diff --git a/build.gradle b/build.gradle
index f6e91faebc..327f7c663c 100644
--- a/build.gradle
+++ b/build.gradle
@@ -85,6 +85,10 @@ configure(javaprojects) {
     apply plugin: 'eclipse' // `gradle eclipse` to generate .classpath/.project
     apply plugin: 'idea' // `gradle idea` to generate .ipr/.iml
 
+    // ensure JDK 5 compatibility
+    sourceCompatibility=1.5
+    targetCompatibility=1.5
+
     // set up dedicated directories for jars and source jars.
     // this makes it easier when putting together the distribution
     libsBinDir = new File(libsDir, 'bin')