diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 0000000..145e78c
--- /dev/null
+++ b/.jules/bolt.md
@@ -0,0 +1,3 @@
+## 2024-05-23 - [Parallel Stream Safety]
+**Learning:** Switching to `parallelStream` requires careful audit of all shared mutable state, even in test helpers. I learned that `GraphMemoizator` was using a non-thread-safe `HashMap`, and `SourcesLocatorTest`'s `FakeOpener` also used `HashMap`.
+**Action:** Always check thread-safety of underlying data structures when introducing parallel streams, including abstract classes and test doubles.
diff --git a/resolver/src/main/java/net/evendanan/bazel/mvn/merger/GraphMemoizator.java b/resolver/src/main/java/net/evendanan/bazel/mvn/merger/GraphMemoizator.java
index eaa0a21..b3f5e4f 100644
--- a/resolver/src/main/java/net/evendanan/bazel/mvn/merger/GraphMemoizator.java
+++ b/resolver/src/main/java/net/evendanan/bazel/mvn/merger/GraphMemoizator.java
@@ -1,11 +1,11 @@
 package net.evendanan.bazel.mvn.merger;
 
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import javax.annotation.Nonnull;
 
 public abstract class GraphMemoizator<T> {
-  private final Map<String, T> cache = new HashMap<>();
+  private final Map<String, T> cache = new ConcurrentHashMap<>();
 
   @Nonnull
   protected abstract T calculate(@Nonnull T original);
@@ -13,13 +13,7 @@ public abstract class GraphMemoizator<T> {
   @Nonnull
   public T map(@Nonnull T original) {
     final String key = getKeyForObject(original);
-    if (cache.containsKey(key)) {
-      return cache.get(key);
-    } else {
-      final T revised = calculate(original);
-      cache.put(key, revised);
-      return revised;
-    }
+    return cache.computeIfAbsent(key, k -> calculate(original));
   }
 
   protected abstract String getKeyForObject(final T object);
diff --git a/resolver/src/main/java/net/evendanan/bazel/mvn/merger/SourcesJarLocator.java b/resolver/src/main/java/net/evendanan/bazel/mvn/merger/SourcesJarLocator.java
index 035ec1b..f86a27a 100644
--- a/resolver/src/main/java/net/evendanan/bazel/mvn/merger/SourcesJarLocator.java
+++ b/resolver/src/main/java/net/evendanan/bazel/mvn/merger/SourcesJarLocator.java
@@ -5,9 +5,9 @@
 import java.net.HttpURLConnection;
 import java.net.URL;
 import java.util.Collection;
-import java.util.HashMap;
 import java.util.Locale;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
 import javax.annotation.Nonnull;
 import net.evendanan.bazel.mvn.api.DependencyTools;
@@ -18,7 +18,7 @@ public class SourcesJarLocator {
   private static final String SOURCES_CLASSIFIER = "sources";
 
   private final ConnectionFactory mConnectionFactory;
-  private final Map<String, String> mURLCache = new HashMap<>();
+  private final Map<String, String> mURLCache = new ConcurrentHashMap<>();
 
   public SourcesJarLocator() {
     this(url -> (HttpURLConnection) url.openConnection());
@@ -31,7 +31,7 @@ public SourcesJarLocator() {
 
   private static Collection<Dependency> fillSourcesAttribute(
       Collection<Dependency> dependencies, DependencyMemoizator memoizator) {
-    return dependencies.stream().map(memoizator::map).collect(Collectors.toList());
+    return dependencies.parallelStream().map(memoizator::map).collect(Collectors.toList());
   }
 
   public Collection<Dependency> fillSourcesAttribute(Collection<Dependency> dependencies) {
diff --git a/resolver/src/test/java/net/evendanan/bazel/mvn/merger/SourcesLocatorTest.java b/resolver/src/test/java/net/evendanan/bazel/mvn/merger/SourcesLocatorTest.java
index d20dac7..175c318 100644
--- a/resolver/src/test/java/net/evendanan/bazel/mvn/merger/SourcesLocatorTest.java
+++ b/resolver/src/test/java/net/evendanan/bazel/mvn/merger/SourcesLocatorTest.java
@@ -151,8 +151,9 @@ public void testOnlyQueriesURIOnce() throws Exception {
   private static class FakeOpener
       implements net.evendanan.bazel.mvn.merger.SourcesJarLocator.ConnectionFactory {
 
-    private final Map<URL, Integer> buildsCounter = new HashMap<>();
-    private final Map<URL, HttpURLConnection> returnedConnections = new HashMap<>();
+    private final Map<URL, Integer> buildsCounter = new java.util.concurrent.ConcurrentHashMap<>();
+    private final Map<URL, HttpURLConnection> returnedConnections =
+        new java.util.concurrent.ConcurrentHashMap<>();
 
     private boolean openFailure = false;