SetOnce
A convenient class which offers a semi-immutable object wrapper implementation which allows one to set the value of an object exactly once, and retrieve it many times.
用来封装对象的一个类，只能赋值一次，可以多次读取值，比如private SetOnce<IndexWriter> writer = new SetOnce<>();
那么writer.set(IndexWriter);只能被执行一边，哪怕是同一个IndexWriter;
AlreadySetException
If set(Object) is called more than once, SetOnce.AlreadySetException is thrown and the operation will fail.
writer.set(IndexWriter);被调用多过一次就抛这个异常。
写了个定时更新索引的任务
TimerTask task = new Task(new Runnable() { public void run() { index();//建索引 indexWriter = new IndexWriter(Directory,IndexWriterConfig);//初始化已关闭的indexWriter } });
Exception in thread "main" org.apache.lucene.util.SetOnce$AlreadySetException: The object cannot be set twice! at org.apache.lucene.util.SetOnce.set(SetOnce.java:69) at org.apache.lucene.index.IndexWriterConfig.setIndexWriter(IndexWriterConfig.java:148) at org.apache.lucene.index.IndexWriter.<init>(IndexWriter.java:687)
分析构造方法new IndexWriter(Directory,IndexWriterConfig);
1 public IndexWriter(Directory d, IndexWriterConfig conf) throws IOException { 2 conf.setIndexWriter(this); // prevent reuse by other instances 3 …… 4 …… 5 …… 6 }
重点在第二行conf.setIndexWriter(this);
1 private SetOnce<IndexWriter> writer = new SetOnce<>(); 2 3 /** 4 * Sets the {@link IndexWriter} this config is attached to. 5 * 6 * @throws AlreadySetException 7 * if this config is already attached to a writer. 8 */ 9 IndexWriterConfig setIndexWriter(IndexWriter writer) { 10 this.writer.set(writer); 11 return this; 12 }
是这样，IndexWriterConfig里的wirter变量，是用SetOnce封装的，当再用用一个IndexWriterConfig去构造IndexWriter时，writer被赋值两次所以报错了
重新构造一个IndexWriterConfig
TimerTask task = new Task(new Runnable() { public void run() { index();//建索引 IndexWriterConfig config = new IndexWriterConfig(Version.Lucene_36,Analyzer); indexWriter = new IndexWriter(Directory,IndexWriterConfig);//初始化已关闭的indexWriter } });
异常——org.apache.lucene.util.SetOnce$AlreadySetException
原文：http://www.cnblogs.com/erbin/p/3947631.html