How to list classes from JAR file in Java

August 21, 2020 No comments Java IO Jar Files

1. Introduction

In this tutorial, we are going to present a way to read files from the JAR archive in Java. This article will cover the method available in Java 8.

2. Using java.util.jar.JarFile to read JAR file

The best way to read the contents of a JAR archive is to use JarFile utility class available in Java 8. This class can read any Java Archive File that can be opened with RandomAccessFile.

The JarFile constructor accepts a single parameter with the Jar file.

The following example code shows how to print all .class files from Jar located in /tmp/commons-io-2.4.jar. 

package com.frontbackend.java.util;

import java.io.File;
import java.io.IOException;
import java.util.jar.JarFile;

public class ReadClassesFromJarFile {

    public static void main(String[] args) throws IOException {
        try (JarFile jar = new JarFile(new File("/tmp/commons-io-2.4.jar"))) {
            EnumerationUtils.enumerationAsStream(jar.entries())
                            .filter(jarEntry -> jarEntry.getName()
                                                        .endsWith(".class"))
                            .forEach(jarEntry -> System.out.println(jarEntry.getName()));

        }
    }
}

We used java.util.jar.JarFile.entries() method to get all resources from the JAR file. This method returns the Enumeration object that provides not so modern way to iterate over collections, so we created utility class EnumerationUtils that will convert Enumeration to the Stream object. 

package com.frontbackend.java.util;

import java.util.Enumeration;
import java.util.Iterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public final class EnumerationUtils {

    private EnumerationUtils() {
    }

    public static <T> Stream<T> enumerationAsStream(Enumeration<T> e) {
        return StreamSupport.stream(Spliterators.spliteratorUnknownSize(new Iterator<T>() {
            public T next() {
                return e.nextElement();
            }

            public boolean hasNext() {
                return e.hasMoreElements();
            }
        }, Spliterator.ORDERED), false);
    }
}

For version 2.4 of commons-io library this snippet will produce the following output:

org/apache/commons/io/ByteOrderMark.class
org/apache/commons/io/Charsets.class
org/apache/commons/io/comparator/AbstractFileComparator.class
org/apache/commons/io/comparator/CompositeFileComparator.class
org/apache/commons/io/comparator/DefaultFileComparator.class
org/apache/commons/io/comparator/DirectoryFileComparator.class
org/apache/commons/io/comparator/ExtensionFileComparator.class
org/apache/commons/io/comparator/LastModifiedFileComparator.class
org/apache/commons/io/comparator/NameFileComparator.class
org/apache/commons/io/comparator/PathFileComparator.class
org/apache/commons/io/comparator/ReverseComparator.class
org/apache/commons/io/comparator/SizeFileComparator.class
org/apache/commons/io/CopyUtils.class
org/apache/commons/io/DirectoryWalker$CancelException.class
org/apache/commons/io/DirectoryWalker.class
org/apache/commons/io/EndianUtils.class
org/apache/commons/io/FileCleaner.class
org/apache/commons/io/FileCleaningTracker$Reaper.class
org/apache/commons/io/FileCleaningTracker$Tracker.class
org/apache/commons/io/FileCleaningTracker.class
org/apache/commons/io/FileDeleteStrategy$ForceFileDeleteStrategy.class
org/apache/commons/io/FileDeleteStrategy.class
org/apache/commons/io/FileExistsException.class
org/apache/commons/io/filefilter/AbstractFileFilter.class
org/apache/commons/io/filefilter/AgeFileFilter.class
org/apache/commons/io/filefilter/AndFileFilter.class
org/apache/commons/io/filefilter/CanReadFileFilter.class
org/apache/commons/io/filefilter/CanWriteFileFilter.class
org/apache/commons/io/filefilter/ConditionalFileFilter.class
org/apache/commons/io/filefilter/DelegateFileFilter.class
org/apache/commons/io/filefilter/DirectoryFileFilter.class
org/apache/commons/io/filefilter/EmptyFileFilter.class
org/apache/commons/io/filefilter/FalseFileFilter.class
org/apache/commons/io/filefilter/FileFileFilter.class
org/apache/commons/io/filefilter/FileFilterUtils.class
org/apache/commons/io/filefilter/HiddenFileFilter.class
org/apache/commons/io/filefilter/IOFileFilter.class
org/apache/commons/io/filefilter/MagicNumberFileFilter.class
org/apache/commons/io/filefilter/NameFileFilter.class
org/apache/commons/io/filefilter/NotFileFilter.class
org/apache/commons/io/filefilter/OrFileFilter.class
org/apache/commons/io/filefilter/PrefixFileFilter.class
org/apache/commons/io/filefilter/RegexFileFilter.class
org/apache/commons/io/filefilter/SizeFileFilter.class
org/apache/commons/io/filefilter/SuffixFileFilter.class
org/apache/commons/io/filefilter/TrueFileFilter.class
org/apache/commons/io/filefilter/WildcardFileFilter.class
org/apache/commons/io/filefilter/WildcardFilter.class
org/apache/commons/io/FilenameUtils.class
org/apache/commons/io/FileSystemUtils.class
org/apache/commons/io/FileUtils.class
org/apache/commons/io/HexDump.class
org/apache/commons/io/input/AutoCloseInputStream.class
org/apache/commons/io/input/BOMInputStream$1.class
org/apache/commons/io/input/BOMInputStream.class
org/apache/commons/io/input/BoundedInputStream.class
org/apache/commons/io/input/BrokenInputStream.class
org/apache/commons/io/input/CharSequenceInputStream.class
org/apache/commons/io/input/CharSequenceReader.class
org/apache/commons/io/input/ClassLoaderObjectInputStream.class
org/apache/commons/io/input/ClosedInputStream.class
org/apache/commons/io/input/CloseShieldInputStream.class
org/apache/commons/io/input/CountingInputStream.class
org/apache/commons/io/input/DemuxInputStream.class
org/apache/commons/io/input/NullInputStream.class
org/apache/commons/io/input/NullReader.class
org/apache/commons/io/input/ProxyInputStream.class
org/apache/commons/io/input/ProxyReader.class
org/apache/commons/io/input/ReaderInputStream.class
org/apache/commons/io/input/ReversedLinesFileReader$1.class
org/apache/commons/io/input/ReversedLinesFileReader$FilePart.class
org/apache/commons/io/input/ReversedLinesFileReader.class
org/apache/commons/io/input/SwappedDataInputStream.class
org/apache/commons/io/input/TaggedInputStream.class
org/apache/commons/io/input/Tailer.class
org/apache/commons/io/input/TailerListener.class
org/apache/commons/io/input/TailerListenerAdapter.class
org/apache/commons/io/input/TeeInputStream.class
org/apache/commons/io/input/XmlStreamReader.class
org/apache/commons/io/input/XmlStreamReaderException.class
org/apache/commons/io/IOCase.class
org/apache/commons/io/IOExceptionWithCause.class
org/apache/commons/io/IOUtils.class
org/apache/commons/io/LineIterator.class
org/apache/commons/io/monitor/FileAlterationListener.class
org/apache/commons/io/monitor/FileAlterationListenerAdaptor.class
org/apache/commons/io/monitor/FileAlterationMonitor.class
org/apache/commons/io/monitor/FileAlterationObserver.class
org/apache/commons/io/monitor/FileEntry.class
org/apache/commons/io/output/BrokenOutputStream.class
org/apache/commons/io/output/ByteArrayOutputStream.class
org/apache/commons/io/output/ClosedOutputStream.class
org/apache/commons/io/output/CloseShieldOutputStream.class
org/apache/commons/io/output/CountingOutputStream.class
org/apache/commons/io/output/DeferredFileOutputStream.class
org/apache/commons/io/output/DemuxOutputStream.class
org/apache/commons/io/output/FileWriterWithEncoding.class
org/apache/commons/io/output/LockableFileWriter.class
org/apache/commons/io/output/NullOutputStream.class
org/apache/commons/io/output/NullWriter.class
org/apache/commons/io/output/ProxyOutputStream.class
org/apache/commons/io/output/ProxyWriter.class
org/apache/commons/io/output/StringBuilderWriter.class
org/apache/commons/io/output/TaggedOutputStream.class
org/apache/commons/io/output/TeeOutputStream.class
org/apache/commons/io/output/ThresholdingOutputStream.class
org/apache/commons/io/output/WriterOutputStream.class
org/apache/commons/io/output/XmlStreamWriter.class
org/apache/commons/io/TaggedIOException.class
org/apache/commons/io/ThreadMonitor.class

If you want to include commons-io dependency for your maven project, check the following link - you can find here all available versions.

3. Conclusion

In this short tutorial, we presented a way to get all files from Java Archive file using utility class available in Java 8. Note that this example code will work for not-signed files.

As always the example code use in this article is available in our GitHub repository.

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