94 lines
3.4 KiB
Markdown
94 lines
3.4 KiB
Markdown
# 通过 Spring 5 中 Supplier 来获取 Bean
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Spring5 中开始提供了 `Supplier`，可以通过接口回调获取到一个 Bean 的实例，这种方式显然性能更好一些。
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如下：
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```java
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AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
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GenericBeanDefinition definition = new GenericBeanDefinition();
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definition.setBeanClass(Book.class);
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definition.setInstanceSupplier((Supplier<Book>) () -> {
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Book book = new Book();
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book.setName("深入浅出 Spring Security");
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book.setAuthor("江南一点雨");
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return book;
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});
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ctx.registerBeanDefinition("b1", definition);
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ctx.refresh();
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Book b = ctx.getBean("b1", Book.class);
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System.out.println("b = " + b);
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```
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关键就是通过调用 `BeanDefinition` 的 `setInstanceSupplier` 方法去设置回调。当然，上面这段代码还可以通过 *Lambda* 进一步简化：
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```java
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public class BookSupplier {
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public Book getBook() {
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Book book = new Book();
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book.setName("深入浅出 Spring Security");
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book.setAuthor("江南一点雨");
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return book;
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}
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}
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```
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然后调用这个方法即可：
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```java
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AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
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GenericBeanDefinition definition = new GenericBeanDefinition();
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definition.setBeanClass(Book.class);
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BookSupplier bookSupplier = new BookSupplier();
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definition.setInstanceSupplier(bookSupplier::getBook);
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ctx.registerBeanDefinition("b1", definition);
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ctx.refresh();
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Book b = ctx.getBean("b1", Book.class);
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System.out.println("b = " + b);
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```
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这是不是更有一点 *Lambda* 的感觉了?
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在 Spring 源码中，处理获取 Bean 实例的时候，有如下一个分支，就是处理 `Supplier` 这种情况的：
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```
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AbstractAutowireCapableBeanFactory#createBeanInstance
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protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
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// Make sure bean class is actually resolved at this point.
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Class<?> beanClass = resolveBeanClass(mbd, beanName);
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if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
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throw new BeanCreationException(mbd.getResourceDescription(), beanName,
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"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
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}
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Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
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if (instanceSupplier != null) {
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return obtainFromSupplier(instanceSupplier, beanName);
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}
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if (mbd.getFactoryMethodName() != null) {
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return instantiateUsingFactoryMethod(beanName, mbd, args);
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}
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//...
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return instantiateBean(beanName, mbd);
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}
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@Nullable
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private Object obtainInstanceFromSupplier(Supplier<?> supplier, String beanName) {
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String outerBean = this.currentlyCreatedBean.get();
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this.currentlyCreatedBean.set(beanName);
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try {
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if (supplier instanceof InstanceSupplier<?> instanceSupplier) {
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return instanceSupplier.get(RegisteredBean.of((ConfigurableListableBeanFactory) this, beanName));
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}
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if (supplier instanceof ThrowingSupplier<?> throwableSupplier) {
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return throwableSupplier.getWithException();
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}
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return supplier.get();
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}
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}
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```
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上面 `obtainFromSupplier` 这个方法，最终会调用到第二个方法。第二个方法中的 `supplier.get();` 其实最终就调用到我们自己写的 `getBook` 方法了。
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如上，这是从 Spring5 开始结合 Lamdba 的一种 Bean 注入方式。 |