解决@Autowired注入空指针问题(利用Bean的生命周期)

我就写出了下面这样的代码进行抽取

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									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;

									    }

									}

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									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：

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									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

解决@Autowired注入空指针问题(利用Bean的生命周期)

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

解决@Autowired注入空指针问题(利用Bean的生命周期)

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

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									@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

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									/**

									 * @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()

									        );

									    }

									}