#10 effective java: item 7

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/cache/CacheKey.java

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+package com.example.effectivejava.chapter01.item07.cache;
+
+import java.time.LocalDateTime;
+
+public class CacheKey {
+
+    private Integer value;
+
+    private LocalDateTime created;
+
+    public CacheKey(Integer value) {
+        this.value = value;
+        this.created = LocalDateTime.now();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        return this.value.equals(o);
+    }
+
+    @Override
+    public int hashCode() {
+        return this.value.hashCode();
+    }
+
+    public LocalDateTime getCreated() {
+        return created;
+    }
+
+    @Override
+    public String toString() {
+        return "CacheKey{" +
+                "value=" + value +
+                ", created=" + created +
+                '}';
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/cache/Post.java

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+package com.example.effectivejava.chapter01.item07.cache;
+
+public class Post {
+
+    private Integer id;
+
+    private String title;
+
+    private String content;
+
+    public Integer getId() {
+        return id;
+    }
+
+    public void setId(Integer id) {
+        this.id = id;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public void setTitle(String title) {
+        this.title = title;
+    }
+
+    public String getContent() {
+        return content;
+    }
+
+    public void setContent(String content) {
+        this.content = content;
+    }
+
+    @Override
+    public void finalize() {
+        System.out.println("gc called");
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/cache/PostRepository.java

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+package com.example.effectivejava.chapter01.item07.cache;
+
+import java.util.Map;
+import java.util.WeakHashMap;
+
+public class PostRepository {
+
+    private Map<CacheKey, Post> cache;
+
+    public PostRepository() {
+        this.cache = new WeakHashMap<>();
+    }
+
+    public Post getPostById(CacheKey key) {
+        if (cache.containsKey(key)) {
+            return cache.get(key);
+        } else {
+            // TODO DB에서 읽어오거나 REST API를 통해 읽어올 수 있습니다.
+            Post post = new Post();
+            cache.put(key, post);
+            return post;
+        }
+    }
+
+    public Map<CacheKey, Post> getCache() {
+        return cache;
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/listener/ChatRoom.java

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+package com.example.effectivejava.chapter01.item07.listener;
+
+import java.lang.ref.WeakReference;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+public class ChatRoom {
+
+    private List<WeakReference<User>> users;
+
+    public ChatRoom() {
+        this.users = new ArrayList<>();
+    }
+
+    public void addUser(User user) {
+        this.users.add(new WeakReference<>(user));
+    }
+
+    public void sendMessage(String message) {
+        users.forEach(wr -> Objects.requireNonNull(wr.get()).receive(message));
+    }
+
+    public List<WeakReference<User>> getUsers() {
+        return users;
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/listener/User.java

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+package com.example.effectivejava.chapter01.item07.listener;
+
+public class User {
+
+    public void receive(String message) {
+        System.out.println(message);
+    }
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/stack/EmptyStackException.java

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+package com.example.effectivejava.chapter01.item07.stack;
+
+// (36쪽의 Stack 코드에서 던지는 예외)
+public class EmptyStackException extends IllegalStateException {
+}

effective-java/src/main/java/com/example/effectivejava/chapter01/item07/stack/Stack.java

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+package com.example.effectivejava.chapter01.item07.stack;
+
+import java.util.Arrays;
+
+// 코드 7-1 메모리 누수가 일어나는 위치는 어디인가? (36쪽)
+public class Stack {
+    private Object[] elements;
+    private int size = 0;
+    private static final int DEFAULT_INITIAL_CAPACITY = 16;
+
+    public Stack() {
+        elements = new Object[DEFAULT_INITIAL_CAPACITY];
+    }
+
+    public void push(Object e) {
+        ensureCapacity();
+        elements[size++] = e;
+    }
+
+//    public Object pop() {
+//        if (size == 0)
+//            throw new EmptyStackException();
+//        return elements[--size];
+//    }
+
+    /**
+     * 원소를 위한 공간을 적어도 하나 이상 확보한다.
+     * 배열 크기를 늘려야 할 때마다 대략 두 배씩 늘린다.
+     */
+    private void ensureCapacity() {
+        if (elements.length == size)
+            elements = Arrays.copyOf(elements, 2 * size + 1);
+    }
+
+    // 코드 7-2 제대로 구현한 pop 메서드 (37쪽)
+    public Object pop() {
+        if (size == 0)
+            throw new EmptyStackException();
+        Object result = elements[--size];
+        elements[size] = null; // 다 쓴 참조 해제
+        return result;
+    }
+
+    public static void main(String[] args) {
+        Stack stack = new Stack();
+        for (String arg : args)
+            stack.push(arg);
+
+        while (true)
+            System.err.println(stack.pop());
+    }
+}

effective-java/src/test/java/com/example/effectivejava/chapter01/item07/cache/PostRepositoryTest.java

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+package com.example.effectivejava.chapter01.item07.cache;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class PostRepositoryTest {
+
+    @Test
+    void cache() throws InterruptedException {
+        PostRepository postRepository = new PostRepository();
+        CacheKey key1 = new CacheKey(1);
+        postRepository.getPostById(key1);
+
+        assertFalse(postRepository.getCache().isEmpty());
+
+        key1 = null;
+        // TODO run gc
+        System.out.println("run gc");
+        System.gc();
+        System.out.println("wait");
+        Thread.sleep(3000L);
+
+        assertTrue(postRepository.getCache().isEmpty());
+    }
+
+    @Test
+    void backgroundThread() throws InterruptedException {
+        ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
+        PostRepository postRepository = new PostRepository();
+        CacheKey key1 = new CacheKey(1);
+        postRepository.getPostById(key1);
+
+        Runnable removeOldCache = () -> {
+            System.out.println("running removeOldCache task");
+            Map<CacheKey, Post> cache = postRepository.getCache();
+            Set<CacheKey> cacheKeys = cache.keySet();
+            Optional<CacheKey> key = cacheKeys.stream().min(Comparator.comparing(CacheKey::getCreated));
+            key.ifPresent((k) -> {
+                System.out.println("removing " + k);
+                cache.remove(k);
+            });
+        };
+
+        System.out.println("The time is : " + new Date());
+
+        executor.scheduleAtFixedRate(removeOldCache,
+                1, 3, TimeUnit.SECONDS);
+
+        Thread.sleep(20000L);
+
+        executor.shutdown();
+    }
+}

effective-java/src/test/java/com/example/effectivejava/chapter01/item07/listener/ChatRoomTest.java

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+package com.example.effectivejava.chapter01.item07.listener;
+
+import org.junit.jupiter.api.Test;
+
+import java.lang.ref.WeakReference;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class ChatRoomTest {
+
+    @Test
+    void charRoom() throws InterruptedException {
+        ChatRoom chatRoom = new ChatRoom();
+        User user1 = new User();
+        User user2 = new User();
+
+        chatRoom.addUser(user1);
+        chatRoom.addUser(user2);
+
+        chatRoom.sendMessage("hello");
+
+        user1 = null;
+
+        System.gc();
+
+        Thread.sleep(5000L);
+
+        List<WeakReference<User>> users = chatRoom.getUsers();
+        assertEquals(1, users.size());
+    }
+}