spring bean的实例化

spring bean的实例化,第1张

spring中bean的实例化主要有两种:factoryMethod(@Bean)和构造器的方式

第一种:factoryMethod:

spring.xml




  
   
   
       
   

    
  
import org.springframework.stereotype.Component;

@Data
@Component
//@Lazy
public class Student {
    @Autowired
    private  Teacher teacher;

    private String name;


    private  int age;
}
package com.spring.bean;

/**
 * @Classname FactoryMethodBean
 * @Description TODO
 * @Author
 * Date 2020/12/20 13:55
 * Version 1.0
 */
public class FactoryMethodBean {

    //第一种配置方式必须保证factoryMethod是非静态方法,第二种反之
    public  static  Object factoryMethod() {
        return new Student();
    }
    //当有重载方法是会进行参数的匹配
//    public Object factoryMethod(String param) {
//        return new Student();
//    }
}
@org.junit.Test
public void test(){

    ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

    Student student =(Student) context.getBean("demo");

    System.out.println(student);

}
 源码详解:

public BeanWrapper instantiateUsingFactoryMethod(
			String beanName, RootBeanDefinition mbd, @Nullable Object[] explicitArgs) {

		//包装bean
		BeanWrapperImpl bw = new BeanWrapperImpl();
		this.beanFactory.initBeanWrapper(bw);

		Object factoryBean;
		Class factoryClass;
		boolean isStatic;

		//获取factory-bean属性的值
		String factoryBeanName = mbd.getFactoryBeanName();
		if (factoryBeanName != null) {
			if (factoryBeanName.equals(beanName)) {
				throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
						"factory-bean reference points back to the same bean definition");
			}
			//获取factoryBeanName的实例
			factoryBean = this.beanFactory.getBean(factoryBeanName);
			if (mbd.isSingleton() && this.beanFactory.containsSingleton(beanName)) {
				throw new ImplicitlyAppearedSingletonException();
			}
			factoryClass = factoryBean.getClass();
			//判断是否为static
			isStatic = false;
		}
		else {
			// It's a static factory method on the bean class.
			if (!mbd.hasBeanClass()) {
				throw new BeanDefinitionStoreException(mbd.getResourceDescription(), beanName,
						"bean definition declares neither a bean class nor a factory-bean reference");
			}
			factoryBean = null;
			factoryClass = mbd.getBeanClass();
			//判断是否为static
			isStatic = true;
		}

		Method factoryMethodToUse = null;
		ArgumentsHolder argsHolderToUse = null;
		Object[] argsToUse = null;

		if (explicitArgs != null) {
			argsToUse = explicitArgs;
		}
		else {
			Object[] argsToResolve = null;
			synchronized (mbd.constructorArgumentLock) {
				factoryMethodToUse = (Method) mbd.resolvedConstructorOrFactoryMethod;
				if (factoryMethodToUse != null && mbd.constructorArgumentsResolved) {
					// Found a cached factory method...
					argsToUse = mbd.resolvedConstructorArguments;
					if (argsToUse == null) {
						argsToResolve = mbd.preparedConstructorArguments;
					}
				}
			}
			if (argsToResolve != null) {
				argsToUse = resolvePreparedArguments(beanName, mbd, bw, factoryMethodToUse, argsToResolve, true);
			}
		}

		if (factoryMethodToUse == null || argsToUse == null) {
			// Need to determine the factory method...
			// Try all methods with this name to see if they match the given arguments.
			factoryClass = ClassUtils.getUserClass(factoryClass);

			List candidates = null;
			if (mbd.isFactoryMethodUnique) {
				if (factoryMethodToUse == null) {
					factoryMethodToUse = mbd.getResolvedFactoryMethod();
				}
				if (factoryMethodToUse != null) {
					candidates = Collections.singletonList(factoryMethodToUse);
				}
			}
			if (candidates == null) {
				candidates = new ArrayList<>();
				//获取到factoryClass的所有方法
				Method[] rawCandidates = getCandidateMethods(factoryClass, mbd);
				//遍历判断添加到candidates容器中
				for (Method candidate : rawCandidates) {
					//若配置了factory-bean则方法必须是非static,否则必须是static
					if (Modifier.isStatic(candidate.getModifiers()) == isStatic && mbd.isFactoryMethod(candidate)) {
						candidates.add(candidate);
					}
				}
			}

			if (candidates.size() == 1 && explicitArgs == null && !mbd.hasConstructorArgumentValues()) {
				Method uniqueCandidate = candidates.get(0);//方法的参数个数
				if (uniqueCandidate.getParameterCount() == 0) {
					mbd.factoryMethodToIntrospect = uniqueCandidate;
					synchronized (mbd.constructorArgumentLock) {
						mbd.resolvedConstructorOrFactoryMethod = uniqueCandidate;
						mbd.constructorArgumentsResolved = true;
						mbd.resolvedConstructorArguments = EMPTY_ARGS;
					}
					//通过反射实例化
					bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, uniqueCandidate, EMPTY_ARGS));
					return bw;
				}
			}

			//有两个及两个以上重载的方法,则进行排序(根据参数个数)
			if (candidates.size() > 1) {  // explicitly skip immutable singletonList
				candidates.sort(AutowireUtils.EXECUTABLE_COMPARATOR);
			}

			ConstructorArgumentValues resolvedValues = null;
			boolean autowiring = (mbd.getResolvedAutowireMode() == AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR);
			int minTypeDiffWeight = Integer.MAX_VALUE;
			Set ambiguousFactoryMethods = null;

			int minNrOfArgs;
			if (explicitArgs != null) {
				minNrOfArgs = explicitArgs.length;
			}
			else {
				// We don't have arguments passed in programmatically, so we need to resolve the
				// arguments specified in the constructor arguments held in the bean definition.
                //查看有几个参数
				if (mbd.hasConstructorArgumentValues()) {
					ConstructorArgumentValues cargs = mbd.getConstructorArgumentValues();
					resolvedValues = new ConstructorArgumentValues();
					minNrOfArgs = resolveConstructorArguments(beanName, mbd, bw, cargs, resolvedValues);
				}
				else {
					minNrOfArgs = 0;
				}
			}

			LinkedList causes = null;

			for (Method candidate : candidates) {
				int parameterCount = candidate.getParameterCount();
                //校验参数个数
				if (parameterCount >= minNrOfArgs) {
					ArgumentsHolder argsHolder;

					Class[] paramTypes = candidate.getParameterTypes();//获取入参的类型
					if (explicitArgs != null) {
						// Explicit arguments given -> arguments length must match exactly.
						if (paramTypes.length != explicitArgs.length) {
							continue;
						}
						argsHolder = new ArgumentsHolder(explicitArgs);
					}
					else {
						// Resolved constructor arguments: type conversion and/or autowiring necessary.
						try {
							String[] paramNames = null;
							ParameterNameDiscoverer pnd = this.beanFactory.getParameterNameDiscoverer();
							if (pnd != null) {
								paramNames = pnd.getParameterNames(candidate);//获取入参的名称数组
							}
							argsHolder = createArgumentArray(beanName, mbd, resolvedValues, bw,
									paramTypes, paramNames, candidate, autowiring, candidates.size() == 1);
						}
						catch (UnsatisfiedDependencyException ex) {
							if (logger.isTraceEnabled()) {
								logger.trace("Ignoring factory method [" + candidate + "] of bean '" + beanName + "': " + ex);
							}
							// Swallow and try next overloaded factory method.
							if (causes == null) {
								causes = new LinkedList<>();
							}
							causes.add(ex);
							continue;
						}
					}

					int typeDiffWeight = (mbd.isLenientConstructorResolution() ?
							argsHolder.getTypeDifferenceWeight(paramTypes) : argsHolder.getAssignabilityWeight(paramTypes));
					// Choose this factory method if it represents the closest match.
					if (typeDiffWeight < minTypeDiffWeight) {
						factoryMethodToUse = candidate;
						argsHolderToUse = argsHolder;
						argsToUse = argsHolder.arguments;
						minTypeDiffWeight = typeDiffWeight;
						ambiguousFactoryMethods = null;
					}
					// Find out about ambiguity: In case of the same type difference weight
					// for methods with the same number of parameters, collect such candidates
					// and eventually raise an ambiguity exception.
					// However, only perform that check in non-lenient constructor resolution mode,
					// and explicitly ignore overridden methods (with the same parameter signature).
					else if (factoryMethodToUse != null && typeDiffWeight == minTypeDiffWeight &&
							!mbd.isLenientConstructorResolution() &&
							paramTypes.length == factoryMethodToUse.getParameterCount() &&
							!Arrays.equals(paramTypes, factoryMethodToUse.getParameterTypes())) {
						if (ambiguousFactoryMethods == null) {
							ambiguousFactoryMethods = new LinkedHashSet<>();
							ambiguousFactoryMethods.add(factoryMethodToUse);
						}
						ambiguousFactoryMethods.add(candidate);
					}
				}
			}

			if (factoryMethodToUse == null || argsToUse == null) {
				if (causes != null) {
					UnsatisfiedDependencyException ex = causes.removeLast();
					for (Exception cause : causes) {
						this.beanFactory.onSuppressedException(cause);
					}
					throw ex;
				}
				List argTypes = new ArrayList<>(minNrOfArgs);
				if (explicitArgs != null) {
					for (Object arg : explicitArgs) {
						argTypes.add(arg != null ? arg.getClass().getSimpleName() : "null");
					}
				}
				else if (resolvedValues != null) {
					Set valueHolders = new LinkedHashSet<>(resolvedValues.getArgumentCount());
					valueHolders.addAll(resolvedValues.getIndexedArgumentValues().values());
					valueHolders.addAll(resolvedValues.getGenericArgumentValues());
					for (ValueHolder value : valueHolders) {
						String argType = (value.getType() != null ? ClassUtils.getShortName(value.getType()) :
								(value.getValue() != null ? value.getValue().getClass().getSimpleName() : "null"));
						argTypes.add(argType);
					}
				}
				String argDesc = StringUtils.collectionToCommaDelimitedString(argTypes);
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"No matching factory method found: " +
						(mbd.getFactoryBeanName() != null ?
							"factory bean '" + mbd.getFactoryBeanName() + "'; " : "") +
						"factory method '" + mbd.getFactoryMethodName() + "(" + argDesc + ")'. " +
						"Check that a method with the specified name " +
						(minNrOfArgs > 0 ? "and arguments " : "") +
						"exists and that it is " +
						(isStatic ? "static" : "non-static") + ".");
			}
			else if (void.class == factoryMethodToUse.getReturnType()) {
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"Invalid factory method '" + mbd.getFactoryMethodName() +
						"': needs to have a non-void return type!");
			}
			else if (ambiguousFactoryMethods != null) {
				throw new BeanCreationException(mbd.getResourceDescription(), beanName,
						"Ambiguous factory method matches found in bean '" + beanName + "' " +
						"(hint: specify index/type/name arguments for simple parameters to avoid type ambiguities): " +
						ambiguousFactoryMethods);
			}

			if (explicitArgs == null && argsHolderToUse != null) {
				mbd.factoryMethodToIntrospect = factoryMethodToUse;
				argsHolderToUse.storeCache(mbd, factoryMethodToUse);
			}
		}

		//通过反射实例化
		bw.setBeanInstance(instantiate(beanName, mbd, factoryBean, factoryMethodToUse, argsToUse));
		return bw;
	}
第二种:构造器的方式         1.无参构造器         2.添加@Autowire的构造器(多个会进行排序(按照入参多少),若@Autowire的value值为true,会报错)         3.只有一个构造器时且参数是容器中的bean时也会创建实例

注意点:  若为String,int,Integer等类型,会报错org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'student' defined in file xxxxxxxxxx; Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'java.lang.String' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}

源码截图:

1无参构造器实例化如下:

  

2-3实例化如下:

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原文地址: http://outofmemory.cn/langs/756404.html

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