为什么我在使用 ProGuard 混淆时会出错，但在没有混淆时却没有？

Question

NPE - http://pastebin.com/gzuf3x1k（当我执行混淆文件时出现）。 （带有 LineNumber 的 StackTrace：http://pastebin.com/ddY3Ei9G）

（混淆后的堆栈跟踪：http://pastebin.com/svipdakN）

addRefreshedVersionsListener(VersionManager.java)：

public void addRefreshedVersionsListener(RefreshedVersionsListener listener){
    this.refreshedVersionsListeners.add(listener);
}

getResourceFiles(VersionManager.java)：

private Set<Downloadable> getResourceFiles(Proxy proxy, File baseDirectory, CompleteVersion version)
    {
        Set result = new HashSet();
        InputStream inputStream = null;
        File assets = new File(baseDirectory, "assets");
        File objectsFolder = new File(assets, "objects");
        File indexesFolder = new File(assets, "indexes");
        String indexName = version.getAssets();
        long start = System.nanoTime();
        if (indexName == null) {
            indexName = "legacy";
        }
        File indexFile = new File(indexesFolder, indexName + ".json");
        try
        {
            URL indexUrl = new URL("https://s3.amazonaws.com/Minecraft.Download/indexes/" + indexName + ".json");
            inputStream = indexUrl.openConnection(proxy).getInputStream();
            String json = IOUtils.toString(inputStream);
            FileUtils.writeStringToFile(indexFile, json);
            AssetIndex index = (AssetIndex)this.gson.fromJson(json, AssetIndex.class);
            for (AssetIndex.AssetObject object : index.getUniqueObjects())
            {
                String filename = object.getHash().substring(0, 2) + "/" + object.getHash();
                File file = new File(objectsFolder, filename);
                if ((!file.isFile()) || (FileUtils.sizeOf(file) != object.getSize()))
                {
                    Downloadable downloadable = new AssetDownloadable(proxy, new URL("http://resources.download.minecraft.net/" + filename), file, false, object.getHash(), object.getSize());
                    downloadable.setExpectedSize(object.getSize());
                    result.add(downloadable);
                }
            }
            long end = System.nanoTime();
            long delta = end - start;
            Launcher.getInstance().println("Delta time to compare resources: " + delta / 1000000L + " ms ");
        }
        catch (IOException|JsonSyntaxException ex)
        {
            Launcher.getInstance().println("Couldn't download resources", ex);
        }
        finally
        {
            IOUtils.closeQuietly(inputStream);
        }
        return result;
    }

& 第一个原因 [at net.minecraft.launcher.updater.VersionManager.refreshVersions(VersionManager.java)]

   public void refreshVersions() throws IOException {
      this.clearCache();
      File[] files = this.baseVersionsDir.listFiles();
      if(files != null) {
         File[] i$ = files;
         int version = files.length;

         for(int type = 0; type < version; ++type) {
            File directory = i$[type];
            String id = directory.getName();
            File jsonFile = new File(directory, id + ".json");
            if(directory.isDirectory() && jsonFile.exists()) {
               try {
                  String ex = "versions/" + id + "/" + id + ".json";
                  CompleteVersion version1 = (CompleteVersion)this.gson.fromJson(this.getContent(ex), CompleteVersion.class);
                  if(version1.getId().equals(id)) {
                     this.addVersion(version1);
                  } else if(Launcher.getInstance() != null) {
                     Launcher.getInstance().println("Ignoring: " + ex + "; it contains id: \'" + version1.getId() + "\' expected \'" + id + "\'");
                  }
               } catch (RuntimeException var10) {
                  if(Launcher.getInstance() == null) {
                     throw new JsonSyntaxException("Loading file: " + jsonFile.toString(), var10);
                  }
                  Launcher.getInstance().println("Couldn\'t load local version " + jsonFile.getAbsolutePath(), var10);
               }
            }
         }

堆栈跟踪上的原因位置：

CompleteVersion version1 = (CompleteVersion)this.gson.fromJson(this.getContent(ex), CompleteVersion.class);
                  if(version1.getId().equals(id)) {
                     this.addVersion(version1);
                  } else if(Launcher.getInstance() != null) {
                     Launcher.getInstance().println("Ignoring: " + ex + "; it contains id: \'" + version1.getId() + "\' expected \'" + id + "\'");
                  }

出于某种奇怪的原因，GSON API 中的 fromJson 方法总是会发生这种情况（在混淆后执行时）。

CompleteVersion 类：http://pastebin.com/dF6aXCjS

Answer 1

我猜这是因为当 GSON 尝试解析 JSON 时，它使用 CompleteVersion 类中的属性名称。如果这些名称被破坏，GSON 将无法在 JSON 中找到属性名称，因此属性将简单地为空。

Another StackOverflow question 似乎有几个解决这个问题的方法。我个人从未使用过 ProGuard，但看起来您可以尝试在 proguard.cfg 中添加类似 -keep class net.minecraft.launcher.versions.CompleteVersion { *; } 的内容，这样 ProGuard 就不会触及 CompleteVersion 类。