1. Gambaran keseluruhan
Dalam artikel ini, kita akan menjelajahi refleksi Java, yang memungkinkan kita memeriksa atau / dan mengubah atribut runtime kelas, antaramuka, bidang, dan metode. Ini sangat berguna apabila kita tidak mengetahui nama mereka pada waktu penyusunan.
Selain itu, kita dapat mewujudkan objek baru, memanggil kaedah, dan mendapatkan atau menetapkan nilai medan menggunakan pantulan.
2. Penyediaan Projek
Untuk menggunakan pantulan Java, kita tidak perlu memasukkan balang khas, konfigurasi khas, atau pergantungan Maven. JDK dihantar dengan sekumpulan kelas yang digabungkan dalam pakej java.lang.reflect khusus untuk tujuan ini.
Jadi yang perlu kita lakukan adalah membuat import berikut ke dalam kod kita:
import java.lang.reflect.*;dan kita selamat pergi.
Untuk mendapatkan akses ke kelas, kaedah, dan informasi lapangan dari contoh, kami memanggil kaedah getClass yang mengembalikan representasi kelas runtime objek. Objek kelas yang dikembalikan menyediakan kaedah untuk mengakses maklumat mengenai kelas.
3. Contoh Ringkas
Untuk membasahi kaki kita, kita akan melihat contoh yang sangat asas yang memeriksa bidang objek Java sederhana pada waktu runtime.
Mari buat kelas Orang sederhana dengan hanya nama dan bidang umur dan tidak ada kaedah sama sekali. Inilah kelas Orang:
public class Person { private String name; private int age; }Kami sekarang akan menggunakan pantulan Java untuk mengetahui nama semua bidang dalam kelas ini. Untuk menghargai daya pantulan, kita akan membina objek Person dan menggunakan Object sebagai jenis rujukan:
@Test public void givenObject_whenGetsFieldNamesAtRuntime_thenCorrect() { Object person = new Person(); Field[] fields = person.getClass().getDeclaredFields(); List actualFieldNames = getFieldNames(fields); assertTrue(Arrays.asList("name", "age") .containsAll(actualFieldNames)); }Ini menunjukkan ujian kita bahawa kita mampu untuk mendapatkan pelbagai F ield objek dari kami orang objek, walaupun rujukan kepada objek adalah sejenis induk objek itu.
Dalam contoh di atas, kami hanya berminat dengan nama-nama bidang tersebut, tetapi masih banyak lagi yang dapat dilakukan dan kami akan melihat contoh-contoh selanjutnya di bahagian-bahagian berikutnya.
Perhatikan bagaimana kita menggunakan kaedah pembantu untuk mengekstrak nama bidang yang sebenarnya, ini adalah kod yang sangat asas:
private static List getFieldNames(Field[] fields) { List fieldNames = new ArrayList(); for (Field field : fields) fieldNames.add(field.getName()); return fieldNames; }4. Kes Penggunaan Refleksi Java
Sebelum kita meneruskan ke pelbagai ciri pantulan Java, kita akan membincangkan beberapa kegunaan umum yang mungkin kita dapati untuknya. Pantulan Java sangat kuat dan dapat digunakan dalam beberapa cara.
Sebagai contoh, dalam banyak kes, kita mempunyai konvensi penamaan untuk jadual pangkalan data. Kami boleh memilih untuk menambahkan konsistensi dengan menetapkan terlebih dahulu nama jadual kami dengan tbl_ , sehingga jadual dengan data pelajar disebut tbl_student_data .
Dalam kes seperti itu, kita dapat menamakan objek Java yang menyimpan data pelajar sebagai Student atau StudentData. Kemudian menggunakan paradigma CRUD, kami mempunyai satu titik masuk untuk setiap operasi sehingga operasi Buat hanya menerima parameter Objek .
Kami kemudian menggunakan pantulan untuk mengambil nama objek dan nama bidang. Pada titik ini, kita dapat memetakan data ini ke tabel DB dan menetapkan nilai medan objek ke nama bidang DB yang sesuai.
5. Memeriksa Kelas Java
Di bahagian ini, kita akan meneroka komponen paling asas dalam API refleksi Java. Objek kelas Java, seperti yang kami sebutkan sebelumnya, memberi kami akses ke perincian dalaman objek apa pun.
Kami akan memeriksa perincian dalaman seperti nama kelas objek, pengubah, medan, kaedah, antara muka yang dilaksanakan, dll.
5.1. Bersiap sedia
Untuk mendapatkan pegangan tegas pada API refleksi, seperti yang diterapkan pada kelas Java dan mempunyai contoh dengan variasi, kami akan membuat kelas Animal abstrak yang menerapkan antara muka Eating . Antaramuka ini mentakrifkan tingkah laku makan dari sebarang objek Haiwan konkrit yang kita buat.
Jadi pertama, inilah antara muka Makan :
public interface Eating { String eats(); }dan kemudian konkrit Animal pelaksanaan Makan muka:
public abstract class Animal implements Eating { public static String CATEGORY = "domestic"; private String name; protected abstract String getSound(); // constructor, standard getters and setters omitted }Mari juga buat antara muka lain yang disebut Locomotion yang menerangkan bagaimana haiwan bergerak:
public interface Locomotion { String getLocomotion(); }Kami sekarang akan membuat kelas konkrit yang disebut Kambing yang memperluas Haiwan dan menerapkan Locomotion . Oleh kerana superclass menerapkan Makan , Kambing juga harus menggunakan kaedah antara muka itu:
public class Goat extends Animal implements Locomotion { @Override protected String getSound() { return "bleat"; } @Override public String getLocomotion() { return "walks"; } @Override public String eats() { return "grass"; } // constructor omitted }Dari titik ini dan seterusnya, kami akan menggunakan pantulan Java untuk memeriksa aspek objek Java yang muncul di kelas dan antara muka di atas.
5.2. Nama Kelas
Mari mulakan dengan mendapatkan nama objek dari Kelas :
@Test public void givenObject_whenGetsClassName_thenCorrect() { Object goat = new Goat("goat"); Class clazz = goat.getClass(); assertEquals("Goat", clazz.getSimpleName()); assertEquals("com.baeldung.reflection.Goat", clazz.getName()); assertEquals("com.baeldung.reflection.Goat", clazz.getCanonicalName()); }Perhatikan bahawa kaedah getSimpleName dari Kelas mengembalikan nama asas objek seperti yang akan muncul dalam deklarasinya. Kemudian dua kaedah lain mengembalikan nama kelas yang lengkap termasuk pengisytiharan pakej.
Mari kita lihat juga bagaimana kita dapat membuat objek kelas Kambing jika kita hanya mengetahui nama kelasnya yang memenuhi syarat:
@Test public void givenClassName_whenCreatesObject_thenCorrect(){ Class clazz = Class.forName("com.baeldung.reflection.Goat"); assertEquals("Goat", clazz.getSimpleName()); assertEquals("com.baeldung.reflection.Goat", clazz.getName()); assertEquals("com.baeldung.reflection.Goat", clazz.getCanonicalName()); }Perhatikan bahawa nama yang kita sampaikan ke kaedah static forName harus merangkumi maklumat pakej. Jika tidak, kami akan mendapat ClassNotFoundException .
5.3. Pengubah Kelas
Kita dapat menentukan pengubah yang digunakan dalam kelas dengan memanggil kaedah getModifiers yang mengembalikan Integer. Setiap pengubah adalah bit bendera yang diset atau dibersihkan.
The java.lang.reflect.Modifier tawaran kelas kaedah statik yang menganalisis kembali Integer kehadiran atau ketiadaan pengubahsuai tertentu.
Mari sahkan pengubahsuaian beberapa kelas yang kami tentukan di atas:
@Test public void givenClass_whenRecognisesModifiers_thenCorrect() { Class goatClass = Class.forName("com.baeldung.reflection.Goat"); Class animalClass = Class.forName("com.baeldung.reflection.Animal"); int goatMods = goatClass.getModifiers(); int animalMods = animalClass.getModifiers(); assertTrue(Modifier.isPublic(goatMods)); assertTrue(Modifier.isAbstract(animalMods)); assertTrue(Modifier.isPublic(animalMods)); }Kami dapat memeriksa pengubah dari mana-mana kelas yang terdapat di balang perpustakaan yang kami impor ke dalam projek kami.
Dalam kebanyakan kes, kita mungkin perlu menggunakan pendekatan forName dan bukannya instansiasi penuh kerana itu akan menjadi proses yang mahal untuk kelas kelas memori.
5.4. Maklumat Pakej
Dengan menggunakan pantulan Java, kita juga dapat memperoleh maklumat mengenai pakej kelas atau objek apa pun. Data ini digabungkan di dalam kelas Package yang dikembalikan dengan kaedah call to getPackage pada objek kelas.
Mari jalankan ujian untuk mendapatkan nama pakej:
@Test public void givenClass_whenGetsPackageInfo_thenCorrect() { Goat goat = new Goat("goat"); Class goatClass = goat.getClass(); Package pkg = goatClass.getPackage(); assertEquals("com.baeldung.reflection", pkg.getName()); }5.5. Kelas Super
Kami juga dapat memperoleh superclass dari kelas Java mana pun dengan menggunakan pantulan Java.
In many cases, especially while using library classes or Java's builtin classes, we may not know beforehand the superclass of an object we are using, this subsection will show how to obtain this information.
So let's go ahead and determine the superclass of Goat. Additionally, we also show that java.lang.String class is a subclass of java.lang.Object class:
@Test public void givenClass_whenGetsSuperClass_thenCorrect() { Goat goat = new Goat("goat"); String str = "any string"; Class goatClass = goat.getClass(); Class goatSuperClass = goatClass.getSuperclass(); assertEquals("Animal", goatSuperClass.getSimpleName()); assertEquals("Object", str.getClass().getSuperclass().getSimpleName()); }5.6. Implemented Interfaces
Using Java reflection, we are also able to get the list of interfaces implemented by a given class.
Let's retrieve the class types of the interfaces implemented by the Goat class and the Animal abstract class:
@Test public void givenClass_whenGetsImplementedInterfaces_thenCorrect(){ Class goatClass = Class.forName("com.baeldung.reflection.Goat"); Class animalClass = Class.forName("com.baeldung.reflection.Animal"); Class[] goatInterfaces = goatClass.getInterfaces(); Class[] animalInterfaces = animalClass.getInterfaces(); assertEquals(1, goatInterfaces.length); assertEquals(1, animalInterfaces.length); assertEquals("Locomotion", goatInterfaces[0].getSimpleName()); assertEquals("Eating", animalInterfaces[0].getSimpleName()); }Notice from the assertions that each class implements only a single interface. Inspecting the names of these interfaces, we find that Goat implements Locomotion and Animal implements Eating, just as it appears in our code.
You may have observed that Goat is a subclass of the abstract class Animal and implements the interface method eats(), then, Goat also implements the Eating interface.
It is therefore worth noting that only those interfaces that a class explicitly declares as implemented with the implements keyword appear in the returned array.
So even if a class implements interface methods because its superclass implements that interface, but the subclass does not directly declare that interface with the implements keyword, then that interface will not appear in the array of interfaces.
5.7. Constructors, Methods, and Fields
With Java reflection, we are able to inspect the constructors of any object's class as well as methods and fields.
We will, later on, be able to see deeper inspections on each of these components of a class but for now, it suffices to just get their names and compare them with what we expect.
Let's see how to get the constructor of the Goat class:
@Test public void givenClass_whenGetsConstructor_thenCorrect(){ Class goatClass = Class.forName("com.baeldung.reflection.Goat"); Constructor[] constructors = goatClass.getConstructors(); assertEquals(1, constructors.length); assertEquals("com.baeldung.reflection.Goat", constructors[0].getName()); }We can also inspect the fields of the Animal class like so:
@Test public void givenClass_whenGetsFields_thenCorrect(){ Class animalClass = Class.forName("com.baeldung.reflection.Animal"); Field[] fields = animalClass.getDeclaredFields(); List actualFields = getFieldNames(fields); assertEquals(2, actualFields.size()); assertTrue(actualFields.containsAll(Arrays.asList("name", "CATEGORY"))); }Just like we can inspect the methods of the Animal class:
@Test public void givenClass_whenGetsMethods_thenCorrect(){ Class animalClass = Class.forName("com.baeldung.reflection.Animal"); Method[] methods = animalClass.getDeclaredMethods(); List actualMethods = getMethodNames(methods); assertEquals(4, actualMethods.size()); assertTrue(actualMethods.containsAll(Arrays.asList("getName", "setName", "getSound"))); }Just like getFieldNames, we have added a helper method to retrieve method names from an array of Method objects:
private static List getMethodNames(Method[] methods) { List methodNames = new ArrayList(); for (Method method : methods) methodNames.add(method.getName()); return methodNames; }6. Inspecting Constructors
With Java reflection, we can inspect constructors of any class and even create class objects at runtime. This is made possible by the java.lang.reflect.Constructor class.
Earlier on, we only looked at how to get the array of Constructor objects, from which we were able to get the names of the constructors.
In this section, we will focus on how to retrieve specific constructors. In Java, as we know, no two constructors of a class share exactly the same method signature. So we will use this uniqueness to get one constructor from many.
To appreciate the features of this class, we will create a Bird subclass of Animal with three constructors. We will not implement Locomotion so that we can specify that behavior using a constructor argument, to add still more variety:
public class Bird extends Animal { private boolean walks; public Bird() { super("bird"); } public Bird(String name, boolean walks) { super(name); setWalks(walks); } public Bird(String name) { super(name); } public boolean walks() { return walks; } // standard setters and overridden methods }Let's confirm by using reflection that this class has three constructors:
@Test public void givenClass_whenGetsAllConstructors_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Constructor[] constructors = birdClass.getConstructors(); assertEquals(3, constructors.length); }Next, we will retrieve each constructor for the Bird class by passing the constructor's parameter class types in declared order:
@Test public void givenClass_whenGetsEachConstructorByParamTypes_thenCorrect(){ Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Constructor cons1 = birdClass.getConstructor(); Constructor cons2 = birdClass.getConstructor(String.class); Constructor cons3 = birdClass.getConstructor(String.class, boolean.class); }There is no need for assertion since when a constructor with given parameter types in the given order does not exist, we will get a NoSuchMethodException and the test will automatically fail.
In the last test, we will see how to instantiate objects at runtime while supplying their parameters:
@Test public void givenClass_whenInstantiatesObjectsAtRuntime_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Constructor cons1 = birdClass.getConstructor(); Constructor cons2 = birdClass.getConstructor(String.class); Constructor cons3 = birdClass.getConstructor(String.class, boolean.class); Bird bird1 = (Bird) cons1.newInstance(); Bird bird2 = (Bird) cons2.newInstance("Weaver bird"); Bird bird3 = (Bird) cons3.newInstance("dove", true); assertEquals("bird", bird1.getName()); assertEquals("Weaver bird", bird2.getName()); assertEquals("dove", bird3.getName()); assertFalse(bird1.walks()); assertTrue(bird3.walks()); }We instantiate class objects by calling the newInstance method of Constructor class and passing the required parameters in declared order. We then cast the result to the required type.
It's also possible to call the default constructor using the Class.newInstance() method. However, this method has been deprecated since Java 9, and we shouldn't use it in modern Java projects.
For bird1, we use the default constructor which from our Bird code, automatically sets the name to bird and we confirm that with a test.
We then instantiate bird2 with only a name and test as well, remember that when we don't set locomotion behavior as it defaults to false, as seen in the last two assertions.
7. Inspecting Fields
Previously, we only inspected the names of fields, in this section, we will show how toget and set their values at runtime.
There are two main methods used to inspect fields of a class at runtime: getFields() and getField(fieldName).
The getFields() method returns all accessible public fields of the class in question. It will return all the public fields in both the class and all superclasses.
For instance, when we call this method on the Bird class, we will only get the CATEGORY field of its superclass, Animal, since Bird itself does not declare any public fields:
@Test public void givenClass_whenGetsPublicFields_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Field[] fields = birdClass.getFields(); assertEquals(1, fields.length); assertEquals("CATEGORY", fields[0].getName()); }This method also has a variant called getField which returns only one Field object by taking the name of the field:
@Test public void givenClass_whenGetsPublicFieldByName_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Field field = birdClass.getField("CATEGORY"); assertEquals("CATEGORY", field.getName()); }We are not able to access private fields declared in superclasses and not declared in the child class. This is why we are not able to access the name field.
However, we can inspect private fields declared in the class we are dealing with by calling the getDeclaredFields method:
@Test public void givenClass_whenGetsDeclaredFields_thenCorrect(){ Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Field[] fields = birdClass.getDeclaredFields(); assertEquals(1, fields.length); assertEquals("walks", fields[0].getName()); }We can also use its other variant in case we know the name of the field:
@Test public void givenClass_whenGetsFieldsByName_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Field field = birdClass.getDeclaredField("walks"); assertEquals("walks", field.getName()); }If we get the name of the field wrong or type in an in-existent field, we will get a NoSuchFieldException.
We get the field type as follows:
@Test public void givenClassField_whenGetsType_thenCorrect() { Field field = Class.forName("com.baeldung.reflection.Bird") .getDeclaredField("walks"); Class fieldClass = field.getType(); assertEquals("boolean", fieldClass.getSimpleName()); }Next, we will look at how to access field values and modify them. To be able to get the value of a field, let alone setting it, we must first set it's accessible by calling setAccessible method on the Field object and pass boolean true to it:
@Test public void givenClassField_whenSetsAndGetsValue_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Bird bird = (Bird) birdClass.getConstructor().newInstance(); Field field = birdClass.getDeclaredField("walks"); field.setAccessible(true); assertFalse(field.getBoolean(bird)); assertFalse(bird.walks()); field.set(bird, true); assertTrue(field.getBoolean(bird)); assertTrue(bird.walks()); }In the above test, we ascertain that indeed the value of the walks field is false before setting it to true.
Notice how we use the Field object to set and get values by passing it the instance of the class we are dealing with and possibly the new value we want the field to have in that object.
One important thing to note about Field objects is that when it is declared as public static, then we don't need an instance of the class containing them, we can just pass null in its place and still obtain the default value of the field, like so:
@Test public void givenClassField_whenGetsAndSetsWithNull_thenCorrect(){ Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Field field = birdClass.getField("CATEGORY"); field.setAccessible(true); assertEquals("domestic", field.get(null)); }8. Inspecting Methods
In a previous example, we used reflection only to inspect method names. However, Java reflection is more powerful than that.
With Java reflection, we can invoke methods at runtime and pass them their required parameters, just like we did for constructors. Similarly, we can also invoke overloaded methods by specifying parameter types of each.
Just like fields, there are two main methods that we use for retrieving class methods. The getMethods method returns an array of all public methods of the class and superclasses.
This means that with this method, we can get public methods of the java.lang.Object class like toString, hashCode, and notifyAll:
@Test public void givenClass_whenGetsAllPublicMethods_thenCorrect(){ Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Method[] methods = birdClass.getMethods(); List methodNames = getMethodNames(methods); assertTrue(methodNames.containsAll(Arrays .asList("equals", "notifyAll", "hashCode", "walks", "eats", "toString"))); }To get only public methods of the class we are interested in, we have to use getDeclaredMethods method:
@Test public void givenClass_whenGetsOnlyDeclaredMethods_thenCorrect(){ Class birdClass = Class.forName("com.baeldung.reflection.Bird"); List actualMethodNames = getMethodNames(birdClass.getDeclaredMethods()); List expectedMethodNames = Arrays .asList("setWalks", "walks", "getSound", "eats"); assertEquals(expectedMethodNames.size(), actualMethodNames.size()); assertTrue(expectedMethodNames.containsAll(actualMethodNames)); assertTrue(actualMethodNames.containsAll(expectedMethodNames)); }Each of these methods has the singular variation which returns a single Method object whose name we know:
@Test public void givenMethodName_whenGetsMethod_thenCorrect() throws Exception { Bird bird = new Bird(); Method walksMethod = bird.getClass().getDeclaredMethod("walks"); Method setWalksMethod = bird.getClass().getDeclaredMethod("setWalks", boolean.class); assertTrue(walksMethod.canAccess(bird)); assertTrue(setWalksMethod.canAccess(bird)); }Notice how we retrieve individual methods and specify what parameter types they take. Those that don't take parameter types are retrieved with an empty variable argument, leaving us with only a single argument, the method name.
Next, we will show how to invoke a method at runtime. We know by default that the walks attribute of the Bird class is false, we want to call its setWalks method and set it to true:
@Test public void givenMethod_whenInvokes_thenCorrect() { Class birdClass = Class.forName("com.baeldung.reflection.Bird"); Bird bird = (Bird) birdClass.getConstructor().newInstance(); Method setWalksMethod = birdClass.getDeclaredMethod("setWalks", boolean.class); Method walksMethod = birdClass.getDeclaredMethod("walks"); boolean walks = (boolean) walksMethod.invoke(bird); assertFalse(walks); assertFalse(bird.walks()); setWalksMethod.invoke(bird, true); boolean walks2 = (boolean) walksMethod.invoke(bird); assertTrue(walks2); assertTrue(bird.walks()); }Perhatikan bagaimana kita mula-mula menggunakan kaedah jalan kaki dan memasukkan jenis pengembalian ke jenis data yang sesuai dan kemudian periksa nilainya. Kami kemudian menggunakan kaedah setWalks untuk menukar nilai itu dan menguji lagi.
9. Kesimpulannya
Dalam tutorial ini, kami telah membahas Java Reflection API dan melihat bagaimana menggunakannya untuk memeriksa kelas, antaramuka, medan, dan kaedah pada waktu berjalan tanpa pengetahuan sebelumnya tentang dalaman mereka dengan menyusun masa.
Kod sumber lengkap dan contoh untuk tutorial ini boleh didapati di GitHub.