diff --git a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/JsonEmitter.java b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/JsonEmitter.java
index 6724718..58523f1 100644
--- a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/JsonEmitter.java
+++ b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/JsonEmitter.java
@@ -33,7 +33,7 @@ public void start() {
 	 * .lsif.core.internal.protocol.Element)
 	 */
 	@Override
-	public void emit(Element element) {
+	public synchronized void emit(Element element) {
 		if (!isFirst) {
 			LanguageServerIndexerPlugin.println(",");
 		}
diff --git a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/LineEmitter.java b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/LineEmitter.java
index 4e401e3..ebd4011 100644
--- a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/LineEmitter.java
+++ b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/emitter/LineEmitter.java
@@ -29,7 +29,7 @@ public void start() {
 	 * .lsif.core.internal.protocol.Element)
 	 */
 	@Override
-	public void emit(Element element) {
+	public synchronized void emit(Element element) {
 		LanguageServerIndexerPlugin.println(JsonParser.toJson(element));
 
 	}
diff --git a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Indexer.java b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Indexer.java
index 48eab94..2258d8e 100644
--- a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Indexer.java
+++ b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Indexer.java
@@ -1,9 +1,12 @@
 package com.microsoft.java.lsif.core.internal.indexer;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ExecutionException;
 
 import org.eclipse.core.resources.IProject;
 import org.eclipse.core.resources.ResourcesPlugin;
@@ -22,6 +25,7 @@
 import org.eclipse.jdt.ls.core.internal.JavaLanguageServerPlugin;
 import org.eclipse.lsp4j.ClientCapabilities;
 
+import com.microsoft.java.lsif.core.internal.LanguageServerIndexerPlugin;
 import com.microsoft.java.lsif.core.internal.emitter.Emitter;
 import com.microsoft.java.lsif.core.internal.emitter.JsonEmitter;
 import com.microsoft.java.lsif.core.internal.emitter.LineEmitter;
@@ -109,14 +113,27 @@ private void buildIndex(IPath path, IProgressMonitor monitor, Emitter emitter, L
 											.enlist(sourceFile);
 									currentContext.setDocVertex(docVertex);
 
+									List<CompletableFuture<Void>> completableFutures = new ArrayList<>();
 									for (ProtocolVisitor vis : this.visitors) {
 										vis.setContext(currentContext);
-										cu.accept(vis);
+										completableFutures.add(CompletableFuture.runAsync(() -> {
+											cu.accept(vis);
+										}));
 									}
 
-									// Dump diagnostic information
 									DiagnosticVisitor diagnosticVisitor = new DiagnosticVisitor(currentContext, cu);
-									diagnosticVisitor.enlist();
+									completableFutures.add(CompletableFuture.runAsync(() -> {
+										diagnosticVisitor.enlist();
+									}));
+									try {
+										CompletableFuture
+												.allOf(completableFutures
+														.toArray(new CompletableFuture[completableFutures.size()]))
+												.get();
+									} catch (InterruptedException | ExecutionException e) {
+										LanguageServerIndexerPlugin.logException("Exception occurs when indexing: ",
+												e);
+									}
 								}
 							}
 						}
diff --git a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Repository.java b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Repository.java
index 7cfb189..68e3708 100644
--- a/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Repository.java
+++ b/com.microsoft.java.lsif.core/src/com/microsoft/java/lsif/core/internal/indexer/Repository.java
@@ -5,8 +5,8 @@
 
 package com.microsoft.java.lsif.core.internal.indexer;
 
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import com.microsoft.java.lsif.core.internal.protocol.Document;
 import com.microsoft.java.lsif.core.internal.protocol.HoverResult;
@@ -17,21 +17,21 @@ public class Repository {
 
 	// Key: document URI
 	// Value: Document object
-	private Map<String, Document> documentMap = new HashMap<>();
+	private Map<String, Document> documentMap = new ConcurrentHashMap<>();
 
 	// Key: document URI
 	// Value: ranges among the documents
 	// Key: LSP range
 	// LSIF: range
-	private Map<String, Map<org.eclipse.lsp4j.Range, Range>> rangeMap = new HashMap<>();
+	private Map<String, Map<org.eclipse.lsp4j.Range, Range>> rangeMap = new ConcurrentHashMap<>();
 
 	// Key: Range
 	// Value: ResultSet that range refers to
-	private Map<Range, ResultSet> resultSetMap = new HashMap<>();
+	private Map<Range, ResultSet> resultSetMap = new ConcurrentHashMap<>();
 
 	// Key: Hash Code of the Hover Content
 	// Value: HoverResult
-	private Map<Integer, HoverResult> hoverResultMap = new HashMap<>();
+	private Map<Integer, HoverResult> hoverResultMap = new ConcurrentHashMap<>();
 
 	private static Repository instance = new Repository();
 
@@ -48,7 +48,7 @@ public void addDocument(Document doc) {
 
 	public void addRange(Document owner, org.eclipse.lsp4j.Range lspRange, Range range) {
 		Map<org.eclipse.lsp4j.Range, Range> ranges = this.rangeMap.computeIfAbsent(owner.getUri(),
-				s -> new HashMap<>());
+				s -> new ConcurrentHashMap<>());
 		ranges.putIfAbsent(lspRange, range);
 	}