Refactor synchronized to Lock in ConfigTreePropertySource

The synchronized guards an I/O operation. This code path can be
triggered every time a user calls .getProperty() on the Environment.

See gh-36670

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/ConfigTreePropertySource.java

@@ -29,6 +29,8 @@ import java.util.EnumSet;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeMap;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Stream;
 
 import org.springframework.boot.convert.ApplicationConversionService;
@@ -258,6 +260,8 @@ public class ConfigTreePropertySource extends EnumerablePropertySource<Path> imp
 
 		private final Path path;
 
+		private final Lock resourceLock = new ReentrantLock();
+
 		private final Resource resource;
 
 		private final Origin origin;
@@ -341,11 +345,15 @@ public class ConfigTreePropertySource extends EnumerablePropertySource<Path> imp
 				}
 				if (this.content == null) {
 					assertStillExists();
-					synchronized (this.resource) {
+					this.resourceLock.lock();
+					try {
 						if (this.content == null) {
 							this.content = FileCopyUtils.copyToByteArray(this.resource.getInputStream());
 						}
 					}
+					finally {
+						this.resourceLock.unlock();
+					}
 				}
 				return this.content;
 			}