終於捕獲或返回實例或空引用

Question

使用GNU NNTP，我怎樣才能讓getMessage(String newsgroup, int i)返回一個真正的javax.mail.Message或null這樣的Message？

init: 
Deleting: /home/thufir/NetBeansProjects/USENET/build/built-jar.properties 
deps-jar: 
Updating property file: /home/thufir/NetBeansProjects/USENET/build/built-jar.properties 
Deleted 1 out of date files in 0 seconds 
Compiling 2 source files to /home/thufir/NetBeansProjects/USENET/build/classes 
warning: Supported source version 'RELEASE_6' from annotation processor 'org.eclipse.persistence.internal.jpa.modelgen.CanonicalModelProcessor' less than -source '1.7' 
warning: The following options were not recognized by any processor: '[eclipselink.canonicalmodel.use_static_factory]' 
1 warning 
Copying 1 file to /home/thufir/NetBeansProjects/USENET/build/classes 
compile: 
run: 
DEBUG: nntp: newsrc loading /home/thufir/.newsrc 
DEBUG: nntp: newsrc load: 6 groups in 31ms 
[EL Info]: 2012-07-31 05:21:51.235--ServerSession(1478593)--EclipseLink, version: Eclipse Persistence Services - 2.3.0.v20110604-r9504 
[EL Info]: 2012-07-31 05:21:52.523--ServerSession(1478593)--file:/home/thufir/NetBeansProjects/USENET/build/classes/_USENETPU login successful 
[EL Warning]: 2012-07-31 05:21:52.672--ServerSession(1478593)--Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.DatabaseException 
Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Table 'usenet.HEADERFIELD' doesn't exist 
Error Code: 1146 
Call: ALTER TABLE HEADERFIELD DROP FOREIGN KEY FK_HEADERFIELD_ARTICLE_ID 
Query: DataModifyQuery(sql="ALTER TABLE HEADERFIELD DROP FOREIGN KEY FK_HEADERFIELD_ARTICLE_ID") 
[EL Warning]: 2012-07-31 05:21:52.687--ServerSession(1478593)--Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.DatabaseException 
Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown table 'ARTICLE' 
Error Code: 1051 
Call: DROP TABLE ARTICLE 
Query: DataModifyQuery(sql="DROP TABLE ARTICLE") 
[EL Warning]: 2012-07-31 05:21:52.797--ServerSession(1478593)--Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.3.0.v20110604-r9504): org.eclipse.persistence.exceptions.DatabaseException 
Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown table 'HEADERFIELD' 
Error Code: 1051 
Call: DROP TABLE HEADERFIELD 
Query: DataModifyQuery(sql="DROP TABLE HEADERFIELD") 
Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.driver.FetchBean <init> 
INFO: [gwene.ca.craigslist.vancouver.labour.general, gwene.com.androidcentral, gwene.com.blogspot.emacsworld, gwene.com.blogspot.googlecode, gwene.com.blogspot.googlereader, gwene.com.economist] 
Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 5 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 6 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 7 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 8 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 9 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 10 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 9 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

Jul 31, 2012 5:21:53 AM net.bounceme.dur.usenet.model.Usenet getMessage 
SEVERE: null 
javax.mail.MessageRemovedException: No such article: 10 
    at gnu.mail.providers.nntp.NNTPFolder.getMessage(NNTPFolder.java:330) 
    at net.bounceme.dur.usenet.model.Usenet.getMessage(Usenet.java:72) 
    at net.bounceme.dur.usenet.driver.FetchBean.<init>(FetchBean.java:40) 
    at net.bounceme.dur.usenet.driver.FetchBean.main(FetchBean.java:24) 

BUILD SUCCESSFUL (total time: 10 seconds) 

枚舉：

package net.bounceme.dur.usenet.model; 

import java.util.*; 
import java.util.logging.Level; 
import java.util.logging.Logger; 
import javax.mail.*; 

public enum Usenet { 

    INSTANCE; 
    private final Logger LOG = Logger.getLogger(Usenet.class.getName()); 
    private Properties props = new Properties(); 
    private Folder root = null; 
    private Store store = null; 
    private List<Folder> folders = new ArrayList<>(); 
    private Folder folder = null; 

    Usenet() { 
     LOG.fine("controller.."); 
     props = PropertiesReader.getProps(); 
     try { 
      connect(); 
     } catch (Exception ex) { 
      Logger.getLogger(Usenet.class.getName()).log(Level.SEVERE, "FAILED TO LOAD MESSAGES", ex); 
     } 
    } 

    private void connect() throws Exception { 
     LOG.fine("Usenet.connect.."); 
     Session session = Session.getDefaultInstance(props); 
     session.setDebug(true); 
     store = session.getStore(new URLName(props.getProperty("nntp.host"))); 
     store.connect(); 
     root = store.getDefaultFolder(); 
     setFolders(Arrays.asList(root.listSubscribed())); 
    } 

    public void foo(String ng) throws Exception { 
     folder = root.getFolder(ng); 
     LOG.fine("opened the folder!!!!!"); 
     folder.open(Folder.READ_ONLY); 
    } 

    public List<Message> getMessages(String newsgroup) throws Exception { 
     LOG.fine("fetching.." + newsgroup); 
     folder = root.getFolder(newsgroup); 
     LOG.fine("opened the folder!!!!!"); 
     folder.open(Folder.READ_ONLY); 
     LOG.fine("opened: " + folder.getFullName()); 
     LOG.fine("opened: " + folder.getFullName()); 
     LOG.fine("opened: " + folder.getFullName()); 
     List<Message> messages = Arrays.asList(folder.getMessages()); 
     LOG.fine("..fetched " + folder); 
     return messages; 
    } 

    public List<Folder> getFolders() { 
     LOG.fine("folders " + folders); 
     return Collections.unmodifiableList(folders); 
    } 

    private void setFolders(List<Folder> folders) { 
     this.folders = folders; 
    } 

    public Message getMessage(String newsgroup, int i) { 
     Message message = null; 
     try { 
      LOG.fine("fetching.." + newsgroup); 
      folder = root.getFolder(newsgroup); 
      folder.open(Folder.READ_ONLY); 
      return folder.getMessage(i); 
     } catch (MessagingException ex) { 
      Logger.getLogger(Usenet.class.getName()).log(Level.SEVERE, null, ex); 
     } finally { 
      return message;//crummy 
     } 
    } 
} 

我如何處理這個異常，使程序不崩潰？

Answer 1

您嘗試處理之前，做一個簡單的布爾檢查：

if(!message.isExpunged) 
{ 
    //process your message here 
}  
else{ 
    //log a message about it being expunged 
} 

API

公共布爾isExpunged（）

檢查此消息是否被刪掉。除了getMessageNumber（）以外的所有其他方法在已清除的Message對象上都是無效的。

由於包含文件夾上的explict expunge（）請求而被清除的郵件將立即從文件夾中刪除。由其他源外部清除的郵件標記爲「已清除」，並且對於isExpunged（）方法返回true，但直到在文件夾上完成顯式清除（）之後才從文件夾中刪除它們。

有關清除處理的更多詳細信息，請參閱expunge（）的說明。