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java/spring.md

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+# 通过 Spring 5 中 Supplier 来获取 Bean
+
+
+
+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。
+
+如下：
+
+```java
+AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+GenericBeanDefinition definition = new GenericBeanDefinition();
+definition.setBeanClass(Book.class);
+definition.setInstanceSupplier((Supplier<Book>) () -> {
+    Book book = new Book();
+    book.setName("深入浅出 Spring Security");
+    book.setAuthor("江南一点雨");
+    return book;
+});
+ctx.registerBeanDefinition("b1", definition);
+ctx.refresh();
+Book b = ctx.getBean("b1", Book.class);
+System.out.println("b = " + b);
+```
+
+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：
+
+```java
+public class BookSupplier {
+    public Book getBook() {
+        Book book = new Book();
+        book.setName("深入浅出 Spring Security");
+        book.setAuthor("江南一点雨");
+        return book;
+    }
+}
+```
+
+然后调用这个方法即可：
+
+```java
+AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+GenericBeanDefinition definition = new GenericBeanDefinition();
+definition.setBeanClass(Book.class);
+BookSupplier bookSupplier = new BookSupplier();
+definition.setInstanceSupplier(bookSupplier::getBook);
+ctx.registerBeanDefinition("b1", definition);
+ctx.refresh();
+Book b = ctx.getBean("b1", Book.class);
+System.out.println("b = " + b);
+```
+
+这是不是更有一点 *Lambda* 的感觉了?
+
+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：
+
+```
+AbstractAutowireCapableBeanFactory#createBeanInstance
+protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
+    // Make sure bean class is actually resolved at this point.
+    Class<?> beanClass = resolveBeanClass(mbd, beanName);
+    if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
+        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
+        "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
+    }
+    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
+    if (instanceSupplier != null) {
+        return obtainFromSupplier(instanceSupplier, beanName);
+    }
+    if (mbd.getFactoryMethodName() != null) {
+        return instantiateUsingFactoryMethod(beanName, mbd, args);
+    }
+    //...
+    return instantiateBean(beanName, mbd);
+}
+
+@Nullable
+private Object obtainInstanceFromSupplier(Supplier<?> supplier, String beanName) {
+    String outerBean = this.currentlyCreatedBean.get();
+    this.currentlyCreatedBean.set(beanName);
+    try {
+        if (supplier instanceof InstanceSupplier<?> instanceSupplier) {
+            return instanceSupplier.get(RegisteredBean.of((ConfigurableListableBeanFactory) this, beanName));
+        }
+        if (supplier instanceof ThrowingSupplier<?> throwableSupplier) {
+            return throwableSupplier.getWithException();
+        }
+        return supplier.get();
+    }
+}
+```
+
+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。
+
+如上，这是从 Spring5 开始结合 Lamdba 的一种 Bean 注入方式。