diff --git a/build.gradle b/build.gradle
index f3fdf3233d0..44d081bdf7d 100644
--- a/build.gradle
+++ b/build.gradle
@@ -24,6 +24,9 @@ plugins {
 
     id 'checkstyle'
 
+    // Homepage: https://github.com/kelloggm/checkerframework-gradle-plugin
+    id 'org.checkerframework' version '0.6.40'
+
     id 'project-report'
 
     id 'idea'
@@ -38,6 +41,8 @@ apply plugin: XjcPlugin
 
 apply from: 'eclipse.gradle'
 
+apply plugin: 'org.checkerframework'
+
 group = "org.jabref"
 version = project.findProperty('projVersion') ?: '100.0.0'
 
@@ -322,6 +327,11 @@ dependencies {
     rewrite("org.openrewrite.recipe:rewrite-testing-frameworks")
     rewrite("org.openrewrite.recipe:rewrite-migrate-java")
 
+    // checker framework
+    compileOnly 'org.checkerframework:checker-qual:3.44.0'
+    testCompileOnly 'org.checkerframework:checker-qual:3.44.0'
+    checkerFramework 'org.checkerframework:checker:3.44.0'
+
     configurations.checkstyle {
         resolutionStrategy.capabilitiesResolution.withCapability("com.google.collections:google-collections") {
             select("com.google.guava:guava:0")
@@ -614,6 +624,13 @@ checkstyle {
     sourceSets = []
 }
 
+checkerFramework {
+    checkers = [
+        'org.checkerframework.checker.nullness.NullnessChecker'
+    ]
+    excludeTests = true
+}
+
 rewrite {
     activeRecipe(
         'org.jabref.config.rewrite.cleanup'
diff --git a/src/main/java/module-info.java b/src/main/java/module-info.java
index 352ef69c95b..fa4ceb494ac 100644
--- a/src/main/java/module-info.java
+++ b/src/main/java/module-info.java
@@ -143,6 +143,7 @@
     uses org.eclipse.jgit.lib.GpgSigner;
 
     requires transitive org.jspecify;
+    requires org.checkerframework.checker.qual;
 
     // other libraries
     requires org.antlr.antlr4.runtime;
diff --git a/src/main/java/org/jabref/logic/pdf/search/PdfIndexer.java b/src/main/java/org/jabref/logic/pdf/search/PdfIndexer.java
index 45750ac5b84..1986c6c109b 100644
--- a/src/main/java/org/jabref/logic/pdf/search/PdfIndexer.java
+++ b/src/main/java/org/jabref/logic/pdf/search/PdfIndexer.java
@@ -55,8 +55,7 @@ public class PdfIndexer {
 
     private final FilePreferences filePreferences;
 
-    @Nullable
-    private final Directory indexDirectory;
+    private final @Nullable Directory indexDirectory;
 
     private IndexReader reader;