聊聊 Java 中绕不开的 Synchronized 关键字-二
Java并发
synchronized 的一些学习记录
jdk1.6 以后对 synchronized 进行了一些优化，包括偏向锁，轻量级锁，重量级锁等
这些锁的加锁方式大多跟对象头有关，我们可以查看 jdk 代码
首先对象头的位置注释1
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20// Bit-format of an object header (most significant first, big endian layout below):
//
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)1
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7enum { locked_value = 0,
unlocked_value = 1,
monitor_value = 2,
marked_value = 3,
biased_lock_pattern = 5
};
我们可以用 java jol库来查看对象头，通过一段简单的代码来看下1
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6public class ObjectHeaderDemo {
public static void main(String[] args) throws InterruptedException {
L l = new L();
System.out.println(ClassLayout.parseInstance(l).toPrintable());
}
}
然后可以看到打印输出，当然这里因为对齐方式，我们看到的其实顺序是反过来的，按最后三位去看，我们这是 001，好像偏向锁都没开，这里使用的是 jdk1.8，默认开始偏向锁的，其实这里有涉及到了一个配置，jdk1.8 中偏向锁会延迟 4 秒开启，可以通过添加启动参数 -XX:+PrintFlagsFinal，看到
因为在初始化的时候防止线程竞争有大量的偏向锁撤销升级，所以会延迟 4s 开启
我们再来延迟 5s 看看1
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7public class ObjectHeaderDemo {
public static void main(String[] args) throws InterruptedException {
TimeUnit.SECONDS.sleep(5);
L l = new L();
System.out.println(ClassLayout.parseInstance(l).toPrintable());
}
}
可以看到偏向锁设置已经开启了，我们来是一下加个偏向锁1
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13public class ObjectHeaderDemo {
public static void main(String[] args) throws InterruptedException {
TimeUnit.SECONDS.sleep(5);
L l = new L();
System.out.println(ClassLayout.parseInstance(l).toPrintable());
synchronized (l) {
System.out.println("1\n" + ClassLayout.parseInstance(l).toPrintable());
}
synchronized (l) {
System.out.println("2\n" + ClassLayout.parseInstance(l).toPrintable());
}
}
}
看下运行结果
可以看到是加上了 101 = 5 也就是偏向锁，后面是线程 id
当我再使用一个线程来竞争这个锁的时候1
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30public class ObjectHeaderDemo {
public static void main(String[] args) throws InterruptedException {
TimeUnit.SECONDS.sleep(5);
L l = new L();
System.out.println(ClassLayout.parseInstance(l).toPrintable());
synchronized (l) {
System.out.println("1\n" + ClassLayout.parseInstance(l).toPrintable());
}
Thread thread1 = new Thread() {
public void run() {
try {
TimeUnit.SECONDS.sleep(5L);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (l) {
System.out.println("thread1 获取锁成功");
System.out.println(ClassLayout.parseInstance(l).toPrintable());
try {
TimeUnit.SECONDS.sleep(5L);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
thread1.start();
}
}
可以看到变成了轻量级锁，在线程没有争抢，只是进行了切换，就会使用轻量级锁，当两个线程在竞争了，就又会升级成重量级锁1
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49public class ObjectHeaderDemo {
public static void main(String[] args) throws InterruptedException {
TimeUnit.SECONDS.sleep(5);
L l = new L();
System.out.println(ClassLayout.parseInstance(l).toPrintable());
synchronized (l) {
System.out.println("1\n" + ClassLayout.parseInstance(l).toPrintable());
}
Thread thread1 = new Thread() {
public void run() {
try {
TimeUnit.SECONDS.sleep(5L);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (l) {
System.out.println("thread1 获取锁成功");
System.out.println(ClassLayout.parseInstance(l).toPrintable());
try {
TimeUnit.SECONDS.sleep(5L);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
Thread thread2 = new Thread() {
public void run() {
try {
TimeUnit.SECONDS.sleep(5L);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (l) {
System.out.println("thread2 获取锁成功");
System.out.println(ClassLayout.parseInstance(l).toPrintable());
}
}
};
thread1.start();
thread2.start();
}
}
class L {
private boolean myboolean = true;
}
可以看到变成了重量级锁。