获取和设置字段值

给定一个类的实例,可以使用反射来设置该类中字段的值。通常仅在特殊情况下无法以常规方式设置值时才执行此操作。因为这样的访问通常会违反该类的设计意图,所以应绝对谨慎地使用它。

Book类说明如何设置 long,array 和 enum 字段类型的值。 Field中描述了获取和设置其他原始类型的方法。

import java.lang.reflect.Field;
import java.util.Arrays;
import static java.lang.System.out;

enum Tweedle { DEE, DUM }

public class Book {
    public long chapters = 0;
    public String[] characters = { "Alice", "White Rabbit" };
    public Tweedle twin = Tweedle.DEE;

    public static void main(String... args) {
	Book book = new Book();
	String fmt = "%6S:  %-12s = %s%n";

	try {
	    Class<?> c = book.getClass();

	    Field chap = c.getDeclaredField("chapters");
	    out.format(fmt, "before", "chapters", book.chapters);
  	    chap.setLong(book, 12);
	    out.format(fmt, "after", "chapters", chap.getLong(book));

	    Field chars = c.getDeclaredField("characters");
	    out.format(fmt, "before", "characters",
		       Arrays.asList(book.characters));
	    String[] newChars = { "Queen", "King" };
	    chars.set(book, newChars);
	    out.format(fmt, "after", "characters",
		       Arrays.asList(book.characters));

	    Field t = c.getDeclaredField("twin");
	    out.format(fmt, "before", "twin", book.twin);
	    t.set(book, Tweedle.DUM);
	    out.format(fmt, "after", "twin", t.get(book));

        // production code should handle these exceptions more gracefully
	} catch (NoSuchFieldException x) {
	    x.printStackTrace();
	} catch (IllegalAccessException x) {
	    x.printStackTrace();
	}
    }
}

这是相应的输出:

$ java Book
BEFORE:  chapters     = 0
 AFTER:  chapters     = 12
BEFORE:  characters   = [Alice, White Rabbit]
 AFTER:  characters   = [Queen, King]
BEFORE:  twin         = DEE
 AFTER:  twin         = DUM

Note:

通过反射设置字段的值具有一定的性能开销,因为必须进行各种操作,例如验证访问权限。从运行时的角度来看,效果是相同的,并且操作是原子的,就好像直接在类代码中更改了值一样。

使用反射可能会导致某些运行时优化丢失。例如,以下代码很可能由 Java 虚拟机进行优化:

int x = 1;
x = 2;
x = 3;

使用Field.set*()的等效代码可能不会。