Replace synchronized blocks and methods with locks

This commit replaces synchronized blocks and methods
that are used frequently or that guard blocking I/O
operations with locks. This is required to prevent
virtual threads pinning, as explained in JEP 444 [1].

Note that synchronized blocks and methods that are used
infrequently (like AutomaticJobRegistrar#start/stop) or
that guard in-memory operations were not replaced as this
is not required, see JEP 444 [1].

Resolves to #4399

---
[1]: https://openjdk.org/jeps/444

spring-batch-samples/src/main/java/org/springframework/batch/sample/common/StagingItemReader.java

@@ -20,6 +20,8 @@ import java.io.InputStream;
 import java.io.ObjectInputStream;
 import java.util.Iterator;
 import java.util.List;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import javax.sql.DataSource;
 
@@ -50,7 +52,7 @@ public class StagingItemReader<T>
 
 	private StepExecution stepExecution;
 
-	private final Object lock = new Object();
+	private final Lock lock = new ReentrantLock();
 
 	private volatile boolean initialized = false;
 
@@ -75,7 +77,8 @@ public class StagingItemReader<T>
 
 	private List<Long> retrieveKeys() {
 
-		synchronized (lock) {
+		this.lock.lock();
+		try {
 
 			return jdbcTemplate.query(
 
@@ -86,6 +89,9 @@ public class StagingItemReader<T>
 					stepExecution.getJobExecution().getJobId(), StagingItemWriter.NEW);
 
 		}
+		finally {
+			this.lock.unlock();
+		}
 
 	}