java - 如何在构造函数中使用@Value？

Question

我想使用构造函数注入(inject)将字符串注入(inject)到 bean 中。基本上我有以下类(class):

@Component
@StepScope
public class MyClass {

    public MyClass (@Value("#{jobParameters['directory']}") final String directory) {
        // ...
    }

}

当我运行我的应用程序时，我得到以下堆栈跟踪:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'scopedTarget.byteArrayItemReader' defined in file [...]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [my.package.MyClass]: No default constructor found; nested exception is java.lang.NoSuchMethodException: my.package.MyClass.<init>()
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1105) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1050) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:510) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory$2.getObject(AbstractBeanFactory.java:345) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.batch.core.scope.StepScope.get(StepScope.java:113) ~[spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:340) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.aop.target.SimpleBeanTargetSource.getTarget(SimpleBeanTargetSource.java:35) ~[spring-aop-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.getTarget(CglibAopProxy.java:686) ~[spring-aop-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:636) ~[spring-aop-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at my.package.MyClass$$EnhancerBySpringCGLIB$$cc4837cc.close(<generated>) ~[classes/:na]
    at org.springframework.batch.item.support.CompositeItemStream.close(CompositeItemStream.java:85) ~[spring-batch-infrastructure-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.step.tasklet.TaskletStep.close(TaskletStep.java:305) ~[spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.step.AbstractStep.execute(AbstractStep.java:271) ~[spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.SimpleStepHandler.handleStep(SimpleStepHandler.java:148) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.flow.JobFlowExecutor.executeStep(JobFlowExecutor.java:64) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.flow.support.state.StepState.handle(StepState.java:67) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.flow.support.SimpleFlow.resume(SimpleFlow.java:169) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.flow.support.SimpleFlow.start(SimpleFlow.java:144) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.flow.FlowJob.doExecute(FlowJob.java:134) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.job.AbstractJob.execute(AbstractJob.java:306) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.batch.core.launch.support.SimpleJobLauncher$1.run(SimpleJobLauncher.java:135) [spring-batch-core-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at my.package.QueueTaskExecutor$TaskRunner.run(QueueTaskExecutor.java:115) [classes/:na]
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [na:1.8.0_66]
    at java.util.concurrent.FutureTask.run(Unknown Source) [na:1.8.0_66]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [na:1.8.0_66]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [na:1.8.0_66]
    at java.lang.Thread.run(Unknown Source) [na:1.8.0_66]
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [my.package.MyClass]: No default constructor found; nested exception is java.lang.NoSuchMethodException: my.package.MyClass.<init>()
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:85) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:1098) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    ... 28 common frames omitted
Caused by: java.lang.NoSuchMethodException: my.package.MyClass.<init>()
    at java.lang.Class.getConstructor0(Unknown Source) ~[na:1.8.0_66]
    at java.lang.Class.getDeclaredConstructor(Unknown Source) ~[na:1.8.0_66]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:80) ~[spring-beans-4.2.3.RELEASE.jar:4.2.3.RELEASE]
    ... 29 common frames omitted

那么如何使用构造函数正确注入(inject)字符串值？

Answer 1

如果需要通过构造函数参数向bean注入(inject)值，则必须在构造函数上使用@Autowired注释，否则将使用默认构造函数。所以，它必须是这样的:

@Autowired
public MyClass (@Value("#{jobParameters['directory']}") final String directory) {
    // ...
}