Java导入错误

Question

I'm trying to import org.apache.commons.fileupload.* but I am being told that it does not exist.

I am downloading this JAR: http://commons.apache.org/fileupload/

And placing it on the classpath. So what am I doing wrong here?

Answer 1

Is the jar in your classpath ? Also check out this discussion on the downsides of wildcard imports.

Answer 2

Do you see your class in the jar? To find out if your class exists in a jar, do the following:

# linux
jar tvf jarname.jar | grep classname

# win
jar tvf jarname.jar | findstr classname

To find out if your class exists in any of a number of jars, you can do this:

# linux
for f in `find . -name *.jar`; do echo $f; jar tvf $f | grep classname; done | less

Answer 3

Most likely you're thinking of %CLASSPATH% environment variable. You shouldn't do that. The JAR file has to go in /WEB-INF/lib folder of the dynamic web application project. That folder is by default covered by the webapp's classpath. A bit decent IDE (Eclipse, Netbeans, etc) will automagically add it to the Build Path whenever you drop a JAR file in that folder.

When you're compiling using plain vanilla javac.exe in command console, then you have to specify it in the -cp argument.

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Update : Assuming that you're using Windows and are sitting in source root folder, here's how javac.exe should look like:

javac -cp .;/path/to/tomcat/lib/*;/path/to/WEB-INF/lib/* com/example/Servlet.java

Note: the wildcard only works on JDK 1.6 or newer. Otherwise you've to specify all libraries separately.

Answer 4

Since you're writing a servlet, you've probably created a web project. Place your fileupload jar inside the WEB-INF/lib folder and let refresh your project in the IDE to be placed automatically in your project build path.

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Edit Seeing you're using command-line, make sure that you provide the full jar file path to the CLASSPATH (including the jar name and its extension, separated by semicolon).