Java 反射調用的一種最佳化

寫一些Java架構的時候，經常需要通過反射get或者set某個bean的field，比較普通的做法是擷取field後調用java.lang.reflect.Field.get(Object)，但每次都這樣調用，能否有最佳化的空間呢？

 

答案是有。

第一種：

      由於每次都是重複的調用，所以想到了緩衝每個bean的field，但這樣做還是不夠，所以想到了寫一個code generator。通過產生代碼的方式，get或者set每個bean的時候直接調用該bean的getter或者setter，這個實現聽起來很牛逼，其實就是用asm產生一個類在用一個classloader載入進來每次調用直接invoke就可以了。

      可單純為了一個反射調用做這麼多，總感覺是大炮打了蚊子。

 

第二種：

      多謝@RednaxelaFX   的指點，找到了更簡單的做法：sun.misc.Unsafe

      使用也非常的簡單：首先通過sun.misc.Unsafe.objectFieldOffset(Field) 擷取field的offset，然後使用sun.misc.Unsafe.getObject(Object, long)擷取某個執行個體上的field的值。

      Hessian在現實對Java對象的序列化和還原序列化的時候都是使用這種方式！具體可以參見com.caucho.hessian.io.HessianInput、com.caucho.hessian.io.HessianOutput。這幾種使用方式的效率究竟相差多大？後續再找個時間來測試下！

     對sun.misc.Unsafe 簡單的測試代碼如下： 

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;}}}

【原文】http://hi.baidu.com/hellolq/item/26493d1efe2b066a71d5e89d