Java 反射调用的一种优化

写一些Java框架的时候，经常需要通过反射get或者set某个bean的field，比较普通的做法是获取field后调用java.lang.reflect.Field.get(Object)，但每次都这样调用，能否有优化的空间呢？

答案是有。

第一种：

      由于每次都是重复的调用，所以想到了缓存每个bean的field，但这样做还是不够，所以想到了写一个code generator。通过生成代码的方式，get或者set每个bean的时候直接调用该bean的getter或者setter，这个实现听起来很牛逼，其实就是用asm生成一个类在用一个classloader加载进来每次调用直接invoke就可以了。

      可单纯为了一个反射调用做这么多，总感觉是大炮打了蚊子。

第二种：

      多谢@RednaxelaFX   的指点，找到了更简单的做法：sun.misc.Unsafe

      使用也非常的简单：首先通过sun.misc.Unsafe.objectFieldOffset(Field) 获取field的offset，然后使用sun.misc.Unsafe.getObject(Object, long)获取某个实例上的field的值。

      简单的测试代码如下：

import java.io.Serializable;
import java.lang.reflect.Field;
import sun.misc.Unsafe;
/**
 * @author haitao.yao Dec 14, 2010
 */
public class ReflectionCompare {
    private static final int count = 10000000;
    /**
     * @param args
     */
    public static void main(String[] args) {
        long duration = testIntCommon();
        System.out.println("int common test for  " + count
                + " times, duration: " + duration);
        duration = testUnsafe();
        System.out.println("int unsafe test for  " + count
                + " times, duration: " + duration);
    }
    private static long testUnsafe() {
        long start = System.currentTimeMillis();
        sun.misc.Unsafe unsafe = getUnsafe();
        int temp = count;
        Field field = getIntField();
        long offset = unsafe.objectFieldOffset(field);
        while (temp-- > 0) {
            unsafe.getInt(new TestBean(), offset);
        }
        return System.currentTimeMillis() - start;
    }
    private static long testIntCommon() {
        long start = System.currentTimeMillis();
        int temp = count;
        getIntField().setAccessible(true);
        while (temp-- > 0) {
            TestBean bean = new TestBean();
            try {
                getIntField().get(bean);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return System.currentTimeMillis() - start;
    }
    private static final sun.misc.Unsafe unsafe;
    static {
        sun.misc.Unsafe value = null;
        try {
            Class<?> clazz = Class.forName("sun.misc.Unsafe");
            Field field = clazz.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            value = (Unsafe) field.get(null);
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException("error to get theUnsafe", e);
        }
        unsafe = value;
    }
    public static final sun.misc.Unsafe getUnsafe() {
        return unsafe;
    }
    private static final Field intField;
    private static final Field stringField;
    static {
        try {
            intField = TestBean.class.getDeclaredField("age");
            stringField = TestBean.class.getDeclaredField("name");
        } catch (Exception e) {
            e.printStackTrace();
            throw new IllegalStateException("failed to init testbean field", e);
        }
    }
    public static final Field getIntField() {
        return intField;
    }
    public static final Field getStringField() {
        return stringField;
    }

    /**
     * @author haitao.yao
     * Dec 14, 2010
     */
    static class TestBean implements Serializable{
        /**
         *
         */
        private static final long serialVersionUID = -5994966479456252766L;

        private String name;
        private int age;
        /**
         * @return the name
         */
        public String getName() {
            return name;
        }
        /**
         * @param name the name to set
         */
        public void setName(String name) {
            this.name = name;
        }
        /**
         * @return the age
         */
        public int getAge() {
            return age;
        }
        /**
         * @param age the age to set
         */
        public void setAge(int age) {
            this.age = age;
        }
    }
}

import java.io.Serializable; import java.lang.reflect.Field; import sun.misc.Unsafe; /** * @author haitao.yao Dec 14, 2010 */ public class ReflectionCompare { private static final int count = 10000000; /** * @param args */ public static void main(String[] args) { long duration = testIntCommon(); System.out.println("int common test for " + count + " times, duration: " + duration); duration = testUnsafe(); System.out.println("int unsafe test for " + count + " times, duration: " + duration); } private static long testUnsafe() { long start = System.currentTimeMillis(); sun.misc.Unsafe unsafe = getUnsafe(); int temp = count; Field field = getIntField(); long offset = unsafe.objectFieldOffset(field); while (temp-- > 0) { unsafe.getInt(new TestBean(), offset); } return System.currentTimeMillis() - start; } private static long testIntCommon() { long start = System.currentTimeMillis(); int temp = count; getIntField().setAccessible(true); while (temp-- > 0) { TestBean bean = new TestBean(); try { getIntField().get(bean); } catch (Exception e) { e.printStackTrace(); } } return System.currentTimeMillis() - start; } private static final sun.misc.Unsafe unsafe; static { sun.misc.Unsafe value = null; try { Class<?> clazz = Class.forName("sun.misc.Unsafe"); Field field = clazz.getDeclaredField("theUnsafe"); field.setAccessible(true); value = (Unsafe) field.get(null); } catch (Exception e) { e.printStackTrace(); throw new RuntimeException("error to get theUnsafe", e); } unsafe = value; } public static final sun.misc.Unsafe getUnsafe() { return unsafe; } private static final Field intField; private static final Field stringField; static { try { intField = TestBean.class.getDeclaredField("age"); stringField = TestBean.class.getDeclaredField("name"); } catch (Exception e) { e.printStackTrace(); throw new IllegalStateException("failed to init testbean field", e); } } public static final Field getIntField() { return intField; } public static final Field getStringField() { return stringField; } /** * @author haitao.yao * Dec 14, 2010 */ static class TestBean implements Serializable{ /** * */ private static final long serialVersionUID = -5994966479456252766L; private String name; private int age; /** * @return the name */ public String getName() { return name; } /** * @param name the name to set */ public void setName(String name) { this.name = name; } /** * @return the age */ public int getAge() { return age; } /** * @param age the age to set */ public void setAge(int age) { this.age = age; } } }

      通过测试发现，效率是普通java.lang.reflect.Field.get(Object)的3倍，当然，性能这个东西，还是自己测试了放心。

      这样做有一个不好的地方：sun.misc.Unsafe在sun的包里，默认情况下，eclipse编译会报错，在Window->Preference->Java->Compiler->Errors/Warnings->Deprecated and restricted API -> Forbidden Reference 修改成Warning或者Ignore就可以了。由于Unsafe在JDK中很多的类库中都在使用，框架代码中使用还是很安全的，如果需要api变动，JDK源代码的修改工作量比我们的大多了 :-0

      至于第一种方法，虽然麻烦，有时间还是可以尝试一下的，有时间了写一下。