解決@Autowired注入空指針問(wèn)題(利用Bean的生命周期)

更新時(shí)間：2022年02月24日 11:52:19 作者：Hermione?Granger

這篇文章主要介紹了解決@Autowired注入空指針問(wèn)題(利用Bean的生命周期)，具有很好的參考價(jià)值，希望對(duì)大家有所幫助。如有錯(cuò)誤或未考慮完全的地方，望不吝賜教

我就寫出了下面這樣的代碼進(jìn)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

本想應(yīng)該毫無(wú)問(wèn)題，但是，報(bào)錯(cuò)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

空指針異常？檢查好幾遍配置沒(méi)錯(cuò)。因?yàn)楣鹃_發(fā)環(huán)境沒(méi)法上網(wǎng)，只好拖到下班google了一下，結(jié)合我比較深厚的基礎(chǔ)（自戀一下），

問(wèn)題輕松解決

這就是答案。上面說(shuō)所有的Spring的@Autowired注解都在構(gòu)造函數(shù)之后，而如果一個(gè)對(duì)象像下面代碼一樣聲明（private XXX = new XXX() 直接在類中聲明）的話，成員變量是在構(gòu)造函數(shù)之前進(jìn)行初始化的，甚至可以作為構(gòu)造函數(shù)的參數(shù)。

即成員變量初始化 -> Constructor -> @Autowired

所以，在這個(gè)時(shí)候如果成員變量初始化時(shí)調(diào)用了利用@Autowired注解初始化的對(duì)象時(shí)，必然會(huì)報(bào)空指針異常的啊。

真相大白了。如果解決呢？那就讓上面我寫的代碼的成員變量threadPool在@Autowired之后執(zhí)行就好了。

要想解決這個(gè)問(wèn)題，首先要知道@Autowired的原理：

AutowiredAnnotationBeanPostProcessor 這個(gè)類

其實(shí)看到這個(gè)繼承結(jié)構(gòu)，我心中已經(jīng)有解決辦法了。具體詳細(xì)為什么，等997的工作結(jié)束（無(wú)奈）我會(huì)在后續(xù)博客里將Spring的注解配置詳細(xì)的捋一遍，到時(shí)候會(huì)講到Bean的生命周期的。

繼承的BeanFactoryAware是在屬性賦值完成，執(zhí)行構(gòu)造方法后,postProcessBeforeInitialization才執(zhí)行,而且,是在其他生命周期之前，而@Autowired注解就是依靠這個(gè)原理進(jìn)行的自動(dòng)注入。想要解決這個(gè)問(wèn)題很簡(jiǎn)單，就是把要賦值的成員變量放到其他生命周期中就可以。

下面介紹其中兩種辦法

第一種JSR250的@PostConstruct

@PostConstruct
public void init() {
	// 這里放要執(zhí)行的賦值
}

第二種是Spring的InitializingBean(定義初始化邏輯)

繼承接口實(shí)現(xiàn)方法即可，這種直接放上完整用法

/**
 * @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()
        );
    }
}

設(shè)置完成后，問(wèn)題解決！

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