Spring源码-lookup-method和replaced-method

lookup-method和replaced-method平时用的很少，主要用来解决单例bean中使用原型bean的场景。

一、应用

lookup-method:

A.java

public class A {
public B getB() {
return null;
}

public void printA(){
System.out.println("AAA:"+this);
this.getB().print();
}
}

B.java

public class B {

public void print() {
System.out.println("BBBBB:"+this);
}
}

loopup.xml

Main.java

public class Main {
public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("classpath:lookup.xml");

A bean = applicationContext.getBean(A.class);
bean.printA();

bean = applicationContext.getBean(A.class);
bean.printA();

}
}

replaced-method：

将上面的B.java修改为：

public class B implements MethodReplacer, ApplicationContextAware {
private ApplicationContext applicationContext;

public void print() {
System.out.println("BBBBB:"+this);
}

@Override
public Object reimplement(Object obj, Method method, Object[] args) throws Throwable {
return this.applicationContext.getBean(B.class);
}

@Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
}

lookup.xml修改为：

cglib例子：

import org.springframework.cglib.proxy.Enhancer;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;

import java.lang.reflect.Method;

public class Test {
public void test() {
System.out.println("调用 test");
}
public static void main(String[] args) {
Enhancer enhancer = new Enhancer();
enhancer.setSuperclass(Test.class);
enhancer.setCallback(new MethodInterceptor() {
@Override
public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
System.out.println("before method run...");
Object result = proxy.invokeSuper(obj, args);
System.out.println("after method run...");
return result;
}
});

Test o = (Test) enhancer.create();
o.test();
}
}

二、Spring源码解读lookup-method和replaced-method
前面Spring解析xml文件的bean标签时：

BeanDefinitionParserDelegate的parseBeanDefinitionElement方法：

public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, @Nullable BeanDefinition containingBean) {

this.parseState.push(new BeanEntry(beanName));

String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}

try {
AbstractBeanDefinition bd = createBeanDefinition(className, parent);

parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

parseMetaElements(ele, bd);
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

parseConstructorArgElements(ele, bd);
parsePropertyElements(ele, bd);
parseQualifierElements(ele, bd);

bd.setResource(this.readerContext.getResource());
bd.setSource(extractSource(ele));

return bd;
}
catch (ClassNotFoundException ex) {
error("Bean class [" + className + "] not found", ele, ex);
}
catch (NoClassDefFoundError err) {
error("Class that bean class [" + className + "] depends on not found", ele, err);
}
catch (Throwable ex) {
error("Unexpected failure during bean definition parsing", ele, ex);
}
finally {
this.parseState.pop();
}

return null;
}

其中：

parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

将lookup-method和replaced-method标签对应的方法名加到beanDefinition的methodOverrides中。

AbstractAutowireCapableBeanFactory的createBean：

@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {

if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;

// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}

// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}

try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}

try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}

其中的mbdToUse.prepareMethodOverrides()处理beanDefinition的methodOverrides。

public void prepareMethodOverrides() throws BeanDefinitionValidationException {
// Check that lookup methods exist and determine their overloaded status.
if (hasMethodOverrides()) {
getMethodOverrides().getOverrides().forEach(this::prepareMethodOverride);
}
}

public boolean hasMethodOverrides() {
return !this.methodOverrides.isEmpty();
}

protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
if (count == 0) {
throw new BeanDefinitionValidationException(
"Invalid method override: no method with name '" + mo.getMethodName() +
"' on class [" + getBeanClassName() + "]");
}
else if (count == 1) {
// Mark override as not overloaded, to avoid the overhead of arg type checking.
mo.setOverloaded(false);
}
}

如果beanDefinition的methodOverrides非空，则设置MethodOverride的overloaded为false。

doCreateBean->createBeanInstance->instantiateBean:

protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {
try {
Object beanInstance;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(
(PrivilegedAction