Refine ApplicationPid for structured logging

Update `ApplicationPid` with `toLong()` and `isAvailable()` methods to
make it easier to use with structured logging.

See gh-41491

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

@@ -38,25 +38,43 @@ public class ApplicationPid {
 	private static final PosixFilePermission[] WRITE_PERMISSIONS = { PosixFilePermission.OWNER_WRITE,
 			PosixFilePermission.GROUP_WRITE, PosixFilePermission.OTHERS_WRITE };
 
-	private final String pid;
+	private final Long pid;
 
 	public ApplicationPid() {
-		this.pid = getPid();
+		this.pid = currentProcessPid();
 	}
 
-	protected ApplicationPid(String pid) {
+	protected ApplicationPid(Long pid) {
 		this.pid = pid;
 	}
 
-	private String getPid() {
+	private Long currentProcessPid() {
 		try {
-			return Long.toString(ProcessHandle.current().pid());
+			return Long.valueOf(ProcessHandle.current().pid());
 		}
 		catch (Throwable ex) {
 			return null;
 		}
 	}
 
+	/**
+	 * Return if the application PID is available.
+	 * @return {@code true} if the PID is available
+	 * @since 3.4.0
+	 */
+	public boolean isAvailable() {
+		return this.pid != null;
+	}
+
+	/**
+	 * Return the application PID as a {@link Long}.
+	 * @return the application PID or {@code null}
+	 * @since 3.4.0
+	 */
+	public Long toLong() {
+		return this.pid;
+	}
+
 	@Override
 	public boolean equals(Object obj) {
 		if (obj == this) {
@@ -75,7 +93,7 @@ public class ApplicationPid {
 
 	@Override
 	public String toString() {
-		return (this.pid != null) ? this.pid : "???";
+		return (this.pid != null) ? String.valueOf(this.pid) : "???";
 	}
 
 	/**
@@ -91,7 +109,7 @@ public class ApplicationPid {
 			assertCanOverwrite(file);
 		}
 		try (FileWriter writer = new FileWriter(file)) {
-			writer.append(this.pid);
+			writer.append(String.valueOf(this.pid));
 		}
 	}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/system/ApplicationPidTests.java

@@ -37,7 +37,7 @@ class ApplicationPidTests {
 
 	@Test
 	void toStringWithPid() {
-		assertThat(new ApplicationPid("123")).hasToString("123");
+		assertThat(new ApplicationPid(123L)).hasToString("123");
 	}
 
 	@Test
@@ -54,7 +54,7 @@ class ApplicationPidTests {
 
 	@Test
 	void writePid() throws Exception {
-		ApplicationPid pid = new ApplicationPid("123");
+		ApplicationPid pid = new ApplicationPid(123L);
 		File file = new File(this.tempDir, "pid");
 		pid.write(file);
 		assertThat(contentOf(file)).isEqualTo("123");
@@ -63,13 +63,37 @@ class ApplicationPidTests {
 	@Test
 	void writeNewPid() throws Exception {
 		// gh-10784
-		ApplicationPid pid = new ApplicationPid("123");
+		ApplicationPid pid = new ApplicationPid(123L);
 		File file = new File(this.tempDir, "pid");
 		file.delete();
 		pid.write(file);
 		assertThat(contentOf(file)).isEqualTo("123");
 	}
 
+	@Test
+	void toLong() {
+		ApplicationPid pid = new ApplicationPid(123L);
+		assertThat(pid.toLong()).isEqualTo(123L);
+	}
+
+	@Test
+	void toLongWhenNotAvailable() {
+		ApplicationPid pid = new ApplicationPid(null);
+		assertThat(pid.toLong()).isNull();
+	}
+
+	@Test
+	void isAvailableWhenAvailable() {
+		ApplicationPid pid = new ApplicationPid(123L);
+		assertThat(pid.isAvailable()).isTrue();
+	}
+
+	@Test
+	void isAvailableWhenNotAvailable() {
+		ApplicationPid pid = new ApplicationPid(null);
+		assertThat(pid.isAvailable()).isFalse();
+	}
+
 	@Test
 	void getPidFromJvm() {
 		assertThat(new ApplicationPid().toString()).isNotEmpty();

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/system/MockApplicationPid.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2012-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.system;
+
+/**
+ * Factory to create mock {@link ApplicationPid} instances for testing.
+ *
+ * @author Phillip Webb
+ */
+public final class MockApplicationPid {
+
+	private MockApplicationPid() {
+	}
+
+	public static ApplicationPid of(long value) {
+		return new ApplicationPid(value);
+	}
+
+}