【Java基础】Java反射——Private Fields and Methods

Despite the common belief it is actually possible to access private fields and methods of other classes via Java Reflection. It is not even that difficult. This can be very handy during unit testing. This text will show you how.

Note: This only works when running the code as a standalone Java application, like you do with unit tests and regular applications. If you try to do this inside a Java Applet, you will need to fiddle around with the SecurityManager. But, since that is not something you need to do very often, it is left out of this text so far.

Note: There has been a lot of talk about disabling the ability to access private fields via reflection from Java 9. From my experiments it seems to still be possible in Java 9, but be aware that this might change in a future Java version.

Accessing Private Fields

To access a private field you will need to call the Class.getDeclaredField(String name) or Class.getDeclaredFields() method. The methods Class.getField(String name) and Class.getFields() methods only return public fields, so they won't work. Here is a simple example of a class with a private field, and below that the code to access that field via Java Reflection:

public class PrivateObject {
  private String privateString = null;
  public PrivateObject(String privateString) {
    this.privateString = privateString;
  }
}
PrivateObject privateObject = new PrivateObject("The Private Value");
Field privateStringField = PrivateObject.class.
            getDeclaredField("privateString");
privateStringField.setAccessible(true);
String fieldValue = (String) privateStringField.get(privateObject);
System.out.println("fieldValue = " + fieldValue);

This code example will print out the text "fieldValue = The Private Value", which is the value of the private field privateString of the PrivateObject instance created at the beginning of the code sample.

Notice the use of the method PrivateObject.class.getDeclaredField("privateString"). It is this method call that returns the private field. This method only returns fields declared in that particular class, not fields declared in any superclasses.

Notice the line in bold too. By calling Field.setAcessible(true) you turn off the access checks for this particular Field instance, for reflection only. Now you can access it even if it is private, protected or package scope, even if the caller is not part of those scopes. You still can't access the field using normal code. The compiler won't allow it.

Accessing Private Methods

To access a private method you will need to call the Class.getDeclaredMethod(String name, Class[] parameterTypes) or Class.getDeclaredMethods() method. The methods Class.getMethod(String name, Class[] parameterTypes) and Class.getMethods() methods only return public methods, so they won't work. Here is a simple example of a class with a private method, and below that the code to access that method via Java Reflection:

public class PrivateObject {
  private String privateString = null;
  public PrivateObject(String privateString) {
    this.privateString = privateString;
  }
  private String getPrivateString(){
    return this.privateString;
  }
}
PrivateObject privateObject = new PrivateObject("The Private Value");
Method privateStringMethod = PrivateObject.class.
        getDeclaredMethod("getPrivateString", null);
privateStringMethod.setAccessible(true);
String returnValue = (String)
        privateStringMethod.invoke(privateObject, null);
System.out.println("returnValue = " + returnValue);

This code example will print out the text "returnValue = The Private Value", which is the value returned by the method getPrivateString() when invoked on the PrivateObject instance created at the beginning of the code sample.

Notice the use of the method PrivateObject.class.getDeclaredMethod("privateString"). It is this method call that returns the private method. This method only returns methods declared in that particular class, not methods declared in any superclasses.

Notice the line in bold too. By calling Method.setAcessible(true) you turn off the access checks for this particular Method instance, for reflection only. Now you can access it even if it is private, protected or package scope, even if the caller is not part of those scopes. You still can't access the method using normal code. The compiler won't allow it.