java 为什么这里会出现 javax.naming.NamingException？
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why does the javax.naming.NamingException occur here?
提问by Suhail Gupta
when i run the following :
当我运行以下命令时：
package NonServletFiles;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import javax.sql.DataSource;
import javax.naming.*;
public class GetTagsFromDatabase {
public GetTagsFromDatabase() {
}
public String[] getTags() {
String tags[] = null;
try {
Context context = new InitialContext();
DataSource ds = (DataSource)context.lookup("java:comp/env/jdbc/photog"); // <<----- line 23
Connection connection = ds.getConnection();
String sqlQuery = "select NAMEOFTHETAG from tagcollection";
PreparedStatement statement = connection.prepareStatement(sqlQuery);
ResultSet set = statement.executeQuery();
int i = 0;
while(set.next()) {
tags[i] = set.getString("NameOfTheTag");
System.out.println(tags[i]);
i++;
}
}catch(Exception exc) {
exc.printStackTrace();
}
return tags;
}
public static void main(String args[]) {
new GetTagsFromDatabase().getTags(); // <<----- line 43
}
}
I get the following exceptions :
我得到以下例外：
javax.naming.NamingException: Lookup failed for 'java:comp/env/jdbc/photog' in SerialContext[myEnv={java.naming.factory.initial=com.sun.enterprise.naming.impl.SerialInitContextFactory, java.naming.factory.url.pkgs=com.sun.enterprise.naming, java.naming.factory.state=com.sun.corba.ee.impl.presentation.rmi.JNDIStateFactoryImpl} [Root exception is javax.naming.NamingException: Invocation exception: Got null ComponentInvocation ]
at com.sun.enterprise.naming.impl.SerialContext.lookup(SerialContext.java:518)
at com.sun.enterprise.naming.impl.SerialContext.lookup(SerialContext.java:455)
at javax.naming.InitialContext.lookup(InitialContext.java:411)
at NonServletFiles.GetTagsFromDatabase.getTags(GetTagsFromDatabase.java:23)
at NonServletFiles.GetTagsFromDatabase.main(GetTagsFromDatabase.java:43)
Caused by: javax.naming.NamingException: Invocation exception: Got null ComponentInvocation
at com.sun.enterprise.naming.impl.GlassfishNamingManagerImpl.getComponentId(GlassfishNamingManagerImpl.java:873)
at com.sun.enterprise.naming.impl.GlassfishNamingManagerImpl.lookup(GlassfishNamingManagerImpl.java:742)
at com.sun.enterprise.naming.impl.JavaURLContext.lookup(JavaURLContext.java:172)
at com.sun.enterprise.naming.impl.SerialContext.lookup(SerialContext.java:498)
... 4 more
I don't know the reason for this exception,all other servlets that need to connect to the database with the url java:comp/env/jdbc/photogwork fine.
我不知道此异常的原因，所有其他需要使用 url 连接到数据库的 servlet 都可以java:comp/env/jdbc/photog正常工作。
回答by BalusC
The stacktrace hints that you're using Glassfish. Remove the java:comp/env/part. It's the default JNDI context root already. Only in Tomcat you need to specify it explicitly. Also, you should be invoking this in webapp context, not as a plain Java Application with main().
堆栈跟踪提示您正在使用 Glassfish。取下java:comp/env/零件。它已经是默认的 JNDI 上下文根。仅在 Tomcat 中您需要明确指定它。此外，您应该在 webapp 上下文中调用它，而不是作为带有main().
Unrelatedto the concrete problem, do you really need to get the DataSourceeverytime? I'd create a helper class which obtains it only once on webapp's startup or in a static initializer. It's application wide and threadsafe. Only the Connectionindeed needs to be obtained (and closed! you're not closing it, so you're leaking DB resources) everytime you need to fire a SQL query.
与具体问题无关，你真的需要得到DataSourceeverytime吗？我会创建一个助手类，它只在 webapp 启动时或在静态初始化程序中获取一次。它是应用程序范围和线程安全的。Connection每次需要触发 SQL 查询时，只需要获取（并关闭！你没有关闭它，所以你正在泄漏数据库资源）。
