The Java Tutorials have been written for JDK 8.Java教程是为JDK 8编写的。Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available.本页中描述的示例和实践没有利用后续版本中引入的改进，并且可能使用不再可用的技术。See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases.有关Java SE 9及其后续版本中更新的语言特性的摘要，请参阅Java语言更改。
See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.有关所有JDK版本的新功能、增强功能以及已删除或不推荐的选项的信息，请参阅JDK发行说明。
There are several modifiers that may be part of a field declaration:
public, protected, and privatetransient and volatilestaticfinalThe method Field.getModifiers() can be used to return the integer representing the set of declared modifiers for the field. The bits representing the modifiers in this integer are defined in java.lang.reflect.Modifier.
The FieldModifierSpyField.isSynthetic() and Field.isEnumCostant() respectively.
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import static java.lang.System.out;
enum Spy { BLACK , WHITE }
public class FieldModifierSpy {
volatile int share;
int instance;
class Inner {}
public static void main(String... args) {
try {
Class<?> c = Class.forName(args[0]);
int searchMods = 0x0;
for (int i = 1; i < args.length; i++) {
searchMods |= modifierFromString(args[i]);
}
Field[] flds = c.getDeclaredFields();
out.format("Fields in Class '%s' containing modifiers: %s%n",
c.getName(),
Modifier.toString(searchMods));
boolean found = false;
for (Field f : flds) {
int foundMods = f.getModifiers();
// Require all of the requested modifiers to be present
if ((foundMods & searchMods) == searchMods) {
out.format("%-8s [ synthetic=%-5b enum_constant=%-5b ]%n",
f.getName(), f.isSynthetic(),
f.isEnumConstant());
found = true;
}
}
if (!found) {
out.format("No matching fields%n");
}
// production code should handle this exception more gracefully
} catch (ClassNotFoundException x) {
x.printStackTrace();
}
}
private static int modifierFromString(String s) {
int m = 0x0;
if ("public".equals(s)) m |= Modifier.PUBLIC;
else if ("protected".equals(s)) m |= Modifier.PROTECTED;
else if ("private".equals(s)) m |= Modifier.PRIVATE;
else if ("static".equals(s)) m |= Modifier.STATIC;
else if ("final".equals(s)) m |= Modifier.FINAL;
else if ("transient".equals(s)) m |= Modifier.TRANSIENT;
else if ("volatile".equals(s)) m |= Modifier.VOLATILE;
return m;
}
}Sample output follows:
$ java FieldModifierSpy FieldModifierSpy volatile Fields in Class 'FieldModifierSpy' containing modifiers: volatile share [ synthetic=false enum_constant=false ] $ java FieldModifierSpy Spy public Fields in Class 'Spy' containing modifiers: public BLACK [ synthetic=false enum_constant=true ] WHITE [ synthetic=false enum_constant=true ] $ java FieldModifierSpy FieldModifierSpy\$Inner final Fields in Class 'FieldModifierSpy$Inner' containing modifiers: final this$0 [ synthetic=true enum_constant=false ] $ java FieldModifierSpy Spy private static final Fields in Class 'Spy' containing modifiers: private static final $VALUES [ synthetic=true enum_constant=false ]
Notice that some fields are reported even though they are not declared in the original code. This is because the compiler will generate some synthetic fields which are needed during runtime. To test whether a field is synthetic, the example invokes Field.isSynthetic(). The set of synthetic fields is compiler-dependent; however commonly used fields include this$0 for inner classes (that is, nested classes that are not static member classes) to reference the outermost enclosing class and $VALUES used by enums to implement the implicitly defined static method values(). The names of synthetic class members are not specified and may not be the same in all compiler implementations or releases. These and other synthetic fields will be included in the array returned by Class.getDeclaredFields() but not identified by Class.getField() since synthetic members are not typically public.
Because Field implements the interface java.lang.reflect.AnnotatedElement, it is possible to retrieve any runtime annotation with java.lang.annotation.RetentionPolicy.RUNTIME. For an example of obtaining annotations see the section Examining Class Modifiers and Types.