Spring BeanFactory 依賴註入

Spring 系列目錄(https://www.cnblogs.com/binarylei/p/10117436.html)

一、autowire 五種註入方式測試

(1) 環境準備

public class Company {
    private Department department;
    private List<Employee> employees;

    public Company() {
    }
    public Company(Department department) {
        this.department = department;
    }
    public void setDepartment(Department department) {
        this.department = department;
    }
    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }
}

public class Employee {
    private String name;
    public void setName(String name) {
        this.name = name;
    }
}

public class Department {
    private String name;
    public void setName(String name) {
        this.name = name;
    }
}
 

(2) xml 配置

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="company1" autowire="byName" class="com.github.binarylei.Company"/>
    <bean id="company2" autowire="byType" class="com.github.binarylei.Company"/>
    <bean id="company3" autowire="no" class="com.github.binarylei.Company"/>
    <bean id="company4" autowire="constructor" class="com.github.binarylei.Company">
        <constructor-arg index="0" ref="department"/>
    </bean>
    <bean id="company5" autowire="default" class="com.github.binarylei.Company"/>

    <bean id="employee1" class="com.github.binarylei.spring.Employee">
        <property name="name" value="employee1"/>
    </bean>
    <bean id="employee2" class="com.github.binarylei.spring.Employee">
        <property name="name" value="employee2"/>
    </bean>

    <bean id="department" class="com.github.binarylei.spring.Department">
        <property name="name" value="department"/>
    </bean>
</beans>
 

(3) 測試一把

@Test
public void test() {
    DefaultListableBeanFactory lbf = new DefaultListableBeanFactory();
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(lbf);
    reader.loadBeanDefinitions(new ClassPathResource("spring-context-di.xml", getClass()));

    // 1. 名稱註入
    Company companyByName = (Company) lbf.getBean("company1");
    // 2. 類型註入，支持 List 方式註入，如果本地容器找到多個則直接拋出異常
    Company companyByType = (Company) lbf.getBean("company2");
    // 3. no
    Company companyByNo = (Company) lbf.getBean("company3");
    // 4. 構造器註入
    Company companyByConstructor = (Company) lbf.getBean("company4");
    // 5. 默認
    Company companyDefault = (Company) lbf.getBean("company5");
}
 

二、Spring 屬性註入源碼分析

2.1 屬性註入 - populateBean

Spring 屬性註入在 populateBean 方法中完成，有兩種註入方式：beanName 或 type 兩種。

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
    if (bw == null) {
        if (mbd.hasPropertyValues()) {
            throw new BeanCreationException("Cannot apply property values to null instance");
        } else {
            return;
        }
    }

    // 1. 後置處理器 InstantiationAwareBeanPostProcessor，可以先略過
    boolean continueWithPropertyPopulation = true;
    if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                    continueWithPropertyPopulation = false;
                    break;
                }
            }
        }
    }
    if (!continueWithPropertyPopulation) {
        return;
    }

    // 2. 依賴查找。根據 beanName 或 type 查找可註入的屬性值。
    PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
    if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
        MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
        if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) {
            autowireByName(beanName, mbd, bw, newPvs);
        }
        if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) {
            autowireByType(beanName, mbd, bw, newPvs);
        }
        pvs = newPvs;
    }

    boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
    boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

    // 3. 後置處理器攔截，對屬性值進行處理 InstantiationAwareBeanPostProcessor
    PropertyDescriptor[] filteredPds = null;
    if (hasInstAwareBpps) {
        if (pvs == null) {
            pvs = mbd.getPropertyValues();
        }
        for (BeanPostProcessor bp : getBeanPostProcessors()) {
            if (bp instanceof InstantiationAwareBeanPostProcessor) {
                InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
                if (pvsToUse == null) {
                    if (filteredPds == null) {
                        filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
                    }
                    pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                    if (pvsToUse == null) {
                        return;
                    }
                }
                pvs = pvsToUse;
            }
        }
    }

    // 4. 依賴校驗。是否所有的字段已經全部匹配上了，根據需要是否要拋出異常
    if (needsDepCheck) {
        if (filteredPds == null) {
            // 過濾不需要進行屬性註入的字段，如 String、BeanFactory...
            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
        }
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }

    // 5. 依賴註入。至些屬性已經全部準備好了，可以進行屬性註入。
    if (pvs != null) {
        applyPropertyValues(beanName, mbd, bw, pvs);
    }
}

上面的代碼看這很復雜，其實拋開後置處理器 InstantiationAwareBeanPostProcessor 就做了三件事，其中屬性的查找，尤其是根據類型的查找最為復雜：

1. 依賴查找。根據 beanName 或 type 查找可註入的依賴值。
2. 依賴校驗。是否所有的字段已經全部匹配上了，根據需要是否要拋出異常
3. 依賴註入。至些依賴已經全部準備好了，可以進行屬性註入。

依賴註入實際上是委托給了 BeanWrapperImpl 完成，本文的重點的依賴的查找，尤其是根據類型匹配的情況。

2.2 名稱查找 - autowireByName

毫無疑問，直接從 BeanFactory 中取出這個 bean 就可以了

protected void autowireByName(
        String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
    for (String propertyName : propertyNames) {
        if (containsBean(propertyName)) {
            Object bean = getBean(propertyName);
            pvs.add(propertyName, bean);
            registerDependentBean(propertyName, beanName);
        }
    }
}

2.3 類型查找 - autowireByType

protected void autowireByType(
            String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
    TypeConverter converter = getCustomTypeConverter();
    if (converter == null) {
        converter = bw;
    }

    Set<String> autowiredBeanNames = new LinkedHashSet<>(4);
    String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
    for (String propertyName : propertyNames) {
        try {
            PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);
            if (Object.class != pd.getPropertyType()) {
                MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);
                // 類型查找時允許對 FactoryBean 提前實例化對象，大部分情況一都是 true。
                // 至於為什麽實現了 PriorityOrdered 接口的 bean 要排除，以後再研究一下。？？？
                boolean eager = !PriorityOrdered.class.isInstance(bw.getWrappedInstance());
                DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);
                // 核心代碼就這一句，類型查找委托給了子類的 resolveDependency 完成
                Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);
                if (autowiredArgument != null) {
                    pvs.add(propertyName, autowiredArgument);
                }
                for (String autowiredBeanName : autowiredBeanNames) {
                    registerDependentBean(autowiredBeanName, beanName);
                }
                autowiredBeanNames.clear();
            }
        }
        catch (BeansException ex) {
            throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, propertyName, ex);
        }
    }
}

resolveDependency(desc, beanName, autowiredBeanNames, converter) 可以說是 AbstractAutowireCapableBeanFactory 最重要的模板方法了，子類 DefaultListableBeanFactory 進行了實現，作用就是根據類型查找依賴。

參考：

1 . 《Spring中循環引用的處理》：https://www.iflym.com/index.php/code/201208280001.html

每天用心記錄一點點。內容也許不重要，但習慣很重要！

Spring BeanFactory 依賴註入