diff --git a/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java b/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java
index ab0a447619..7a3ee76a4f 100644
--- a/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java
+++ b/spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java
@@ -376,7 +376,7 @@ public class JarFile extends java.util.jar.JarFile {
 	 * {@link URLStreamHandler} will be located to deal with jar URLs.
 	 */
 	public static void registerUrlProtocolHandler() {
-		String handlers = System.getProperty(PROTOCOL_HANDLER);
+		String handlers = System.getProperty(PROTOCOL_HANDLER, "");
 		System.setProperty(PROTOCOL_HANDLER, ("".equals(handlers) ? HANDLERS_PACKAGE
 				: handlers + "|" + HANDLERS_PACKAGE));
 		resetCachedUrlHandlers();
diff --git a/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java b/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java
index d238656d30..f4cc67b9ff 100644
--- a/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java
+++ b/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java
@@ -54,6 +54,9 @@ import static org.mockito.Mockito.verify;
  * @author Andy Wilkinson
  */
 public class JarFileTests {
+	private static final String PROTOCOL_HANDLER = "java.protocol.handler.pkgs";
+
+	private static final String HANDLERS_PACKAGE = "org.springframework.boot.loader";
 
 	@Rule
 	public ExpectedException thrown = ExpectedException.none();
@@ -420,4 +423,42 @@ public class JarFileTests {
 		url.openConnection().getInputStream();
 	}
 
+	@Test
+	public void registerUrlProtocolHandlerWithNoExistingRegistration() {
+		String original = System.getProperty(PROTOCOL_HANDLER);
+		try {
+			System.clearProperty(PROTOCOL_HANDLER);
+			JarFile.registerUrlProtocolHandler();
+			String protocolHandler = System.getProperty(PROTOCOL_HANDLER);
+			assertThat(protocolHandler).isEqualTo(HANDLERS_PACKAGE);
+		}
+		finally {
+			if (original == null) {
+				System.clearProperty(PROTOCOL_HANDLER);
+			}
+			else {
+				System.setProperty(PROTOCOL_HANDLER, original);
+			}
+		}
+	}
+
+	@Test
+	public void registerUrlProtocolHandlerAddsToExistingRegistration() {
+		String original = System.getProperty(PROTOCOL_HANDLER);
+		try {
+			System.setProperty(PROTOCOL_HANDLER, "com.example");
+			JarFile.registerUrlProtocolHandler();
+			String protocolHandler = System.getProperty(PROTOCOL_HANDLER);
+			assertThat(protocolHandler).isEqualTo("com.example|" + HANDLERS_PACKAGE);
+		}
+		finally {
+			if (original == null) {
+				System.clearProperty(PROTOCOL_HANDLER);
+			}
+			else {
+				System.setProperty(PROTOCOL_HANDLER, original);
+			}
+		}
+	}
+
 }