Friendly prompt that it cannot download some libraries successfully.

HMCL/src/main/java/org/jackhuang/hmcl/game/LauncherHelper.java

@@ -25,6 +25,7 @@ import org.jackhuang.hmcl.auth.AuthenticationException;
 import org.jackhuang.hmcl.auth.CredentialExpiredException;
 import org.jackhuang.hmcl.download.DefaultDependencyManager;
 import org.jackhuang.hmcl.download.MaintainTask;
+import org.jackhuang.hmcl.download.game.LibraryDownloadException;
 import org.jackhuang.hmcl.launch.*;
 import org.jackhuang.hmcl.mod.CurseCompletionException;
 import org.jackhuang.hmcl.mod.CurseCompletionTask;
@@ -190,6 +191,8 @@ public final class LauncherHelper {
                                     message = i18n("launch.failed.creating_process") + ex.getLocalizedMessage();
                                 } else if (ex instanceof NotDecompressingNativesException) {
                                     message = i18n("launch.failed.decompressing_natives") + ex.getLocalizedMessage();
+                                } else if (ex instanceof LibraryDownloadException) {
+                                    message = i18n("launch.failed.download_library", ((LibraryDownloadException) ex).getLibrary().getName()) + "\n" + StringUtils.getStackTrace(ex.getCause());
                                 } else {
                                     message = StringUtils.getStackTrace(ex);
                                 }

HMCL/src/main/resources/assets/lang/I18N.properties

@@ -139,6 +139,7 @@ launch.failed=Unable to launch
 launch.failed.cannot_create_jvm=Java virtual machine cannot be created. Java arguments may have problems. You can enable the no args mode in the settings.
 launch.failed.creating_process=Failed to create process, maybe your java path is wrong, please modify your java path.
 launch.failed.decompressing_natives=Unable to decompress native libraries.
+launch.failed.download_library=Unable to download library %s.
 launch.failed.executable_permission=Unable to add permission to the launch script
 launch.failed.exited_abnormally=Game exited abnormally, please visit the log, or ask someone for help.
 launch.state.dependencies=Dependencies

HMCL/src/main/resources/assets/lang/I18N_zh.properties

@@ -139,6 +139,7 @@ launch.failed=啓動失敗
 launch.failed.cannot_create_jvm=截獲到無法創建 Java 虛擬機，可能是 Java 參數有問題，可以在設置中開啓無參數模式啓動。
 launch.failed.creating_process=啓動失敗，在創建新進程時發生錯誤，可能是 Java 路徑錯誤。
 launch.failed.decompressing_natives=未能解壓遊戲本地庫。
+launch.failed.download_library=未能下载游戏依赖 %s.
 launch.failed.executable_permission=未能爲啓動文件添加執行權限。
 launch.failed.exited_abnormally=遊戲非正常退出，請查看日志文件，或聯繫他人尋求幫助。
 launch.state.dependencies=正在處理遊戲依賴

HMCL/src/main/resources/assets/lang/I18N_zh_CN.properties

@@ -139,6 +139,7 @@ launch.failed=启动失败
 launch.failed.cannot_create_jvm=截获到无法创建 Java 虚拟机，可能是 Java 参数有问题，可以在设置中开启无参数模式启动。
 launch.failed.creating_process=启动失败，在创建新进程时发生错误，可能是 Java 路径错误。
 launch.failed.decompressing_natives=未能解压游戏本地库。
+launch.failed.download_library=未能下载游戏依赖 %s.
 launch.failed.executable_permission=未能为启动文件添加执行权限。
 launch.failed.exited_abnormally=游戏非正常退出，请查看日志文件，或联系他人寻求帮助。
 launch.state.dependencies=正在处理游戏依赖

HMCLCore/src/main/java/org/jackhuang/hmcl/download/game/LibraryDownloadException.java

@@ -0,0 +1,34 @@
+/*
+ * Hello Minecraft! Launcher.
+ * Copyright (C) 2017  huangyuhui <huanghongxun2008@126.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see {http://www.gnu.org/licenses/}.
+ */
+package org.jackhuang.hmcl.download.game;
+
+import org.jackhuang.hmcl.game.Library;
+
+public class LibraryDownloadException extends Exception {
+    private final Library library;
+
+    public LibraryDownloadException(Library library, Throwable cause) {
+        super("Unable to download library " + library, cause);
+
+        this.library = library;
+    }
+
+    public Library getLibrary() {
+        return library;
+    }
+}

HMCLCore/src/main/java/org/jackhuang/hmcl/download/game/LibraryDownloadTask.java

@@ -55,7 +55,7 @@ public final class LibraryDownloadTask extends Task {
 
     @Override
     public Collection<? extends Task> getDependents() {
-        return library.getChecksums() != null ? Collections.singleton(xzTask) : Collections.emptySet();
+        return library.getChecksums() != null ? Collections.singleton(xzTask) : Collections.singleton(task);
     }
 
     @Override
@@ -65,20 +65,23 @@ public final class LibraryDownloadTask extends Task {
 
     @Override
     public void execute() throws Exception {
-        if (isDependentsSucceeded() && library.getChecksums() != null) {
+        if (!isDependentsSucceeded()) {
+            // Since FileDownloadTask wraps the actual exception with another IOException.
+            // We should extract it letting the error message clearer.
+            Throwable t = library.getChecksums() != null ? xzTask.getLastException() : task.getLastException();
+            if (t.getCause() != null && t.getCause() != t)
+                throw new LibraryDownloadException(library, t.getCause());
+            else
+                throw new LibraryDownloadException(library, t);
+        }
+
+        if (library.getChecksums() != null) {
             unpackLibrary(jar, FileUtils.readBytes(xzFile));
             if (!checksumValid(jar, library.getChecksums()))
                 throw new IOException("Checksum failed for " + library);
-
-            downloaded = true;
         }
     }
 
-    @Override
-    public Collection<? extends Task> getDependencies() {
-        return downloaded ? Collections.emptySet() : Collections.singleton(task);
-    }
-
     private static boolean checksumValid(File libPath, List<String> checksums) {
         try {
             if ((checksums == null) || (checksums.isEmpty())) {

HMCLCore/src/main/java/org/jackhuang/hmcl/task/Task.java

@@ -63,6 +63,16 @@ public abstract class Task {
         this.state = state;
     }
 
+    private Throwable lastException = null;
+
+    public Throwable getLastException() {
+        return lastException;
+    }
+
+    void setLastException(Throwable e) {
+        lastException = e;
+    }
+
     /**
      * The scheduler that decides how this task runs.
      */

HMCLCore/src/main/java/org/jackhuang/hmcl/task/TaskExecutor.java

@@ -190,6 +190,7 @@ public final class TaskExecutor {
         } catch (SilentException | RejectedExecutionException e) {
             // do nothing
         } catch (Exception e) {
+            task.setLastException(e);
             lastException = e;
             variables.set(LAST_EXCEPTION_ID, e);
             if (task.getSignificance().shouldLog()) {