📅 最后修改于: 2020-11-14 10:08:41 🧑 作者: Mango
邮局协议(POP)是本地电子邮件客户端使用的应用程序层Internet标准协议，用于通过TCP / IP连接从远程服务器检索电子邮件。 POP支持简单的下载和删除要求，以访问远程邮箱。 POP3服务器侦听众所周知的端口110。
软件包com.sun.mail.pop3是JavaMail API的POP3协议提供程序，它提供对POP3消息存储的访问。下表列出了此软件包中的类：
| Name | Description |
|---|---|
| POP3Folder | A POP3 Folder (can only be “INBOX”). |
| POP3Message | A POP3 Message. |
| POP3SSLStore | A POP3 Message Store using SSL. |
| POP3Store | A POP3 Message Store. |
此提供者上方需要注意的几点：
POP3提供程序仅支持一个名为INBOX的文件夹。由于POP3协议的限制，不允许使用许多JavaMail API功能，例如事件通知，文件夹管理，标志管理等。
使用协议名称pop3或格式为pop3：// user：password @ host：port / INBOX的URL，可以通过JavaMail API访问POP3提供程序。
POP3不支持永久标志。例如，永远不会为POP3邮件设置Flags.Flag.RECENT标志。由应用程序确定POP3邮箱中的哪些邮件是新邮件。
POP3不支持Folder.expunge()方法。若要删除和删除消息，请在消息上设置Flags.Flag.DELETED标志，然后使用Folder.close(true)方法关闭文件夹。
POP3不提供接收日期，因此getReceivedDate方法将返回null。
访问POP3消息的标头后，POP3提供程序将使用TOP命令获取所有标头，然后将其缓存。
当访问POP3消息的内容时，POP3提供程序将使用RETR命令来提取整个消息。
POP3Message.invalidate方法可用于使缓存的数据无效，而无需关闭文件夹。
POP3协议提供程序支持以下属性，可以在JavaMail Session对象中设置这些属性。属性始终设置为字符串; “类型”列描述了如何解释字符串。
| Name | Type | Description |
|---|---|---|
| mail.pop3.user | String | Default user name for POP3. |
| mail.pop3.host | String | The POP3 server to connect to. |
| mail.pop3.port | int | The POP3 server port to connect to, if the connect() method doesn’t explicitly specify one. Defaults to 110. |
| mail.pop3.connectiontimeout | int | Socket connection timeout value in milliseconds. Default is infinite timeout. |
| mail.pop3.timeout | int | Socket I/O timeout value in milliseconds. Default is infinite timeout. |
| mail.pop3.rsetbeforequit | boolean | Send a POP3 RSET command when closing the folder, before sending the QUIT command. Default is false. |
| mail.pop3.message.class | String | Class name of a subclass of com.sun.mail.pop3.POP3Message. The subclass can be used to handle (for example) non-standard Content-Type headers. The subclass must have a public constructor of the form MyPOP3Message(Folder f, int msgno) throws MessagingException. |
| mail.pop3.localaddress | String | Local address (host name) to bind to when creating the POP3 socket. Defaults to the address picked by the Socket class. |
| mail.pop3.localport | int | Local port number to bind to when creating the POP3 socket. Defaults to the port number picked by the Socket class. |
| mail.pop3.apop.enable | boolean | If set to true, use APOP instead of USER/PASS to login to the POP3 server, if the POP3 server supports APOP. APOP sends a digest of the password rather than the clear text password. Defaults to false. |
| mail.pop3.socketFactory | Socket Factory | If set to a class that implements the javax.net.SocketFactory interface, this class will be used to create POP3 sockets. |
| mail.pop3.socketFactory.class | String | If set, specifies the name of a class that implements the javax.net.SocketFactory interface. This class will be used to create POP3 sockets. |
| mail.pop3.socketFactory.fallback | boolean | If set to true, failure to create a socket using the specified socket factory class will cause the socket to be created using the java.net.Socket class. Defaults to true. |
| mail.pop3.socketFactory.port | int | Specifies the port to connect to when using the specified socket factory. If not set, the default port will be used. |
| mail.pop3.ssl.enable | boolean | If set to true, use SSL to connect and use the SSL port by default. Defaults to false for the “pop3” protocol and true for the “pop3s” protocol. |
| mail.pop3.ssl.checkserveridentity | boolean | If set to true, check the server identity as specified by RFC 2595. Defaults to false. |
| mail.pop3.ssl.trust | String | If set, and a socket factory hasn’t been specified, enables use of a MailSSLSocketFactory. If set to “*”, all hosts are trusted. If set to a whitespace separated list of hosts, those hosts are trusted. Otherwise, trust depends on the certificate the server presents. |
| mail.pop3.ssl.socketFactory | SSL Socket Factory | If set to a class that extends the javax.net.ssl.SSLSocketFactory class, this class will be used to create POP3 SSL sockets. |
| mail.pop3.ssl.socketFactory.class | String | If set, specifies the name of a class that extends the javax.net.ssl.SSLSocketFactory class. This class will be used to create POP3 SSL sockets. |
| mail.pop3.ssl.socketFactory.port | int | Specifies the port to connect to when using the specified socket factory. If not set, the default port will be used. |
| mail.pop3.ssl.protocols | string | Specifies the SSL protocols that will be enabled for SSL connections. The property value is a whitespace separated list of tokens acceptable to the javax.net.ssl.SSLSocket.setEnabledProtocols method. |
| mail.pop3.starttls.enable | boolean | If true, enables the use of the STLS command (if supported by the server) to switch the connection to a TLS-protected connection before issuing any login commands. Defaults to false. |
| mail.pop3.starttls.required | boolean | If true, requires the use of the STLS command. If the server doesn’t support the STLS command, or the command fails, the connect method will fail. Defaults to false. |
| mail.pop3.socks.host | string | Specifies the host name of a SOCKS5 proxy server that will be used for connections to the mail server. |
| mail.pop3.socks.port | string | Specifies the port number for the SOCKS5 proxy server. |
| mail.pop3.disabletop | boolean | If set to true, the POP3 TOP command will not be used to fetch message headers. Defaults to false. |
| mail.pop3.forgettopheaders | boolean | If set to true, the headers that might have been retrieved using the POP3 TOP command will be forgotten and replaced by headers retrieved as part of the POP3 RETR command. Defaults to false. |
| mail.pop3.filecache.enable | boolean | If set to true, the POP3 provider will cache message data in a temporary file rather than in memory. Messages are only added to the cache when accessing the message content. Message headers are always cached in memory (on demand). The file cache is removed when the folder is closed or the JVM terminates. Defaults to false. |
| mail.pop3.filecache.dir | String | If the file cache is enabled, this property can be used to override the default directory used by the JDK for temporary files. |
| mail.pop3.cachewriteto | boolean | Controls the behavior of the writeTo method on a POP3 message object. If set to true, and the message content hasn’t yet been cached, and ignoreList is null, the message is cached before being written. Otherwise, the message is streamed directly to the output stream without being cached. Defaults to false. |
| mail.pop3.keepmessagecontent | boolean | If this property is set to true, a hard reference to the cached content will be kept, preventing the memory from being reused until the folder is closed or the cached content is explicitly invalidated (using the invalidate method). Defaults to false. |
通常，应用程序不应直接使用此包中的类。相反，他们应该使用javax.mail包(和子包)定义的API。应用程序永远不要直接构造POP3Store或POP3Folder的实例。相反，他们应该使用Session方法getStore来获取适当的Store对象，并从中获取Folder对象。
在检查电子邮件一章中演示了使用POP3服务器的示例。