Spring IOC（五）依赖注入

Spring IOC（五）依赖注入

Spring 系列目录(https://www.cnblogs.com/binarylei/p/10198698.html)

一、autowire 五种注入方式测试

(1) 环境准备

public class Company {
    private Department department;
    private List employees;

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

(3) 测试一把

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

    // 1. 名称注入
    Company companyByName = (Company) lbf.getBean("company1");
    // 2. 类型注入，支持 List 方式注入，如果本地容器找到多个则直接抛出异常
    Company companyByType = (Company) lbf.getBean("company2");
    // 3. no
    Company companyByNo = (Company) lbf.getBean("company3");
    // 4. 构造器注入
    Company companyByConstructor = (Company) lbf.getBean("company4");
    // 5. 默认
    Company companyDefault = (Company) lbf.getBean("company5");
}

二、Spring 属性注入源码分析

2.1 属性注入 - populateBean

Spring 属性注入在 populateBean 方法中完成，有两种注入方式：beanName 或 type 两种。

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. 后置处理器 InstantiationAwareBeanPostProcessor，可以先略过
    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. 依赖查找。根据 beanName 或 type 查找可注入的属性值。
    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. 后置处理器拦截，对属性值进行处理 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. 依赖校验。是否所有的字段已经全部匹配上了，根据需要是否要抛出异常
    if (needsDepCheck) {
        if (filteredPds == null) {
            // 过滤不需要进行属性注入的字段，如 String、BeanFactory...
            filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
        }
        checkDependencies(beanName, mbd, filteredPds, pvs);
    }

    // 5. 依赖注入。至些属性已经全部准备好了，可以进行属性注入。
    if (pvs != null) {
        applyPropertyValues(beanName, mbd, bw, pvs);
    }
}

上面的代码看这很复杂，其实抛开后置处理器 InstantiationAwareBeanPostProcessor 就做了三件事，其中属性的查找，尤其是根据类型的查找最为复杂：

1. 依赖查找。根据 beanName 或 type 查找可注入的依赖值。
2. 依赖校验。是否所有的字段已经全部匹配上了，根据需要是否要抛出异常
3. 依赖注入。至些依赖已经全部准备好了，可以进行属性注入。

参考：

1 . 《Spring各种依赖注入注解的区别》：https://blog.csdn.net/gaohe7091/article/details/39319363

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