Spring @Autowired注解私有属性（无set方法)

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今天看到一个POJO类中的某个私有属性通过@Autowired自动注入。

按照以前理解的依赖注入需要注入点，可以是set方法也可以是构造方法，但是此属性并没有可以通过外部access的方法，他是如何注入的呢？

在网上搜索了一番，答案如下：

stackoverflow.com/questions/4127365/how-can-spring-hibernate-access-private-members/4127403#4127403

You can set private a variable of another object through reflection. Here is an example on how to do it. Consider the following object with a private variable:

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public class MyBean {
    private String message;
}

Normally the message field wouldn't be accessible from outside MyBean, however, SnoopyClass can set and get its value. I wrote two static methods: setValue which can set a value into a private field called fieldName of an Object bean and a getValue method which can get the value of a private variable called fieldName from an Object bean.

The main method just demonstrates its use by creating an Object of MyBean class, setting the message variable and retrieving it. I've actually tested this code as a standalone application and it works.

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import java.lang.reflect.Field;
public class SnoopyClass {
    private static void setValue(Object bean, String fieldName, Object value)
            throws IllegalArgumentException, IllegalAccessException,
            SecurityException, NoSuchFieldException {
        Field privateVar = bean.getClass().getDeclaredField(fieldName);
        privateVar.setAccessible(true);
        privateVar.set(bean, value);
    }
    private static Object getValue(Object bean, String fieldName)
            throws IllegalArgumentException, IllegalAccessException,
            SecurityException, NoSuchFieldException {
        Field privateVar = bean.getClass().getDeclaredField(fieldName);
        privateVar.setAccessible(true);
        return privateVar.get(bean);
    }
    public static void main(String[] argv)
            throws IllegalArgumentException, SecurityException,
            IllegalAccessException, NoSuchFieldException {
         MyBean instance = new MyBean();
         setValue(instance, "message", "Shht! Don't tell anyone!");
         System.out.println("The message is '" + getValue(instance, "message"));
    }
}

The implementation uses getDeclaredField method on the class of the Object, because this method can look for all fields, even private. In contrast, getField can only access public members. The next step is calling setAccessible on the field to allow reading and writing it. The last step, is simply use the get and set methods provided by the java.lang.reflect.Field class.

This kind of manipulation is allowed only if the security manager allows that. By default Java doesn't install any security manager, so in a standalone program that you launch through your IDE or the command line, you won't have any problems to use this technique. I've also tried, in a Spring Application under Tomcat, and it's still working.

The primary application, at least for me, is being able to set private variables in my unit tests, especially for Spring Beans, without polluting the interface with unneeded setters.

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