Refactor synchronized to Lock in ApplicationTemp

The synchronized guards an I/O operation. Additionally, this adds
double-checked locking to the path instance field.

See gh-36670

Author: mhalbritter

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/system/ApplicationTemp.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2022 the original author or authors.
+ * Copyright 2012-2023 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.
@@ -28,6 +28,8 @@
 import java.security.MessageDigest;
 import java.util.EnumSet;
 import java.util.HexFormat;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -49,6 +51,8 @@ public class ApplicationTemp {
 
 	private final Class<?> sourceClass;
 
+	private final Lock pathLock = new ReentrantLock();
+
 	private volatile Path path;
 
 	/**
@@ -90,9 +94,15 @@ public File getDir(String subDir) {
 
 	private Path getPath() {
 		if (this.path == null) {
-			synchronized (this) {
-				String hash = HexFormat.of().withUpperCase().formatHex(generateHash(this.sourceClass));
-				this.path = createDirectory(getTempDirectory().resolve(hash));
+			this.pathLock.lock();
+			try {
+				if (this.path == null) {
+					String hash = HexFormat.of().withUpperCase().formatHex(generateHash(this.sourceClass));
+					this.path = createDirectory(getTempDirectory().resolve(hash));
+				}
+			}
+			finally {
+				this.pathLock.unlock();
 			}
 		}
 		return this.path;