解决@Autowired注入空指针问题(利用Bean的生命周期)
来源:脚本之家    时间:2022-02-24 14:10:20
目录
我就写出了下面这样的代码进行抽取问题轻松解决下面介绍其中两种办法第一种JSR250的@PostConstruct第二种是Spring的InitializingBean(定义初始化逻辑)

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(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(threadPool1.getCapacity()),
                namedThreadFactory,
                new ThreadPoolExecutor.DiscardPolicy()
        );
    }
}

设置完成后,问题解决!

关键词: 成员变量 生命周期 构造函数 空指针异常 轻松解决

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