【JAVA】Reflection 笔记

1. Use object.getClass() and class.class to get the Class.

note that getclass() is for objects, .class is for classes, for example, a class Student, a object sy, we call Student.class and sy.getClass.

2.get a class object through forName() method. Like

String className = "java.util.Random";
Class cl = Class.forName(className);

3.then we can creat a instance of that class

Object obj = cl.getConstructor().newInstance();

4.get the structure of a class through its name

java.lang.Class 1.0

• Field[] getFields() 1.1

• Field[] getDeclaredFields() 1.1

getFields returns an array containing Field objects for the public fields of this class or its superclasses; getDeclaredField returns an array of Field objects for all fields of this class. The methods return an array of length 0 if there are no such fields or if the Class object

represents a primitive or array type.

• Method[] getMethods() 1.1

• Method[] getDeclaredMethods() 1.1

returns an array containing Method objects: getMethods returns public methods and includes inherited methods; getDeclaredMethods returns all methods of this class or interface but does not include inherited methods.

• Constructor[] getConstructors() 1.1

• Constructor[] getDeclaredConstructors() 1.1

returns an array containing Constructor objects that give you all the public constructors (for getConstructors) or all constructors (for getDeclaredConstructors) of the class represented by this Class object.

• isInterface()

returns true if this Class object describes an interface.

• isEnum() 1.5

returns true if this Class object describes an enum.

• isRecord() 16

returns true if this Class object describes a record.

• RecordComponent[] getRecordComponents() 16

returns an array of RecordComponent objects that describe the record fields, or null if this class is not a record.

• String getPackageName() 9

gets the name of the package containing this type, or the package of the element type if this type is an array type, or "java.lang" if this type is a primitive type.

java.lang.reflect.Field 1.1

java.lang.reflect.Method 1.1

java.lang.reflect.Constructor 1.1

• Class getDeclaringClass()

returns the Class object for the class that defines this constructor, method, or field.

• Class[] getExceptionTypes() (in Constructor and Method classes)

returns an array of Class objects that represent the types of the exceptions thrown by the method.

• int getModifiers()

returns an integer that describes the modifiers of this constructor, method, or field. Use the methods in the Modifier class to analyze the return value.

• String getName()

returns a string that is the name of the constructor, method, or field.

• Class[] getParameterTypes() (in Constructor and Method classes)

returns an array of Class objects that represent the types of the parameters.

• Class getReturnType() (in Method class)

returns a Class object that represents the return type

java.lang.reflect.RecordComponent 16

• String getName()Class<?> getType()

get the name and type of this record component

• Method getAccessor()

returns the Method object for accessing this record component

java.lang.reflect.Modifier 1.1

• static String toString(int modifiers)

returns a string with the modifiers that correspond to the bits set in modifiers.

• static boolean isAbstract(int modifiers)

• static boolean isFinal(int modifiers)

• static boolean isInterface(int modifiers)

• static boolean isNative(int modifiers)

• static boolean isPrivate(int modifiers)

• static boolean isProtected(int modifiers)

• static boolean isPublic(int modifiers)

• static boolean isStatic(int modifiers)

• static boolean isStrict(int modifiers)

• static boolean isSynchronized(int modifiers)

• static boolean isVolatile(int modifiers)

tests the bit in the modifiers value that corresponds to the modifier in the method name.

5.do more

we first get the corresponding Class object.  ->   Class object

and then call getDeclaredFields on the Class object.  ->  Field object

then we can use f.get() to get the actual instance field value  ->  any type value

further more, we can set the value of this field by f.set(obj, value)

for example

var harry = new Employee("Harry Hacker", 50000, 10, 1, 1989);
Class cl = harry.getClass();
 // the class object representing Employee
Field f = cl.getDeclaredField("name");
 // the name field of the Employee class
Object v = f.get(harry);
 // the value of the name field of the harry object, i.e.,
 // the String object "Harry Hacker"

but this has violate the rules of encapsulation, for doing this, we need this

f.setAccessible(true); // now OK to call f.get(harry)

but for doing this, we need to add some VM options in our run configuration. Add these

--add-opens java.base/java.util=ALL-UNNAMED

--add-opens java.base/java.lang=ALL-UNNAMED

 6. get component type of an array

(1) First, get the class object of a.

(2) Confirm that it is indeed an array.

(3) Use the getComponentType method of the Class class (which is defined only for class objects that represent arrays) to find the right type for the array.

(4) Conversely, for any Class object representing a class C, the arrayType

method yields the Class object representing C[].

 public static Object goodCopyOf(Object a, int newLength) {
        Class cl = a.getClass();
        if (!cl.isArray()) return null;
        Class componentType = cl.getComponentType();
        int length = Array.getLength(a);
        Object newArray = Array.newInstance(componentType, newLength);
        System.arraycopy(a, 0, newArray, 0, Math.min(length, newLength));
        return newArray;
    }

7.get and invoke methods

(1) we can get methods through getMethod()  using a class object, method name and parameters  ->  Method object

(2) then we can call method with Object invoke(Object obj, Object... args), obj is the implicit parameter, args are explicit parameters, for example

Method m1 = Employee.class.getMethod("getName");
Method m2 = Employee.class.getMethod("raiseSalary",double.class);

String n = (String) m1.invoke(harry);
double s = (Double) m2.invoke(harry);