Java 類加載之匿名類和主類相互依賴問題

時間 2019-12-07

標籤 java 加載匿名相互依賴問題欄目 Java 简体版

原文原文鏈接

Qestion

/**
 * ClassInitializedOrder for : Java Classload Order Test
 *
 * @author <a href="mailto:magicianisaac@gmail.com">Isaac.Zhang | 若初</a>
 * @since 2019/7/20
 */
// CASE 1  
public class ClassInitializedOrder {
    private static boolean initialized = false;
    static {
        println("static 代碼塊執行。");
        Thread thread = new Thread(() -> initialized = true);
        thread.start();
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        println("main 函數執行。");
        System.out.println("initialized = " + initialized);
    }

    private static void println(Object o){
        System.out.println(o);
    }
}

-------------------------------------------------------------------
// CASE 2    
public class ClassInitializedOrder {
    private static boolean initialized = false;
    static {
        println("static 代碼塊執行。");
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                println("Runnable 代碼塊執行。");
                initialized = true;
            }
        });
        thread.start();
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        println("main 函數執行。");
        System.out.println("initialized = " + initialized);
    }

    private static void println(Object o){
        System.out.println(o);
    }

Answer

A: initialized = true
B: initialized = false
C: 編譯錯誤
D: 以上答案都是錯的

Explain

程序執行的時候，App Classloader 會首先加載ClassInitializedOrder.class, 按照類的順序依次執行。java

private static boolean initialized = false;bash

CASE 1

咱們都知道，static塊會在類加載的時候初始化，那麼下一步會執行到Thread thread = new Thread(() -> initialized = true);咱們先來看一下當前行的字節碼：ide

static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=3, locals=2, args_size=0
         0: iconst_0
         1: putstatic     #7                  // Field initialized:Z
         4: new           #11                 // class java/lang/Thread
         7: dup
         8: invokedynamic #12,  0             // InvokeDynamic #0:run:()Ljava/lang/Runnable;
        13: invokespecial #13                 // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
        16: astore_0
        17: aload_0
        18: invokevirtual #14                 // Method java/lang/Thread.start:()V
        21: aload_0
        22: invokevirtual #15                 // Method java/lang/Thread.join:()V
        25: goto          33
        28: astore_1
        29: aload_1
        30: invokevirtual #17                 // Method java/lang/InterruptedException.printStackTrace:()V
        33: return

分析#12能夠看到當前行的處理須要()也就是改匿名類自己來處理，InvokeDynamic指令的在當前的執行又依賴於當前所處的主類，主類並無執行結束，所以它須要等待主類執行結束，所以會在此停頓，以下：函數

CASE 2

繼續查看字節碼：spa

static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=4, locals=2, args_size=0
         0: iconst_0
         1: putstatic     #1                  // Field initialized:Z
         4: ldc           #14                 // String static 代碼塊執行。
         6: invokestatic  #2                  // Method println:(Ljava/lang/Object;)V
         9: new           #15                 // class java/lang/Thread
        12: dup
        13: new           #16                 // class com/sxzhongf/daily/question/july/ClassInitializedOrder$1
        16: dup
        17: invokespecial #17                 // Method com/sxzhongf/daily/question/july/ClassInitializedOrder$1."<init>":()V
        20: invokespecial #18                 // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
        23: astore_0
        24: aload_0
        25: invokevirtual #19                 // Method java/lang/Thread.start:()V
        28: aload_0
        29: invokevirtual #20                 // Method java/lang/Thread.join:()V
        32: goto          40
        35: astore_1
        36: aload_1
        37: invokevirtual #22                 // Method java/lang/InterruptedException.printStackTrace:()V
        40: return

查看#16,咱們能夠看到這裏變成了new #16 // class com/sxzhongf/daily/question/july/ClassInitializedOrder$1,能夠明顯看到從以前的invokeDynamic 變成了 new 一個匿名類，那麼它的結果呢？線程

依然仍是block.咱們來換一行代碼試試？code

public class ClassInitializedOrder {
    private static boolean initialized = false;
    static {
        println("static 代碼塊執行。");
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                //println("Runnable 代碼塊執行。");
                System.out.println("Runnable 代碼塊執行。");
                //initialized = true;
            }
        });
        thread.start();
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

咱們看到咱們只是修改了一行代碼System.out.println("Runnable 代碼塊執行。");，那麼結果呢？對象

執行成功的返回了。爲何？繼續查看字節碼ip

static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=4, locals=2, args_size=0
         0: iconst_0
         1: putstatic     #9                  // Field initialized:Z
         4: ldc           #14                 // String static 代碼塊執行。
         6: invokestatic  #3                  // Method println:(Ljava/lang/Object;)V
         9: new           #15                 // class java/lang/Thread
        12: dup
        13: new           #16                 // class com/sxzhongf/daily/question/july/ClassInitializedOrder$1
        16: dup
        17: invokespecial #17                 // Method com/sxzhongf/daily/question/july/ClassInitializedOrder$1."<init>":()V
        20: invokespecial #18                 // Method java/lang/Thread."<init>":(Ljava/lang/Runnable;)V
        23: astore_0
        24: aload_0
        25: invokevirtual #19                 // Method java/lang/Thread.start:()V
        28: aload_0
        29: invokevirtual #20                 // Method java/lang/Thread.join:()V
        32: goto          40
        35: astore_1
        36: aload_1
        37: invokevirtual #22                 // Method java/lang/InterruptedException.printStackTrace:()V
        40: return

查看#16,看到的仍是new了一個匿名類，和上一個是同樣的，爲何就能夠成功呢？這個在於當前匿名類中沒有依賴主類的代碼信息。不存在上下依賴，那麼就不會出現相互等待的狀況發生，固然也就不會出現block。ssl

那麼就有朋友會問，爲何會相互等待呢?這裏和咱們join就有關聯了，咱們來看一下它的實現代碼。

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

咱們能夠看到，首先它是synchronized關鍵詞修飾的，那就說明它同時只能被一個線程訪問，再往下看，咱們能發現，join的具體實現，其實就是wait()來實現，當子線程中的程序再等待main線程的實現類初始化完成的時候，又依賴了主線程中的某些元素對象。那麼就會開始等待主線程初始化完成，這個時候，根據classloader加載類的執行順序，在#16就會開始等待，那麼主類沒法初始化完成，形成相互等待現相。