Java 如何获取 .class 文件的包名？

How to get a package name of .class file?

提问by Wicia

I have an unusual problem which is concerned to dynamic loading java .class file at run-time. All I want to do is to load a .class file and basing on it create a Class object.

我有一个不寻常的问题，它与在运行时动态加载 java .class 文件有关。我想要做的就是加载一个 .class 文件并基于它创建一个 Class 对象。

Input: an absolute path of .class file.

输入：.class文件的绝对路径。

Basing on it i want to load class by ClassLoader, so I need a path of root directory where file is located and full class name e.g com.test.MyClass. Basing on mentioned absolute path I can only get a class name but I can't get a package name which is "hiden" in this file.

基于它，我想通过 ClassLoader 加载类，所以我需要文件所在的根目录路径和完整的类名，例如 com.test.MyClass。基于提到的绝对路径，我只能获得一个类名，但我无法获得在此文件中“隐藏”的包名。

Here is code of my "loading class method":

这是我的“加载类方法”的代码：

    public static void loadClass(String directory){

        // Get file root directory
        String rootDirectory = new File(directory).getParent();

        // Get rid of file extension
        String className = getFileNameWithoutExtension(directory);

        URL[] urls = null;
        ClassLoader cl = null;

        try {
            // Convert File to a URL and save them
            urls = new URL[]{new File(rootDirectory).toURI().toURL()};

            // Create a new class loader with the directory
            cl = new URLClassLoader(urls);

            // Load in the class
            dynamicClass = cl.loadClass(className);
        } 
        catch (MalformedURLException e) 
        {

    } 
    catch (ClassNotFoundException e) 
    {

    }
    catch (NoClassDefFoundError e)
    {
        // Basing on error message get the class package name
        String classPackage = getClassPackage(e.getMessage());

        try {             

           // Load the class once more!
           dynamicClass = cl.loadClass(classPackage);
        } 
        catch (ClassNotFoundException ex) 
        {

        }
    }
}

Second method is used to get package name from exception message:

第二种方法用于从异常消息中获取包名：

private static String getClassPackage(String errorMsg){

    // Start and end index of cutting
    int startIndex = errorMsg.lastIndexOf(" ") + 1;
    int endIndex = errorMsg.length() - 1;

    // Let's save a substring
    String classPackage = errorMsg.substring(startIndex, endIndex);

    // Replace char '/' to '.'
    classPackage = classPackage.replace('/', '.');

    return classPackage;
}

Code of method getFileNameWithoutExtension:

方法getFileNameWithoutExtension 的代码：

private static String getFileNameWithoutExtension(String path){
    int start = path.lastIndexOf(File.separator) + 1;
    int end = path.lastIndexOf(DOT);
    end = start < end ? end : path.length();
    String name = path.substring(start, end);

    return name;
}

Where the static final variable is:

其中静态最终变量是：

private static final String DOT = ".";

And here is my question: is it possible to get package name from .class file without using this kind of trick?

这是我的问题：是否可以在不使用这种技巧的情况下从 .class 文件中获取包名？

回答by William Reed

You can use the Foo.class.getPackage().getName()method to determine this.

您可以使用该Foo.class.getPackage().getName()方法来确定这一点。

    public Package getPackage()

Returns:

返回：

the package of the class, or null if no package information is available from the archive or codebase.

类的包，如果存档或代码库中没有可用的包信息，则为 null。

Using getName() :

使用 getName() ：

    public String getName()

Returns:

返回：

The fully-qualified name of this package as defined in section 6.5.3 of The Java? Language Specification, for example, java.lang

The Java 的 6.5.3 节中定义的此包的完全限定名称？语言规范，例如java.lang

回答by ADTC

Since you already have the required data in className, just use it again. You don't need the getClassPackagemethod.

由于您已经在 中拥有所需的数据className，只需再次使用它即可。你不需要这个getClassPackage方法。

    catch (NoClassDefFoundError e)
    {
        // Basing on error message get the class package name
        //But we already have the class name in className variable!
        //String classPackage = getClassPackage(e.getMessage());

        try {             

           // Load the class once more!
           dynamicClass = cl.loadClass(className);
        } 
        catch (ClassNotFoundException ex) 
        {

        }
    }

And if you want to get the package name only (not sure why), you can just get it from the class name:

如果你只想获取包名（不知道为什么），你可以从类名中获取它：

String packageName = className.substring(0, className.lastIndexOf('.'));
dynamicClass = cl.loadClass(packageName);

回答by Akhilesh Dhar Dubey

You can do like this-

你可以这样做-

String packName = new Object(){}.getClass().getPackage().getName();        
System.out.println(packName);