Properties that cannot be bound due to a missing default constructor are hard to diagnose

[wilkinsona]

Using Spring Boot 2.0.3, running the following code:

package com.example.demo;

import java.util.ArrayList;
import java.util.List;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

@SpringBootApplication
public class So51149634Application {

	public static void main(String[] args) {
		SpringApplication.run(So51149634Application.class, "--printers[0].deviceNo=123");
	}

}

class Printer {

	private String deviceNo;

	public Printer(String deviceNo) {
		this.deviceNo = deviceNo;
	}

	public String getDeviceNo() {
		return deviceNo;
	}

	public void setDeviceNo(String deviceNo) {
		this.deviceNo = deviceNo;
	}

}

@Component
@ConfigurationProperties
class PrinterProperties {

	List<Printer> printers = new ArrayList<>();

	public List<Printer> getPrinters() {
		return printers;
	}

	public void setPrinters(List<Printer> printers) {
		this.printers = printers;
	}

}

Results in the following output:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.3.RELEASE)

2018-07-03 12:02:44.836  INFO 94960 --- [           main] com.example.demo.So51149634Application   : Starting So51149634Application on aw-rmbp.local with PID 94960 (/Users/awilkinson/dev/workspaces/spring/spring-boot/2.0.x/so-51149634/target/classes started by awilkinson in /Users/awilkinson/dev/workspaces/spring/spring-boot/2.0.x/so-51149634)
2018-07-03 12:02:44.840  INFO 94960 --- [           main] com.example.demo.So51149634Application   : No active profile set, falling back to default profiles: default
2018-07-03 12:02:44.883  INFO 94960 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@62bd765: startup date [Tue Jul 03 12:02:44 BST 2018]; root of context hierarchy
2018-07-03 12:02:45.255  WARN 94960 --- [           main] s.c.a.AnnotationConfigApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.boot.context.properties.ConfigurationPropertiesBindException: Error creating bean with name 'printerProperties': Could not bind properties to 'PrinterProperties' : prefix=, ignoreInvalidFields=false, ignoreUnknownFields=true; nested exception is org.springframework.boot.context.properties.bind.BindException: Failed to bind properties under 'printers' to java.util.List<com.example.demo.Printer>
2018-07-03 12:02:45.262  INFO 94960 --- [           main] ConditionEvaluationReportLoggingListener : 

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2018-07-03 12:02:45.264 ERROR 94960 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : 

***************************
APPLICATION FAILED TO START
***************************

Description:

Failed to bind properties under 'printers' to java.util.List<com.example.demo.Printer>:

    Reason: Failed to bind properties under 'printers' to java.util.List<com.example.demo.Printer>

Action:

Update your application's configuration

The actual problem is that Printer does not have a default constructor.

Running with --debug reveals the following exception:

org.springframework.boot.context.properties.bind.BindException: Failed to bind properties under 'printers' to java.util.List<com.example.demo.Printer>
	at org.springframework.boot.context.properties.bind.Binder.handleBindError(Binder.java:250) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:226) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindBean$4(Binder.java:333) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:73) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:62) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.JavaBeanBinder.bind(JavaBeanBinder.java:54) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.lambda$null$5(Binder.java:341) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[na:1.8.0_151]
	at java.util.ArrayList$ArrayListSpliterator.tryAdvance(ArrayList.java:1357) ~[na:1.8.0_151]
	at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:126) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:498) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:485) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_151]
	at java.util.stream.FindOps$FindOp.evaluateSequential(FindOps.java:152) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_151]
	at java.util.stream.ReferencePipeline.findFirst(ReferencePipeline.java:464) ~[na:1.8.0_151]
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindBean$6(Binder.java:342) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:441) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder$Context.withBean(Binder.java:427) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder$Context.access$400(Binder.java:381) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bindBean(Binder.java:339) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:278) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:221) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:210) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:192) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.ConfigurationPropertiesBinder.bind(ConfigurationPropertiesBinder.java:82) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.bind(ConfigurationPropertiesBindingPostProcessor.java:107) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.ConfigurationPropertiesBindingPostProcessor.postProcessBeforeInitialization(ConfigurationPropertiesBindingPostProcessor.java:93) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(AbstractAutowireCapableBeanFactory.java:424) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1700) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:581) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:503) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:760) ~[spring-beans-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:869) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.0.7.RELEASE.jar:5.0.7.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:759) [spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:395) [spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:327) [spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1255) [spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1243) [spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at com.example.demo.So51149634Application.main(So51149634Application.java:15) [classes/:na]
Caused by: java.lang.NullPointerException: null
	at java.util.TreeMap.compare(TreeMap.java:1294) ~[na:1.8.0_151]
	at java.util.TreeMap.put(TreeMap.java:538) ~[na:1.8.0_151]
	at java.util.TreeSet.add(TreeSet.java:255) ~[na:1.8.0_151]
	at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) ~[na:1.8.0_151]
	at java.util.LinkedList$LLSpliterator.forEachRemaining(LinkedList.java:1235) ~[na:1.8.0_151]
	at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:580) ~[na:1.8.0_151]
	at java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:270) ~[na:1.8.0_151]
	at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[na:1.8.0_151]
	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471) ~[na:1.8.0_151]
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_151]
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_151]
	at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[na:1.8.0_151]
	at org.springframework.boot.context.properties.bind.IndexedElementsBinder.assertNoUnboundChildren(IndexedElementsBinder.java:146) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.IndexedElementsBinder.bindIndexed(IndexedElementsBinder.java:120) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.IndexedElementsBinder.bindIndexed(IndexedElementsBinder.java:89) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.IndexedElementsBinder.bindIndexed(IndexedElementsBinder.java:71) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.CollectionBinder.bindAggregate(CollectionBinder.java:50) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.AggregateBinder.bind(AggregateBinder.java:58) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.lambda$bindAggregate$2(Binder.java:305) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder$Context.withIncreasedDepth(Binder.java:441) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder$Context.access$100(Binder.java:381) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bindAggregate(Binder.java:304) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bindObject(Binder.java:262) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	at org.springframework.boot.context.properties.bind.Binder.bind(Binder.java:221) ~[spring-boot-2.0.3.RELEASE.jar:2.0.3.RELEASE]
	... 43 common frames omitted

The NullPointerException is occurring because knownIndexedChildren contains two null entries for a key, elementBinder.bind(name, Bindable.of(elementType), source) returns null due to the lack of default constructor on Printer, the loop's broken and the entry isn't removed from knownIndexedChildren, assertNoUnboundChildren then tries to stream the values and add null to a TreeSet which is rejected with a NullPointerException.

[mbhave]

This bug can also be seen when properties for non-scalar collections are not sequential. For example, the test below fails with an NPE.

@Test
public void bindToNonScalarCollectionWhenNonSequentialShouldThrowException() {
	MockConfigurationPropertySource source = new MockConfigurationPropertySource();
	source.put("foo[0].value", "1");
	source.put("foo[1].value", "2");
	source.put("foo[4].value", "4");
	this.sources.add(source);
	try {
		Bindable<List<JavaBean>> target = Bindable.listOf(JavaBean.class);
		this.binder.bind("foo", target);
		fail("No exception thrown");
	}
	catch (BindException ex) {
		ex.printStackTrace();
		Set<ConfigurationProperty> unbound = ((UnboundConfigurationPropertiesException) ex
				.getCause()).getUnboundProperties();
		assertThat(unbound).hasSize(1);
		ConfigurationProperty property = unbound.iterator().next();
		assertThat(property.getName().toString()).isEqualTo("foo[4].value");
		assertThat(property.getValue()).isEqualTo("4");
	}
}