Custom annotation files

core-java-8/src/main/java/com/baeldung/customannotations/Init.java

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+package com.baeldung.customannotations;
+
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+@Retention(RUNTIME)
+@Target(METHOD)
+public @interface Init {
+
+}

core-java-8/src/main/java/com/baeldung/customannotations/JsonElement.java

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+package com.baeldung.customannotations;
+
+import static java.lang.annotation.ElementType.FIELD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+@Retention(RUNTIME)
+@Target({ FIELD })
+public @interface JsonElement {
+    public String key() default "";
+}

core-java-8/src/main/java/com/baeldung/customannotations/JsonSerializable.java

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+package com.baeldung.customannotations;
+
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+@Retention(RUNTIME)
+@Target(TYPE)
+public @interface JsonSerializable {
+
+}

core-java-8/src/main/java/com/baeldung/customannotations/JsonSerializationException.java

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+package com.baeldung.customannotations;
+
+public class JsonSerializationException extends RuntimeException {
+
+    private static final long serialVersionUID = 1L;
+
+    public JsonSerializationException(String message) {
+        super(message);
+    }
+}

core-java-8/src/main/java/com/baeldung/customannotations/ObjectToJsonConverter.java

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+package com.baeldung.customannotations;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+import com.sun.istack.internal.NotNull;
+
+public class ObjectToJsonConverter {
+    public String convertToJson(@NotNull Object object) throws JsonSerializationException {
+        try {
+
+            checkIfSerializable(object);
+            initializeObject(object);
+            return getJsonString(object);
+
+        } catch (Exception e) {
+            throw new JsonSerializationException(e.getMessage());
+        }
+    }
+
+    private void checkIfSerializable(Object object) {
+        if (Objects.isNull(object)) {
+            throw new JsonSerializationException("Can't serialize a null object");
+        }
+
+        Class<?> clazz = object.getClass();
+        if (!clazz.isAnnotationPresent(JsonSerializable.class)) {
+            throw new JsonSerializationException("The class" + clazz.getSimpleName() + " is not annotated with JsonSerializable");
+        }
+    }
+
+    private void initializeObject(Object object) throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {
+        Class<?> clazz = object.getClass();
+        for (Method method : clazz.getDeclaredMethods()) {
+            if (method.isAnnotationPresent(Init.class)) {
+                method.setAccessible(true);
+                method.invoke(object);
+            }
+        }
+    }
+
+    private String getJsonString(Object object) throws IllegalArgumentException, IllegalAccessException {
+        Class<?> clazz = object.getClass();
+        Map<String, String> jsonElementsMap = new HashMap<>();
+        for (Field field : clazz.getDeclaredFields()) {
+            field.setAccessible(true);
+            if (field.isAnnotationPresent(JsonElement.class)) {
+                jsonElementsMap.put(getKey(field), (String) field.get(object));
+            }
+        }
+
+        String jsonString = jsonElementsMap.entrySet()
+            .stream()
+            .map(entry -> "\"" + entry.getKey() + "\":\"" + entry.getValue() + "\"")
+            .collect(Collectors.joining(","));
+        return "{" + jsonString + "}";
+    }
+
+    private String getKey(Field field) {
+        String value = field.getAnnotation(JsonElement.class)
+            .key();
+        return value.isEmpty() ? field.getName() : value;
+    }
+}

core-java-8/src/main/java/com/baeldung/customannotations/Person.java

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+package com.baeldung.customannotations;
+
+@JsonSerializable
+public class Person {
+    @JsonElement
+    private String firstName;
+    @JsonElement
+    private String lastName;
+    @JsonElement(key = "personAge")
+    private String age;
+
+    private String address;
+
+    public Person(String firstName, String lastName) {
+        super();
+        this.firstName = firstName;
+        this.lastName = lastName;
+    }
+
+    public Person(String firstName, String lastName, String age) {
+        this.firstName = firstName;
+        this.lastName = lastName;
+        this.age = age;
+    }
+
+    @Init
+    private void initNames() {
+        this.firstName = this.firstName.substring(0, 1)
+            .toUpperCase() + this.firstName.substring(1);
+        this.lastName = this.lastName.substring(0, 1)
+            .toUpperCase() + this.lastName.substring(1);
+    }
+
+    public String getFirstName() {
+        return firstName;
+    }
+
+    public void setFirstName(String firstName) {
+        this.firstName = firstName;
+    }
+
+    public String getLastName() {
+        return lastName;
+    }
+
+    public void setLastName(String lastName) {
+        this.lastName = lastName;
+    }
+
+    public String getAge() {
+        return age;
+    }
+
+    public void setAge(String age) {
+        this.age = age;
+    }
+
+    public String getAddress() {
+        return address;
+    }
+
+    public void setAddress(String address) {
+        this.address = address;
+    }
+
+}