Explicit handling of nested splits in SplitState

Resolves #3857

spring-batch-core/src/main/java/org/springframework/batch/core/job/builder/FlowBuilder.java

@@ -301,9 +301,9 @@ else if (input instanceof Flow) {
 		return result;
 	}
 
-	private SplitState createState(Collection<Flow> flows, TaskExecutor executor) {
+	private SplitState createState(Collection<Flow> flows, TaskExecutor executor, SplitState parentSplit) {
 		if (!states.containsKey(flows)) {
-			states.put(flows, new SplitState(flows, prefix + "split" + (splitCounter++)));
+			states.put(flows, new SplitState(flows, prefix + "split" + (splitCounter++), parentSplit));
 		}
 		SplitState result = (SplitState) states.get(flows);
 		if (executor != null) {
@@ -627,23 +627,23 @@ public SplitBuilder(FlowBuilder<Q> parent, TaskExecutor executor) {
 		public FlowBuilder<Q> add(Flow... flows) {
 			Collection<Flow> list = new ArrayList<>(Arrays.asList(flows));
 			String name = "split" + (parent.splitCounter++);
-			int counter = 0;
 			State one = parent.currentState;
-			Flow flow = null;
+
+			if (one instanceof SplitState) {
+				parent.currentState = parent.createState(list, executor, (SplitState) one);
+				return parent;
+			}
+
 			if (!(one == null || one instanceof FlowState)) {
-				FlowBuilder<Flow> stateBuilder = new FlowBuilder<>(name + "_" + (counter++));
+				FlowBuilder<Flow> stateBuilder = new FlowBuilder<>(name + "_0");
 				stateBuilder.currentState = one;
-				flow = stateBuilder.build();
+				list.add(stateBuilder.build());
 			}
 			else if (one instanceof FlowState && parent.states.size() == 1) {
 				list.add(((FlowState) one).getFlows().iterator().next());
 			}
 
-			if (flow != null) {
-				list.add(flow);
-			}
-			State next = parent.createState(list, executor);
-			parent.currentState = next;
+			parent.currentState = parent.createState(list, executor, null);
 			return parent;
 		}
 

spring-batch-core/src/main/java/org/springframework/batch/core/job/flow/support/state/SplitState.java

@@ -16,7 +16,9 @@
 package org.springframework.batch.core.job.flow.support.state;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
 import java.util.concurrent.FutureTask;
@@ -31,6 +33,7 @@
 import org.springframework.core.task.SyncTaskExecutor;
 import org.springframework.core.task.TaskExecutor;
 import org.springframework.core.task.TaskRejectedException;
+import org.springframework.lang.Nullable;
 
 /**
  * A {@link State} implementation that splits a {@link Flow} into multiple parallel
@@ -44,6 +47,8 @@ public class SplitState extends AbstractState implements FlowHolder {
 
 	private final Collection<Flow> flows;
 
+	private final SplitState parentSplit;
+
 	private TaskExecutor taskExecutor = new SyncTaskExecutor();
 
 	private final FlowExecutionAggregator aggregator = new MaxValueFlowExecutionAggregator();
@@ -53,8 +58,18 @@ public class SplitState extends AbstractState implements FlowHolder {
 	 * @param name the name of the state.
 	 */
 	public SplitState(Collection<Flow> flows, String name) {
+		this(flows, name, null);
+	}
+
+	/**
+	 * @param flows collection of {@link Flow} instances.
+	 * @param name the name of the state.
+	 * @param parentSplit the parent {@link SplitState}.
+	 */
+	public SplitState(Collection<Flow> flows, String name, @Nullable SplitState parentSplit) {
 		super(name);
 		this.flows = flows;
+		this.parentSplit = parentSplit;
 	}
 
 	/**
@@ -101,6 +116,8 @@ public FlowExecutionStatus handle(final FlowExecutor executor) throws Exception
 
 		}
 
+		FlowExecutionStatus parentSplitStatus = parentSplit == null ? null : parentSplit.handle(executor);
+
 		Collection<FlowExecution> results = new ArrayList<>();
 
 		// Could use a CompletionService here?
@@ -120,7 +137,11 @@ public FlowExecutionStatus handle(final FlowExecutor executor) throws Exception
 			}
 		}
 
-		return doAggregation(results, executor);
+		FlowExecutionStatus flowExecutionStatus = doAggregation(results, executor);
+		if (parentSplitStatus != null) {
+			return Collections.max(Arrays.asList(flowExecutionStatus, parentSplitStatus));
+		}
+		return flowExecutionStatus;
 	}
 
 	protected FlowExecutionStatus doAggregation(Collection<FlowExecution> results, FlowExecutor executor) {

spring-batch-core/src/test/java/org/springframework/batch/core/job/builder/FlowJobBuilderTests.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.
@@ -38,6 +38,7 @@
 import org.springframework.batch.core.job.flow.Flow;
 import org.springframework.batch.core.job.flow.FlowExecutionStatus;
 import org.springframework.batch.core.job.flow.JobExecutionDecider;
+import org.springframework.batch.core.job.flow.support.SimpleFlow;
 import org.springframework.batch.core.launch.JobLauncher;
 import org.springframework.batch.core.repository.JobRepository;
 import org.springframework.batch.core.repository.support.JobRepositoryFactoryBean;
@@ -171,6 +172,22 @@ void testBuildSplit() {
 		assertEquals(2, execution.getStepExecutions().size());
 	}
 
+	@Test
+	void testNestedSplitsWithSingleThread() {
+		SimpleAsyncTaskExecutor taskExecutor = new SimpleAsyncTaskExecutor();
+		taskExecutor.setConcurrencyLimit(1);
+
+		FlowBuilder<SimpleFlow> flowBuilder = new FlowBuilder<>("flow");
+		FlowBuilder.SplitBuilder<SimpleFlow> splitBuilder = flowBuilder.split(taskExecutor);
+		splitBuilder.add(new FlowBuilder<Flow>("subflow1").from(step1).end());
+		splitBuilder.add(new FlowBuilder<Flow>("subflow2").from(step2).end());
+		Job job = new JobBuilder("job").repository(jobRepository).start(flowBuilder.build()).end().build();
+		job.execute(execution);
+
+		assertEquals(BatchStatus.COMPLETED, execution.getStatus());
+		assertEquals(2, execution.getStepExecutions().size());
+	}
+
 	@Test
 	void testBuildSplitUsingStartAndAdd_BATCH_2346() {
 		Flow subflow1 = new FlowBuilder<Flow>("subflow1").from(step2).end();