Java反射的优化

最新推荐文章于 2025-09-12 18:08:49 发布

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最新推荐文章于 2025-09-12 18:08:49 发布

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文章标签： java 开发工具

原文链接：http://blog.51cto.com/hahaeason/1184463

Java的反射机制，相信很多人都已经用过，我们在使用的时候，最大的一个问题除了使用复杂之外，是性能隐忧，本文主要针对性能隐忧进行阐述。

反射为什么慢？

在stackoverflow上，已经有人问了这个问题：http://stackoverflow.com/questions/1392351/java-reflection-why-is-it-so-slow

有一个回复很多人支持，如下

The compiler can do no optimization whatsoever as it can have no real idea about what you are doing. This probably goes for the JIT as well
Everything being invoked/created has to be discovered (i.e. classes looked up by name, methods looked at for matches etc)
Arguments need to be dressed up via boxing/unboxing, packing into arrays, Exceptions wrapped in InvocationTargetExceptions and re-thrown etc.
All the processing that Jon Skeet mentions here.

简单意译：编译器不能对代码对优化，所有的反射操作都需要类似查表的操作，参数需要封装，解封装，异常也重新封装，rethrow等等。而四点具体的内容如下：

Compare that with everything that reflection has to do:

Check that there's a parameterless constructor
Check the accessibility of the parameterless constructor
Check that the caller has access to use reflection at all
Work out (at execution time) how much space needs to be allocated
Call into the constructor code (because it won't know beforehand that the constructor is empty)

基本上都是check这check那，还有无法被优化掉的调用。

所以，这些额外的操作，就可以理解为什么反射比较慢了，但是，究竟哪些环节比较耗时呢？

反射究竟是什么地方慢？

我写了一段测试代码，可能有漏洞，欢迎大家指出！

class A{
public String field = "";
public boolean doSomething(){
return true;
}
}
A object = new A();
Class c = A.class;
int loops = 10000000;
A[] As = new A[loops];
boolean boolRet = false;
long start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
As[i] = new A();
}
System.out.println( loops + " regular instaniations:" + (System.currentTimeMillis() - start) + " milliseconds." );
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
As[i] = (A) c.newInstance();
}
System.out.println( loops + " reflection instaniations:" + (System.currentTimeMillis() - start) + " milliseconds." );
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
boolRet = object.doSomething();
}
System.out.println( loops + " regular method calls:" + (System.currentTimeMillis() - start) + " milliseconds." );
java.lang.reflect.Method method = c.getMethod( "doSomething", null );
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
boolRet = (Boolean) method.invoke( object, null );
}
System.out.println( loops + " reflective method calls without lookup:" + (System.currentTimeMillis() - start) + " milliseconds." );
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
method = c.getMethod( "doSomething", null );
method.invoke( object, null );
}
System.out.println( loops + " reflective method calls with lookup:" + (System.currentTimeMillis() - start) + " milliseconds." );
String fieldValue = "";
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
fieldValue = object.field;
}
System.out.println( loops + " regular field get value:" + (System.currentTimeMillis() - start) + " milliseconds." );
Field field = null;
field = c.getField("field");
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
fieldValue = (String) field.get(object);
}
System.out.println( loops + " reflective field get value without lookup:" + (System.currentTimeMillis() - start) + " milliseconds." );
start = System.currentTimeMillis();
for( int i = 0; i < loops; i++ )
{
field = c.getField("field");
fieldValue = (String) field.get(object);
}
System.out.println( loops + " reflective field get value with lookup:" + (System.currentTimeMillis() - start) + " milliseconds." );

执行后结果如下（JDK1.6）：

10000000 regular instaniations:1557 milliseconds.
10000000 reflection instaniations:2893 milliseconds.

10000000 regular method calls:6 milliseconds.
10000000 reflective method calls without lookup:3460 milliseconds.
10000000 reflective method calls with lookup:10119 milliseconds.

10000000 regular field get value:5 milliseconds.
10000000 reflective field get value without lookup:3897 milliseconds.
10000000 reflective field get value with lookup:12560 milliseconds.

可见：