Merge pull request #6759 from hengyunab

* gh-6759:
  Polish “Avoid null handler package in JarFile protocol handler registration”
  Avoid null handler package in JarFile protocol handler registration

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -449,7 +449,7 @@ public class JarFile extends java.util.jar.JarFile implements Iterable<JarEntryD
 	 * {@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();

spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java

@@ -63,6 +63,9 @@ 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();
@@ -453,4 +456,42 @@ 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, equalTo(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, equalTo("com.example|" + HANDLERS_PACKAGE));
+		}
+		finally {
+			if (original == null) {
+				System.clearProperty(PROTOCOL_HANDLER);
+			}
+			else {
+				System.setProperty(PROTOCOL_HANDLER, original);
+			}
+		}
+	}
+
 }