spring实例化二：SimpleInstantiationStrategy

最新推荐文章于 2025-02-17 18:41:01 发布

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原文链接：http://www.cnblogs.com/sten/p/5762516.html

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#java

本文深入剖析了Spring框架中类的实例化策略，通过InstantiationStrategy接口及其默认实现SimpleInstantiationStrategy，展示了如何根据不同场景选择合适的实例化方法，包括直接实例化、方法注入实例化及工厂方法实例化。

spring对类的实例化，定义了接口InstantiationStrategy，同时先做了个简单实现类SimpleInstantiationStrategy。采用实现部分，抽象部分的策略。为了更好的扩展性，把一部分再次抽象，后面可以采用多种实现方式。

下面具体代码分析：

public class SimpleInstantiationStrategy implements InstantiationStrategy {
 
    // FactoryMethod的ThreadLocal对象，线程所有的变量
    private static final ThreadLocal<Method> currentlyInvokedFactoryMethod = new ThreadLocal<Method>();
 
    // 返回当前线程所有的FactoryMethod变量值
    public static Method getCurrentlyInvokedFactoryMethod() {
        return currentlyInvokedFactoryMethod.get();
    }
 
    // 第一种实例化方法，实现部分，部分抽象
    @Override
    public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
        // bd对象定义里，是否包含MethodOverride列表；spring有两个标签参数会产生MethodOverrides ，分别是 lookup-method,replaced-method 
        // 没有MethodOverride对象，可以直接实例化
        if (bd.getMethodOverrides().isEmpty()) {
            // 实例化对象的构造方法
            Constructor<?> constructorToUse;
            // 锁定对象，使获得实例化构造方法线程安全
            synchronized (bd.constructorArgumentLock) {
                // 查看bd对象里是否含有
                constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
                // 没有就生成
                if (constructorToUse == null) {
                    final Class<?> clazz = bd.getBeanClass();
                    if (clazz.isInterface()) {
                        throw new BeanInstantiationException(clazz, "Specified class is an interface");
                    }
                    try {
                        if (System.getSecurityManager() != null) {
                            constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
                                @Override
                                public Constructor<?> run() throws Exception {
                                    return clazz.getDeclaredConstructor((Class[]) null);
                                }
                            });
                        }
                        else {
                            constructorToUse =    clazz.getDeclaredConstructor((Class[]) null);
                        }
                        // 生成成功后，赋值给bd对象，后面使用
                        bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                    }
                    catch (Exception ex) {
                        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                    }
                }
            }
            // 反射生成对象
            return BeanUtils.instantiateClass(constructorToUse);
        }
        else {
            // 有MethodOverride对象，需要使用另一种实现方式，之类实现
            return instantiateWithMethodInjection(bd, beanName, owner);
        }
    }
 
    // 第一种实例化方法的抽象部分
    protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner) {
        throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");
    }
 
    // 第二种实例化方法，实现部分，抽象部分
    @Override
    public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,
            final Constructor<?> ctor, Object... args) {
        // 查看bd对象是否有MethodOverride对象
        // 没有MethodOverride，则直接实例化对象
        if (bd.getMethodOverrides().isEmpty()) {
            if (System.getSecurityManager() != null) {
                // use own privileged to change accessibility (when security is on)
                AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    @Override
                    public Object run() {
                        ReflectionUtils.makeAccessible(ctor);
                        return null;
                    }
                });
            }
            // 反射实例化对象
            return BeanUtils.instantiateClass(ctor, args);
        }
        else {
            // 有MethodOverride，之类实现实例化方法
            return instantiateWithMethodInjection(bd, beanName, owner, ctor, args);
        }
    }
 
    // 第二种实例化方法的抽象部分
    protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,
            Constructor<?> ctor, Object... args) {
        throw new UnsupportedOperationException("Method Injection not supported in SimpleInstantiationStrategy");
    }
 
    // 第三种实例化方法，全部实现
    @Override
    public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner,
            Object factoryBean, final Method factoryMethod, Object... args) {
 
        try {
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged(new PrivilegedAction<Object>() {
                    @Override
                    public Object run() {
                        ReflectionUtils.makeAccessible(factoryMethod);
                        return null;
                    }
                });
            }
            else {
                ReflectionUtils.makeAccessible(factoryMethod);
            }
 
            // currentlyInvokedFactoryMethod，这块暂时还没看到在哪个地方用到了
            // 先取出原有的 Method
            Method priorInvokedFactoryMethod = currentlyInvokedFactoryMethod.get();
            try {
                // 设置当前的Method
                currentlyInvokedFactoryMethod.set(factoryMethod);
                // 使用factoryMethod实例化对象
                return factoryMethod.invoke(factoryBean, args);
            }
            finally {
                // 实例化完成，恢复现场
                if (priorInvokedFactoryMethod != null) {
                    currentlyInvokedFactoryMethod.set(priorInvokedFactoryMethod);
                }
                else {
                    currentlyInvokedFactoryMethod.remove();
                }
            }
        }
        catch (IllegalArgumentException ex) {
            throw new BeanInstantiationException(factoryMethod.getReturnType(),
                    "Illegal arguments to factory method '" + factoryMethod.getName() + "'; " +
                    "args: " + StringUtils.arrayToCommaDelimitedString(args), ex);
        }
        catch (IllegalAccessException ex) {
            throw new BeanInstantiationException(factoryMethod.getReturnType(),
                    "Cannot access factory method '" + factoryMethod.getName() + "'; is it public?", ex);
        }
        catch (InvocationTargetException ex) {
            String msg = "Factory method '" + factoryMethod.getName() + "' threw exception";
            if (bd.getFactoryBeanName() != null && owner instanceof ConfigurableBeanFactory &&
                    ((ConfigurableBeanFactory) owner).isCurrentlyInCreation(bd.getFactoryBeanName())) {
                msg = "Circular reference involving containing bean '" + bd.getFactoryBeanName() + "' - consider " +
                        "declaring the factory method as static for independence from its containing instance. " + msg;
            }
            throw new BeanInstantiationException(factoryMethod.getReturnType(), msg, ex.getTargetException());
        }
    }
 
}