Java Reflection - Get List of Packages

Question

I am building a Java app that has a number of different packages. I want to be able to tell programmatically which packages exist in the app that start with a specific pre-fix. Is there anyway to do this with the Java reflection APIs? I didn't see anything like this related to the reflection apis.

Example:

com.app.controls.text
com.app.controls.picker
com.app.controls.date
etc

I want to enumerate all of these by knowing the prefix "com.app.controls" and understanding that a new package might get integrated in the future.

Thanks!

Sean Reilly · Accepted Answer · 2018-08-09T09:21:58.137

23

You can do this by using Package.getPackages(), which returns an array of all packages known to the current class loader. You'll have to manually loop through the array and find the ones with the appropriate prefix using getName().

Here's a quick example:

public List<String> findPackageNamesStartingWith(String prefix) {
    return Package.getPackages().stream()
        .map(Package::getName)
        .filter(n -> n.startsWith(prefix))
        .collect(toList());
}

Note that this technique will only return the packages defined in the current class loader. If you need the packages from a different class loader, there are some options:

Arrange things so that your program can run the above code from inside that class loader. This requires a certain organisation to your code base, which may or may not be feasible.
Use reflection to call the (normally protected) method getPackages() on the appropriate class loader. This won't work if the program is running under a security manager.

edited Aug 09 '18 at 09:21

answered Dec 19 '12 at 02:25

Sean Reilly

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Unfortunately the packages aren't known to the current class loader. Is there any other possible way? – Derek Gebhard Dec 19 '12 at 02:55
Probably not, other than listing them explicitly. – Louis Wasserman Dec 19 '12 at 02:57
@DerekGebhard: Two possibilities came to mind. I've added them to the answer. – Sean Reilly Dec 19 '12 at 03:11
`Package.getPackages()` returns an array. You need to use `Arrays.stream(Package.getPackages())` instead of `Package.getPackages().stream()`. – wickund Jan 27 '23 at 13:29

score 0 · Answer 2 · edited May 23 '17 at 12:25

Based on Sean's answer and using reflection to get a list of packages - possibly ignoring empty ones:

/**
 * Finds all package names starting with prefix
 * @return Set of package names
 */
public Set<String> findAllPackagesStartingWith(String prefix) {
    List<ClassLoader> classLoadersList = new LinkedList<ClassLoader>();
    classLoadersList.add(ClasspathHelper.contextClassLoader());
    classLoadersList.add(ClasspathHelper.staticClassLoader());
    Reflections reflections = new Reflections(new ConfigurationBuilder()
            .setScanners(new SubTypesScanner(false), new ResourcesScanner())
            .setUrls(ClasspathHelper.forClassLoader(classLoadersList.toArray(new ClassLoader[0])))
            .filterInputsBy(new FilterBuilder().include(FilterBuilder.prefix("my.base.package"))));
    Set<Class<? extends Object>> classes = reflections.getSubTypesOf(Object.class);

    Set<String> packageNameSet = new TreeSet<String>();
    for (Class classInstance : classes) {
        String packageName = classInstance.getPackage().getName();
        if (packageName.startsWith(prefix)) {
            packageNameSet.add(packageName);
        }
    }
    return packageNameSet;
}

Most of these classes don't exist in my native library. Are you using a third party? — Lord Farquaad, Aug 07 '18 at 15:34
@LordFarquaad this is an old response (January, 2014) I assume your Java version is fairly new compared to what I was using back then. — BLuEGoD, Feb 08 '19 at 13:58

Java Reflection - Get List of Packages

2 Answers2

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