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@Autowired注入空指针问题

Hermione Granger 人气:1

今天做项目的时候遇到一个问题,需要将线程池的参数抽取到yml文件里进行设置。这不是so easy吗?

我就写出了下面这样的代码进行抽取

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;
/**
 * @author BestQiang
 */
@Component
@ConfigurationProperties(prefix = "thread-pool")
public class ThreadPool {
    private int corePoolSize;
    private int maximumPoolSize;
    private long keepAliveTime;
    private int capacity;
    public int getCorePoolSize() {
        return corePoolSize;
    }
    public void setCorePoolSize(int corePoolSize) {
        this.corePoolSize = corePoolSize;
    }
    public int getMaximumPoolSize() {
        return maximumPoolSize;
    }
    public void setMaximumPoolSize(int maximumPoolSize) {
        this.maximumPoolSize = maximumPoolSize;
    }
    public long getKeepAliveTime() {
        return keepAliveTime;
    }
    public void setKeepAliveTime(long keepAliveTime) {
        this.keepAliveTime = keepAliveTime;
    }
    public int getCapacity() {
        return capacity;
    }
    public void setCapacity(int capacity) {
        this.capacity = capacity;
    }
}
package cn.bestqiang.util;
import cn.bestqiang.pojo.ThreadPool;
import com.google.common.util.concurrent.ThreadFactoryBuilder;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import javax.annotation.PostConstruct;
import java.util.concurrent.*;
/**
 * @author Yaqiang Chen
 */
@Component
public class MyThreadUtils {
    @Autowired
    ThreadPool threadPool1;
    private ExecutorService threadPool = new ThreadPoolExecutor(
                threadPool1.getCorePoolSize(),
                threadPool1.getMaximumPoolSize(),
                threadPool1.getKeepAliveTime(),
                TimeUnit.SECONDS,
                new LinkedBlockingDeque<Runnable>(threadPool1.getCapacity()),
                namedThreadFactory,
                new ThreadPoolExecutor.DiscardPolicy()
        );;
    private ThreadFactory namedThreadFactory = new ThreadFactoryBuilder()
            .setNameFormat("pool-%d").build();
    public void execute(Runnable runnable){
        threadPool.submit(runnable);
    }
}

在yml文件的配置如下:

thread-pool:
  core-pool-size: 5
  maximum-pool-size: 20
  keep-alive-time: 1
  capacity: 1024

本想应该毫无问题,但是,报错了:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'myThreadUtils' defined in fileXXXXXXXXXX(省略)Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [cn.itcast.util.MyThreadUtils]: Constructor threw exception; nested exception is java.lang.NullPointerExceptionCaused by: java.lang.NullPointerException: null

空指针异常?检查好几遍配置没错。因为公司开发环境没法上网,只好拖到下班google了一下,结合我比较深厚的基础(自恋一下),

问题轻松解决

这就是答案。上面说所有的Spring的@Autowired注解都在构造函数之后,而如果一个对象像下面代码一样声明(private XXX = new XXX() 直接在类中声明)的话,成员变量是在构造函数之前进行初始化的,甚至可以作为构造函数的参数。

即 成员变量初始化 -> Constructor -> @Autowired

所以,在这个时候如果成员变量初始化时调用了利用@Autowired注解初始化的对象时,必然会报空指针异常的啊。

真相大白了。如果解决呢?那就让上面我写的代码的成员变量threadPool在@Autowired之后执行就好了。

要想解决这个问题,首先要知道@Autowired的原理:

AutowiredAnnotationBeanPostProcessor 这个类

其实看到这个继承结构,我心中已经有解决办法了。具体详细为什么,等997的工作结束(无奈)我会在后续博客里将Spring的注解配置详细的捋一遍,到时候会讲到Bean的生命周期的。

继承的BeanFactoryAware是在属性赋值完成,执行构造方法后,postProcessBeforeInitialization才执行,而且,是在其他生命周期之前,而@Autowired注解就是依靠这个原理进行的自动注入。想要解决这个问题很简单,就是把要赋值的成员变量放到其他生命周期中就可以。

下面介绍其中两种办法

第一种JSR250的@PostConstruct

@PostConstruct
public void init() {
	// 这里放要执行的赋值
}

第二种是Spring的InitializingBean(定义初始化逻辑) 

继承接口实现方法即可,这种直接放上完整用法

/**
 * @author Yaqiang Chen
 */
@Component
public class MyThreadUtils implements InitializingBean {
    @Autowired
    ThreadPool threadPool1;
    private ExecutorService threadPool;
    private ThreadFactory namedThreadFactory = new ThreadFactoryBuilder()
            .setNameFormat("pool-%d").build();
    public void execute(Runnable runnable){
        threadPool.submit(runnable);
    }
    @Override
    public void afterPropertiesSet() throws Exception {
        threadPool = new ThreadPoolExecutor(
                threadPool1.getCorePoolSize(),
                threadPool1.getMaximumPoolSize(),
                threadPool1.getKeepAliveTime(),
                TimeUnit.SECONDS,
                new LinkedBlockingDeque<Runnable>(threadPool1.getCapacity()),
                namedThreadFactory,
                new ThreadPoolExecutor.DiscardPolicy()
        );
    }
}

设置完成后,问题解决!

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