that take their data from a string:

				public void write(String s) 
throws IOException
public void write(String s, int offset, int length) throws IOException

		

Because streams don't know how to deal with character-based data, there are no corresponding methods in the java.io.OutputStream class.

Hello World!

1   byte [] hello  =   { 72 ,  101 ,  108 ,  108 ,  111 ,  32 ,  87 ,  111 ,  114 ,  108 ,  100 ,  33 ,  10 , 13 } ;
2 System.out.write(hello);

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