在 Java 中可以通过反射访问私有字段吗

Yes, it absolutely is - assuming you've got the appropriate security permissions. Use Field.setAccessible(true) first if you're accessing it from a different class.

import java.lang.reflect.*;


class Other
{
private String str;
public void setStr(String value)
{
str = value;
}
}


class Test
{
public static void main(String[] args)
// Just for the ease of a throwaway test. Don't
// do this normally!
throws Exception
{
Other t = new Other();
t.setStr("hi");
Field field = Other.class.getDeclaredField("str");
field.setAccessible(true);
Object value = field.get(t);
System.out.println(value);
}
}

And no, you shouldn't normally do this... it's subverting the intentions of the original author of the class. For example, there may well be validation applied in any situation where the field can normally be set, or other fields may be changed at the same time. You're effectively violating the intended level of encapsulation.

Yes.

  Field f = Test.class.getDeclaredField("str");
f.setAccessible(true);//Very important, this allows the setting to work.
String value = (String) f.get(object);

Then you use the field object to get the value on an instance of the class.

Note that get method is often confusing for people. You have the field, but you don't have an instance of the object. You have to pass that to the get method

Yes it is possible.

You need to use the getDeclaredField method (instead of the getField method), with the name of your private field:

Field privateField = Test.class.getDeclaredField("str");

Additionally, you need to set this Field to be accessible, if you want to access a private field:

privateField.setAccessible(true);

Once that's done, you can use the get method on the Field instance, to access the value of the str field.