如何运行 Java.class 文件？

我编译了一个 HelloWorld 程序，并使用命令提示符运行它。那个。类文件名为 HelloWorld2.class

该文件位于 C: Users Matt workspace HelloWorld2 bin 中 下面是我在命令提示符下输入“ JavaHelloWorld2”得到的结果:

C:\Users\Matt>Java HelloWorld2
Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld2
Caused by: java.lang.ClassNotFoundException: HelloWorld2
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
Could not find the main class: HelloWorld2.  Program will exit.

我还以为你会打印一本《你好世界》呢。我做错了什么？我安装了 JDK。

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最佳答案

If your class does not have a package, you only need to set the classpath to find your compiled class:

java -cp C:\Users\Matt\workspace\HelloWorld2\bin HelloWorld2

If your class has a package, then it needs to be in a directory corresponding to the package name, and the classpath must be set to the root of the directory tree that represents the package.

// Source file HelloWorld2/src/com/example/HelloWorld2.java
package com.example;
...


Compiled class file: HelloWorld2/bin/com/example/HelloWorld2.class


$ java -cp HelloWorld2/bin com.example.HelloWorld2
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This can mean a lot of things, but the most common one is that the class contained in the file doesn't have the same name as the file itself. So, check if your class is also called HelloWorld2.

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To run Java class file from the command line, the syntax is:

java -classpath /path/to/jars <packageName>.<MainClassName>

where packageName (usually starts with either com or org) is the folder name where your class file is present.

For example if your main class name is App and Java package name of your app is com.foo.app, then your class file needs to be in com/foo/app folder (separate folder for each dot), so you run your app as:

$ java com.foo.app.App

Note: $ is indicating shell prompt, ignore it when typing

If your class doesn't have any package name defined, simply run as: java App.

If you've any other jar dependencies, make sure you specified your classpath parameter either with -cp/-classpath or using CLASSPATH variable which points to the folder with your jar/war/ear/zip/-classpath0 files. So on Linux you can prefix the command with: CLASSPATH=/path/to/jars, on Windows you need to add the folder into system variable. If not set, the user class path consists of the current directory (.).

Practical example

Given we've created sample project using Maven as:

$ mvn archetype:generate -DgroupId=com.foo.app -DartifactId=my-app -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false

and we've compiled our project by mvn compile in our my-app/ dir, it'll generate our class file is in target/classes/com/foo/app/App.class.

To run it, we can either specify class path via -cp or going to it directly, check examples below:

$ find . -name "*.class"
./target/classes/com/foo/app/App.class
$ CLASSPATH=target/classes/ java com.foo.app.App
Hello World!
$ java -cp target/classes com.foo.app.App
Hello World!
$ java -classpath .:/path/to/other-jars:target/classes com.foo.app.App
Hello World!
$ cd target/classes && java com.foo.app.App
Hello World!

To double check your class and package name, you can use Java class file disassembler tool, e.g.:

$ javap target/classes/com/foo/app/App.class
Compiled from "App.java"
public class com.foo.app.App {
public com.foo.app.App();
public static void main(java.lang.String[]);
}

Note: javap won't work if the compiled file has been obfuscated.

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  1. Go to the path where you saved the java file you want to compile.
  2. Replace path by typing cmd and press enter.
  3. Command Prompt Directory will pop up containing the path file like C:/blah/blah/foldercontainJava
  4. Enter javac javafile.java
  5. Press Enter. It will automatically generate java class file

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