resolve main class automatically

com.microsoft.java.debug.plugin/plugin.xml

@@ -5,6 +5,7 @@
       <delegateCommandHandler class="com.microsoft.java.debug.plugin.internal.JavaDebugDelegateCommandHandler">
             <command id="vscode.java.startDebugSession"/>
             <command id="vscode.java.resolveClasspath"/>
+            <command id="vscode.java.resolveMainClass"/>
             <command id="vscode.java.buildWorkspace"/>
             <command id="vscode.java.fetchUsageData"/>
             <command id="vscode.java.configLogLevel"/>

com.microsoft.java.debug.plugin/src/main/java/com/microsoft/java/debug/plugin/internal/JavaDebugDelegateCommandHandler.java

@@ -26,6 +26,8 @@ public class JavaDebugDelegateCommandHandler implements IDelegateCommandHandler
 
     public static String RESOLVE_CLASSPATH = "vscode.java.resolveClasspath";
 
+    public static String RESOLVE_MAINCLASS = "vscode.java.resolveMainClass";
+
     public static String BUILD_WORKSPACE = "vscode.java.buildWorkspace";
 
     public static String CONFIG_LOG_LEVEL = "vscode.java.configLogLevel";
@@ -40,6 +42,9 @@ public Object executeCommand(String commandId, List<Object> arguments, IProgress
         } else if (RESOLVE_CLASSPATH.equals(commandId)) {
             ResolveClasspathsHandler handler = new ResolveClasspathsHandler();
             return handler.resolveClasspaths(arguments);
+        } else if (RESOLVE_MAINCLASS.equals(commandId)) {
+            ResolveMainClassHandler handler = new ResolveMainClassHandler();
+            return handler.resolveMainClass();
         } else if (BUILD_WORKSPACE.equals(commandId)) {
             // TODO
         } else if (FETCH_USER_DATA.equals(commandId)) {

com.microsoft.java.debug.plugin/src/main/java/com/microsoft/java/debug/plugin/internal/ResolveMainClassHandler.java

@@ -0,0 +1,114 @@
+/*******************************************************************************
+ * Copyright (c) 2017 Microsoft Corporation and others.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     Microsoft Corporation - initial API and implementation
+ *******************************************************************************/
+
+package com.microsoft.java.debug.plugin.internal;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.logging.Logger;
+import java.util.stream.Collectors;
+
+import org.eclipse.core.resources.IProject;
+import org.eclipse.core.resources.IResource;
+import org.eclipse.core.runtime.CoreException;
+import org.eclipse.jdt.core.IMethod;
+import org.eclipse.jdt.core.JavaModelException;
+import org.eclipse.jdt.core.search.IJavaSearchConstants;
+import org.eclipse.jdt.core.search.IJavaSearchScope;
+import org.eclipse.jdt.core.search.SearchEngine;
+import org.eclipse.jdt.core.search.SearchMatch;
+import org.eclipse.jdt.core.search.SearchParticipant;
+import org.eclipse.jdt.core.search.SearchPattern;
+import org.eclipse.jdt.core.search.SearchRequestor;
+import org.eclipse.jdt.ls.core.internal.managers.ProjectsManager;
+
+import com.microsoft.java.debug.core.Configuration;
+
+public class ResolveMainClassHandler {
+    private static final Logger logger = Logger.getLogger(Configuration.LOGGER_NAME);
+
+    /**
+     * resolve main class and project name.
+     * @return an array of main class and project name
+     * @throws CoreException when there are errors when resolving main class.
+     */
+    public Object resolveMainClass() throws CoreException {
+        return resolveMainClassCore();
+    }
+
+    private List<ResolutionItem> resolveMainClassCore() throws CoreException {
+        IJavaSearchScope searchScope = SearchEngine.createWorkspaceScope();
+        SearchPattern pattern = SearchPattern.createPattern("main(String[]) void", IJavaSearchConstants.METHOD,
+                IJavaSearchConstants.DECLARATIONS, SearchPattern.R_CASE_SENSITIVE | SearchPattern.R_EXACT_MATCH);
+        ArrayList<ResolutionItem> res = new ArrayList<>();
+        SearchRequestor requestor = new SearchRequestor() {
+            @Override
+            public void acceptSearchMatch(SearchMatch match) {
+                Object element = match.getElement();
+                if (element instanceof IMethod) {
+                    IMethod method = (IMethod) element;
+                    try {
+                        if (method.isMainMethod()) {
+                            IResource resource = method.getResource();
+                            if (resource != null) {
+                                IProject project = resource.getProject();
+                                if (project != null) {
+                                    String mainClass = method.getDeclaringType().getFullyQualifiedName();
+                                    String projectName = ProjectsManager.DEFAULT_PROJECT_NAME.equals(project.getName()) ? null : project.getName();
+                                    res.add(new ResolutionItem(mainClass, projectName));
+                                }
+                            }
+                        }
+                    } catch (JavaModelException e) {
+                        // ignore
+                    }
+                }
+            }
+        };
+        SearchEngine searchEngine = new SearchEngine();
+        searchEngine.search(pattern, new SearchParticipant[] {SearchEngine.getDefaultSearchParticipant()},
+                searchScope, requestor, null /* progress monitor */);
+        return res.stream().distinct().collect(Collectors.toList());
+    }
+
+    private class ResolutionItem {
+        private String mainClass;
+        private String projectName;
+
+        public ResolutionItem(String mainClass, String projectName) {
+            this.mainClass = mainClass;
+            this.projectName = projectName;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) {
+                return true;
+            }
+            if (o instanceof ResolutionItem) {
+                ResolutionItem item = (ResolutionItem) o;
+                if (mainClass != null ? !mainClass.equals(item.mainClass) : item.mainClass != null) {
+                    return false;
+                }
+                if (projectName != null ? !projectName.equals(item.projectName) : item.projectName != null) {
+                    return false;
+                }
+                return true;
+            }
+            return false;
+        }
+
+        @Override
+        public int hashCode() {
+            return (mainClass == null ? 0 : mainClass.hashCode()) * 13 + (projectName == null ? 0 : projectName.hashCode());
+        }
+    }
+}