我尝试为
Java字节码实现几个静态分析.他们尝试计算某个方法是否具有特定的属性,例如是一种工厂方法.因为这些分析很难测试,所以我决定写一些Java代码,并用正确的属性直接注释方法.运行分析后,自动检查计算的和注释的属性是否相同是很容易的.
MyAnnotation：
@Retention(RUNTIME)
@Target(METHOD)
public @interface FactoryMethodProperty {
FactoryMethodKeys value() default FactoryMethodKeys.NonFactoryMethod;
}
示例测试代码：
public class PublicFactoryMethod {
private PublicFactoryMethod(){
// I'm private
}
@FactoryMethodProperty
public static void newInstanceAfterOtherConstructorCall(){
new TransFacoryMethod();
new PublicFactoryMethod();
}
@FactoryMethodProperty(FactoryMethodKeys.IsFactoryMethod)
public static PublicFactoryMethod newInstance(){
return new PublicFactoryMethod();
}
}
因为我的测试代码中的大多数方法都不是工厂方法,所以我将默认值设置为枚举值“FactoryMethodKeys.NonFactoryMethod”.但是,当我没有将枚举值显式传递给注释时,它不会被编译为字节码.
字节码：
#23 = Utf8 value
#24 = Utf8 Lorg/opalj/fpa/test/annotations/FactoryMethodKeys;
#25 = Utf8 IsFactoryMethod
{
public static void newInstanceAfterOtherConstructorCall();
descriptor: ()V
flags: ACC_PUBLIC,ACC_STATIC
RuntimeVisibleAnnotations:
0: #16()
Code:
stack=1,locals=0,args_size=0
0: new #17 // class factoryMethodTest/TransFacoryMethod
3: invokespecial #19 // Method factoryMethodTest/TransFacoryMethod."<init>":()V
6: new #1 // class factoryMethodTest/PublicFactoryMethod
9: invokespecial #20 // Method "<init>":()V
12: return
LineNumberTable:
line 49: 0
line 50: 6
line 51: 12
LocalVariableTable:
Start Length Slot Name Signature
public static factoryMethodTest.PublicFactoryMethod newInstance();
descriptor: ()LfactoryMethodTest/PublicFactoryMethod;
flags: ACC_PUBLIC,ACC_STATIC
RuntimeVisibleAnnotations:
0: #16(#23=e#24.#25)
Code:
stack=2,args_size=0
0: new #1 // class factoryMethodTest/PublicFactoryMethod
3: dup
4: invokespecial #20 // Method "<init>":()V
7: areturn
LineNumberTable:
line 55: 0
LocalVariableTable:
Start Length Slot Name Signature
}
我错了什么？为什么默认值完全被忽略？
解决方法
它不需要在那里在运行时,JVM构造一个可以检索的注释实例.该实例将使用默认值进行初始化,该默认值在.class文件中,用于注释本身.这表示为
AnnotationDefault attribute
The @H_502_24@AnnotationDefault attribute is a variable-length attribute in the
attributes table of certain @H_502_24@method_info structures (§4.6),namely
those representing elements of annotation types. The@H_502_24@AnnotationDefault
attribute records the default value for the element represented by the
@H_502_24@method_info structure.Each @H_502_24@method_info structure representing an element of an annotation
type may contain at most one @H_502_24@AnnotationDefault attribute. The Java
Virtual Machine must make this default value available so it can be
applied by appropriate reflective APIs.
