一、synchronized 原理(字節碼角度)
static final Object lock = new Object();
static int counter = 0;
public static void main(String[] args) {
synchronized (lock) {
counter++;
}
}
字節碼文件:
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=3, args_size=1
0: getstatic #2 // <- lock引用 (synchronized開始),拿到lock鎖對象
3: dup //複製了一份對象
4: astore_1 // lock引用 -> slot 1 存儲到一個臨時變量,爲了解鎖用
5: monitorenter // 將 lock對象 MarkWord 置爲 Monitor 指針
6: getstatic #3 // <- i
9: iconst_1 // 準備常數 1
10: iadd // +1
11: putstatic #3 // -> i
14: aload_1 // <- lock引用 獲取之前存儲的臨時變量
15: monitorexit // 將 lock對象 MarkWord 重置, 喚醒 EntryList
16: goto 24
19: astore_2 // e -> slot 2
20: aload_1 // <- lock引用
21: monitorexit // 將 lock對象 MarkWord 重置, 喚醒 EntryList
22: aload_2 // <- slot 2 (e)
23: athrow // throw e
24: return
Exception table: //如果發生異常
from to target type
6 16 19 any //這裏是監控6-19,也就是同步代碼塊
19 22 19 any //執行19-22行,如果發生異常,也會解鎖,因爲在21行將MarkWord重置了。
LineNumberTable:
line 8: 0
line 9: 6
line 10: 14
line 11: 24
LocalVariableTable:
Start Length Slot Name Signature
0 25 0 args [Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 19
locals = [ class "[Ljava/lang/String;", class java/lang/Object ]
stack = [ class java/lang/Throwable ]
frame_type = 250 /* chop */
offset_delta = 4
所以,使用synchronized 如果發生異常了,也不會導致死鎖
二、synchronized 優化原理
2.1輕量級鎖
輕量級鎖的使用場景:如果一個對象雖然有多線程要加鎖,但加鎖的時間是錯開的(也就是沒有競爭),那麼可以使用輕量級鎖來優化。
輕量級鎖對使用者是透明的,即語法仍然是 synchronized
假設有兩個方法同步塊,利用同一個對象加鎖:
static final Object obj = new Object();
public static void method1() {
synchronized( obj ) {
// 同步塊 A
method2();
}
}
public static void method2() {
synchronized( obj ) {
// 同步塊 B
}
}
- 創建鎖記錄(Lock Record)對象,每個線程都的棧幀都會包含一個鎖記錄的結構,內部可以存儲鎖定對象的Mark Word
- 讓鎖記錄中 Object reference 指向鎖對象,並嘗試用 cas 替換 Object 的 Mark Word,將 Mark Word 的值存
入鎖記錄
- 如果 cas 替換成功,對象頭中存儲了 鎖記錄地址和狀態 00 ,表示由該線程給對象加鎖,這時圖示如下
- 如果 cas 失敗,有兩種情況
1、如果是其它線程已經持有了該 Object 的輕量級鎖,這時表明有競爭,進入鎖膨脹過程
2、如果是自己執行了 synchronized 鎖重入,那麼再添加一條 Lock Record 作爲重入的計數
- 當退出 synchronized 代碼塊(解鎖時)如果有取值爲 null 的鎖記錄,表示有重入,這時重置鎖記錄,表示重入計數減一
- 當退出 synchronized 代碼塊(解鎖時)鎖記錄的值不爲 null,這時使用 cas 將 Mark Word 的值恢復給對象頭
1、成功,則解鎖成功
2、失敗,說明輕量級鎖進行了鎖膨脹或已經升級爲重量級鎖,進入重量級鎖解鎖流程
2.2 鎖膨脹
如果在嘗試加輕量級鎖的過程中,CAS 操作無法成功,這時一種情況就是有其它線程爲此對象加上了輕量級鎖(有競爭),這時需要進行鎖膨脹,將輕量級鎖變爲重量級鎖。
- 當 Thread-1 進行輕量級加鎖時,Thread-0 已經對該對象加了輕量級鎖
- 這時 Thread-1 加輕量級鎖失敗,進入鎖膨脹流程
即爲 Object 對象申請 Monitor 鎖,讓 Object 指向重量級鎖地址
然後自己進入 Monitor 的 EntryList BLOCKED
當 Thread-0 退出同步塊解鎖時,使用 cas 將 Mark Word 的值恢復給對象頭,失敗。這時會進入重量級解鎖流程,即按照 Monitor 地址找到 Monitor 對象,設置 Owner 爲 null,喚醒 EntryList 中 BLOCKED 線程
3.3自旋優化
重量級鎖競爭的時候,還可以使用自旋來進行優化,如果當前線程自旋成功(即這時候持鎖線程已經退出了同步塊,釋放了鎖),這時當前線程就可以避免阻塞。
自旋重試成功的情況
自旋失敗的情況
- 自旋會佔用 CPU 時間,單核 CPU 自旋就是浪費,多核 CPU 自旋才能發揮優勢。
- 在 Java 6 之後自旋鎖是自適應的,比如對象剛剛的一次自旋操作成功過,那麼認爲這次自旋成功的可能性會高,就多自旋幾次;反之,就少自旋甚至不自旋,總之,比較智能。
- Java 7 之後不能控制是否開啓自旋功能
3.4偏向鎖
輕量級鎖在沒有競爭時(就自己這個線程),每次重入仍然需要執行 CAS 操作。
Java 6 中引入了偏向鎖來做進一步優化:只有第一次使用 CAS 將線程 ID 設置到對象的 Mark Word 頭,之後發現這個線程 ID 是自己的就表示沒有競爭,不用重新 CAS。以後只要不發生競爭,這個對象就歸該線程所有
例如:
static final Object obj = new Object();
public static void m1() {
synchronized( obj ) {
// 同步塊 A
m2();
}
}
public static void m2() {
synchronized( obj ) {
// 同步塊 B
m3();
}
}
public static void m3() {
synchronized( obj ) {
// 同步塊 C
}
}
3.5偏向鎖狀態
先來回憶一下對象頭
一個對象創建時:
- 如果開啓了偏向鎖(默認開啓),那麼對象創建後,markword 值爲 0x05 即最後 3 位爲 101,這時它的thread、epoch、age 都爲 0
- 偏向鎖是默認是延遲的,不會在程序啓動時立即生效,如果想避免延遲,可以加 VM 參數 -XX:BiasedLockingStartupDelay=0 來禁用延遲
- 如果沒有開啓偏向鎖,那麼對象創建後,markword 值爲 0x01 即最後 3 位爲 001,這時它的 hashcode、age 都爲 0,第一次用到 hashcode 時纔會賦值
測試偏向鎖:
利用 jol 第三方工具來查看對象頭信息(注意這裏我擴展了 jol 讓它輸出更爲簡潔)
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.10</version>
</dependency>
// 添加虛擬機參數 -XX:BiasedLockingStartupDelay=0,防止延遲
public static void main(String[] args) throws IOException {
Dog d = new Dog();
ClassLayout classLayout = ClassLayout.parseInstance(d);
new Thread(() -> {
log.debug("synchronized 前");
System.out.println(classLayout.toPrintableSimple(true));
synchronized (d) {
log.debug("synchronized 中");
System.out.println(classLayout.toPrintableSimple(true));
}
log.debug("synchronized 後");
System.out.println(classLayout.toPrintableSimple(true));
}, "t1").start();
}
輸出:
11:08:58.117 c.TestBiased [t1] - synchronized 前
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000101
11:08:58.121 c.TestBiased [t1] - synchronized 中
00000000 00000000 00000000 00000000 00011111 11101011 11010000 00000101
11:08:58.121 c.TestBiased [t1] - synchronized 後
00000000 00000000 00000000 00000000 00011111 11101011 11010000 00000101
注意
處於偏向鎖的對象解鎖後,線程 id 仍存儲於對象頭中。
測試禁用:
在上面測試代碼運行時在添加 VM 參數 -XX:-UseBiasedLocking
禁用偏向鎖
輸出
11:13:10.018 c.TestBiased [t1] - synchronized 前
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
11:13:10.021 c.TestBiased [t1] - synchronized 中
00000000 00000000 00000000 00000000 00100000 00010100 11110011 10001000
11:13:10.021 c.TestBiased [t1] - synchronized 後
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
測試 hashCode:
正常狀態對象一開始是沒有 hashCode 的,第一次調用才生成。
3.6 偏向鎖撤銷
1、當一個對象處於偏向狀態的時候,調用一下hashcode方法,就會撤銷偏向狀態
2、當有其它線程使用偏向鎖對象時,會將偏向鎖升級爲輕量級鎖
private static void test2() throws InterruptedException {
Dog d = new Dog();
Thread t1 = new Thread(() -> {
synchronized (d) {
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
}
synchronized (TestBiased.class) {
TestBiased.class.notify();
}
// 如果不用 wait/notify 使用 join 必須打開下面的註釋
// 因爲:t1 線程不能結束,否則底層線程可能被 jvm 重用作爲 t2 線程,底層線程 id 是一樣的
/*try {
System.in.read();
} catch (IOException e) {
e.printStackTrace();
}*/
}, "t1");
t1.start();
Thread t2 = new Thread(() -> {
synchronized (TestBiased.class) {
try {
TestBiased.class.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
synchronized (d) {
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
}
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
}, "t2");
t2.start();
}
上述代碼將兩個線程的執行錯開了。
輸出:
[t1] - 00000000 00000000 00000000 00000000 00011111 01000001 00010000 00000101
[t2] - 00000000 00000000 00000000 00000000 00011111 01000001 00010000 00000101
[t2] - 00000000 00000000 00000000 00000000 00011111 10110101 11110000 01000000
[t2] - 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
由上面的第三行可以看出,再次加鎖後變成了輕量級鎖,最後釋放鎖之後,撤銷了偏向鎖。
撤銷 - 調用 wait/notify
public static void main(String[] args) throws InterruptedException {
Dog d = new Dog();
Thread t1 = new Thread(() -> {
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
synchronized (d) {
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
try {
d.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
log.debug(ClassLayout.parseInstance(d).toPrintableSimple(true));
}
}, "t1");
t1.start();
new Thread(() -> {
try {
Thread.sleep(6000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (d) {
log.debug("notify");
d.notify();
}
}, "t2").start();
}
因爲wait/notify只有重量級鎖有,所以撤銷偏向鎖其實不難理解
輸出
[t1] - 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000101
[t1] - 00000000 00000000 00000000 00000000 00011111 10110011 11111000 00000101
[t2] - notify
[t1] - 00000000 00000000 00000000 00000000 00011100 11010100 00001101 11001010
3.7批量重偏向
- 如果對象雖然被多個線程訪問,但沒有競爭,這時偏向了線程 T1 的對象仍有機會重新偏向 T2,重偏向會重置對象的 Thread ID
- 當撤銷偏向鎖閾值超過 20 次後,jvm 會這樣覺得,我是不是偏向錯了呢,於是會在給這些對象加鎖時重新偏向至加鎖線程
private static void test3() throws InterruptedException {
Vector<Dog> list = new Vector<>();
Thread t1 = new Thread(() -> {
for (int i = 0; i < 30; i++) {
Dog d = new Dog();
list.add(d);
synchronized (d) {
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
}
synchronized (list) {
list.notify();
}
}, "t1");
t1.start();
Thread t2 = new Thread(() -> {
synchronized (list) {
try {
list.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
log.debug("===============> ");
for (int i = 0; i < 30; i++) {
Dog d = list.get(i);
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
synchronized (d) {
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
}, "t2");
t2.start();
}
輸出:
[t1] - 0 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 1 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 2 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 3 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 4 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 5 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 6 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 7 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 8 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 9 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 10 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 11 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 12 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 13 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 14 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 15 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 16 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 17 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 18 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 19 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 20 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 21 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 22 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 23 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 24 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 25 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 26 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 27 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 28 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t1] - 29 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - ===============>
[t2] - 0 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 0 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 0 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 1 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 1 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 1 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 2 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 2 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 2 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 3 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 3 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 3 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 4 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 4 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 4 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 5 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 5 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 5 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 6 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 6 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 6 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 7 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 7 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 7 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 8 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 8 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 8 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 9 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 9 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 9 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 10 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 10 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 10 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 11 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 11 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 11 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 12 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 12 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 12 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 13 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 13 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 13 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 14 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 14 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 14 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 15 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 15 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 15 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 16 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 16 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 16 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 17 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 17 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 17 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 18 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 18 00000000 00000000 00000000 00000000 00100000 01011000 11110111 00000000
[t2] - 18 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000001
[t2] - 19 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 19 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 19 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 20 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 20 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 20 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 21 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 21 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 21 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 22 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 22 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 22 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 23 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 23 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 23 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 24 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 24 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 24 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 25 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 25 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 25 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 26 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 26 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 26 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 27 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 27 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 27 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 28 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 28 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 28 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 29 00000000 00000000 00000000 00000000 00011111 11110011 11100000 00000101
[t2] - 29 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
[t2] - 29 00000000 00000000 00000000 00000000 00011111 11110011 11110001 00000101
當執行20次時,偏向的線程變了。偏向了t2線程。
2.8批量撤銷
- 當撤銷偏向鎖閾值超過 40 次後,jvm 會這樣覺得,自己確實偏向錯了,根本就不該偏向。於是整個類的所有對象都會變爲不可偏向的,新建的對象也是不可偏向的。
static Thread t1,t2,t3;
private static void test4() throws InterruptedException {
Vector<Dog> list = new Vector<>();
int loopNumber = 39;
t1 = new Thread(() -> {
for (int i = 0; i < loopNumber; i++) {
Dog d = new Dog();
list.add(d);
synchronized (d) {
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
}
LockSupport.unpark(t2);
}, "t1");
t1.start();
t2 = new Thread(() -> {
LockSupport.park();
log.debug("===============> ");
for (int i = 0; i < loopNumber; i++) {
Dog d = list.get(i);
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
synchronized (d) {
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
LockSupport.unpark(t3);
}, "t2");
t2.start();
t3 = new Thread(() -> {
LockSupport.park();
log.debug("===============> ");
for (int i = 0; i < loopNumber; i++) {
Dog d = list.get(i);
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
synchronized (d) {
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
log.debug(i + "\t" + ClassLayout.parseInstance(d).toPrintableSimple(true));
}
}, "t3");
t3.start();
t3.join();
log.debug(ClassLayout.parseInstance(new Dog()).toPrintableSimple(true));
}
2.9鎖消除
@Fork(1)
@BenchmarkMode(Mode.AverageTime)
@Warmup(iterations=3)
@Measurement(iterations=5)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class MyBenchmark {
static int x = 0;
@Benchmark
public void a() throws Exception {
x++;
}
@Benchmark
public void b() throws Exception {
Object o = new Object();
synchronized (o) {
x++;
}
}
}
java -jar benchmarks.jar
Benchmark Mode Samples Score Score error Units
c.i.MyBenchmark.a avgt 5 1.542 0.056 ns/op
c.i.MyBenchmark.b avgt 5 1.518 0.091 ns/op
java -XX:-EliminateLocks -jar benchmarks.jar //去掉鎖消除優化
Benchmark Mode Samples Score Score error Units
c.i.MyBenchmark.a avgt 5 1.507 0.108 ns/op
c.i.MyBenchmark.b avgt 5 16.976 1.572 ns/op
JIT發現這個鎖對象是局部變量,不會存在共享,乾脆就就將鎖消除了。