CODEWITHSUNDEEP
javajarjava-iooutputstream
Michael
Michael

Reputation: 11

The file gets smaller after reading the jar package and writing it to another file

package com.example.demo.Util;

public class Test {

static HashMap<String,String> map = new HashMap<>();

public static void main(String[] args) throws IOException {
    String data = "12j3h1i7tsa7sgdajk123y8asd: 88888";
    File jarFile = new File(new Test().getJarPath());
    File tempJar = upJarFile(jarFile, "BOOT-INF/classes/application.properties", data);
}


public static File upJarFile(File originalJarFile, String editFilePath, String content) throws IOException {
    File tempFile = File.createTempFile("temp", ".jar");
    JarFile jarFile = new JarFile(originalJarFile);
    Enumeration<JarEntry> entries = jarFile.entries();

    System.out.println("before:"+ originalJarFile.length());

    JarOutputStream jarOutputStream = new JarOutputStream(new FileOutputStream(tempFile));
    while (entries.hasMoreElements()) {
        JarEntry jarEntry = entries.nextElement();
        jarOutputStream.putNextEntry(jarEntry);
        map.put(jarEntry.getName(), String.valueOf(jarEntry.getSize()));
        jarOutputStream.write(new Test().inputStreamToByteArray(jarFile.getInputStream(jarEntry)));
    }
    jarOutputStream.finish();
    jarOutputStream.close();
    System.out.println(tempFile.getPath());

    System.out.println("after:" + tempFile.length());

    return tempFile;
}

public String getJarPath() {
    String path1 = System.getProperty("user.dir");
    File file = new File(path1 + "/target/");
    String jarFile = null;
    for (File file1 : file.listFiles()) {
        if (file1.getName().endsWith(".jar")) {
            jarFile = file1.getPath();
            break;
        }
    }
    return jarFile;
}


public byte[] inputStreamToByteArray(InputStream inputStream) {
    try (ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream()) {
        byte[] buffer = new byte[1024];
        int num;
        while ((num = inputStream.read(buffer)) != -1) {
            byteArrayOutputStream.write(buffer, 0, num);
        }
        byteArrayOutputStream.flush();
        return byteArrayOutputStream.toByteArray();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return new byte[]{};
}

}

As shown in the code above,I just turn the incoming jar packages into streams and write them one by one,But it got smaller when I tested the size of the input package and the size of the output temporary package(before:49651057-->after:49647985)

What could be causing this difference?

Upvotes: 1

Views: 132

Answers (1)

Mike Nakis
Mike Nakis

Reputation: 61995

This can happen due to a number of reasons:

  1. The original JAR file was created with a compression level that is not as high as the default compression level, so the JAR file that you create (with default compression) achieves better compression, and therefore it is smaller. You can verify this by opening both the original and the result JAR files with a ZIP utility (e.g. 7Zip) and examining their checksums and their compressed sizes. If the checksums are identical, but the compressed sizes differ, then the difference is simply due to better compression.

  2. The original JAR file contains unused data. This can happen when sloppy archive creation software updates an archive by appending to it instead of rewriting it from scratch. You can verify this by opening the original ZIP archive with a ZIP utility (e.g. 7Zip) and saving it under a new filename. If the new file is smaller, then the original file contained some unused data.

  3. The original JAR file contains files in subdirectories, which you are not checking. Thus, your output JAR file does not contain all of the files in the original. To fix this, you need to check each entry with jarEntry.isDirectory() and if so, recurse.

Upvotes: 1

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