小编典典
您可以使用MethodHandle。其Javadoc写道：
使用Lookup API中的工厂方法，可以将Core Reflection
API对象表示的任何类成员转换为行为等效的方法句柄。例如，可以使用Lookup.unreflect将反射方法转换为方法句柄。生成的方法句柄通常提供对底层类成员的更直接和有效的访问。
虽然这将减少开销，但如果使用常规(非反射)字节代码指令进行调用，则方法句柄仍会阻止JVM可以采用某些优化(此类方法内联)。这种优化是否有益取决于您使用该方法的方式(如果该代码路径始终调用同一方法，则内联可以提供帮助(如果每次都是不同的方法，则可能不会)。
以下微基准可以使您大致了解反射，方法句柄和直接调用的相对性能：
package tools.bench;
import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.reflect.Method;
import java.math.BigDecimal;
public abstract class Bench {
final String name;
public Bench(String name) {
this.name = name;
}
abstract int run(int iterations) throws Throwable;
private BigDecimal time() {
try {
int nextI = 1;
int i;
long duration;
do {
i = nextI;
long start = System.nanoTime();
run(i);
duration = System.nanoTime() - start;
nextI = (i << 1) | 1;
} while (duration < 100000000 && nextI > 0);
return new BigDecimal((duration) * 1000 / i).movePointLeft(3);
} catch (Throwable e) {
throw new RuntimeException(e);
}
}
@Override
public String toString() {
return name + "\t" + time() + " ns";
}
static class C {
public Integer foo() {
return 1;
}
}
static final MethodHandle sfmh;
static {
try {
Method m = C.class.getMethod("foo");
sfmh = MethodHandles.lookup().unreflect(m);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
public static void main(String[] args) throws Exception {
final C invocationTarget = new C();
final Method m = C.class.getMethod("foo");
final Method am = C.class.getMethod("foo");
am.setAccessible(true);
final MethodHandle mh = sfmh;
Bench[] marks = {
new Bench("reflective invocation (without setAccessible)") {
@Override int run(int iterations) throws Throwable {
int x = 0;
for (int i = 0; i < iterations; i++) {
x += (Integer) m.invoke(invocationTarget);
}
return x;
}
},
new Bench("reflective invocation (with setAccessible)") {
@Override int run(int iterations) throws Throwable {
int x = 0;
for (int i = 0; i < iterations; i++) {
x += (Integer) am.invoke(invocationTarget);
}
return x;
}
},
new Bench("methodhandle invocation") {
@Override int run(int iterations) throws Throwable {
int x = 0;
for (int i = 0; i < iterations; i++) {
x += (Integer) mh.invokeExact(invocationTarget);
}
return x;
}
},
new Bench("static final methodhandle invocation") {
@Override int run(int iterations) throws Throwable {
int x = 0;
for (int i = 0; i < iterations; i++) {
x += (Integer) sfmh.invokeExact(invocationTarget);
}
return x;
}
},
new Bench("direct invocation") {
@Override int run(int iterations) throws Throwable {
int x = 0;
for (int i = 0; i < iterations; i++) {
x += invocationTarget.foo();
}
return x;
}
},
};
for (Bench bm : marks) {
System.out.println(bm);
}
}
}
在我有些过时的笔记本上
java version "1.7.0_02"
Java(TM) SE Runtime Environment (build 1.7.0_02-b13)
Java HotSpot(TM) Client VM (build 22.0-b10, mixed mode, sharing)
打印：
reflective invocation (without setAccessible) 568.506 ns
reflective invocation (with setAccessible) 42.377 ns
methodhandle invocation 27.461 ns
static final methodhandle invocation 9.402 ns
direct invocation 9.363 ns
更新：正如Irreputable所指出的那样，服务器VM具有一些不同的性能特征，因此
，仅当您可以将MethodHandle放在静态final字段中时 ，在 服务器VM中使用MethodHandle才有帮助
，在这种情况下，VM可以内联调用：
reflective invocation (without setAccessible) 9.736 ns
reflective invocation (with setAccessible) 7.113 ns
methodhandle invocation 26.319 ns
static final methodhandle invocation 0.045 ns
direct invocation 0.044 ns
我建议您衡量您的特定用例。
2020-09-18
--Picking Deep Filter Responses for Fine-grained Image Recognition)
--SPDA-CNN: Unifying Semantic Part Detection and Abstraction for Fine-grained Recognition)
--Part-Stacked CNN for Fine-Grained Visual Categorization)
