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This article shows you how to understand locks and concurrency in java. The content is concise and easy to understand, which will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

1. The concept of object header and lock 1.1 object header

In the implementation of java virtual machine, each object has an object header, which is used to store the system information of the object. There is a section in the object header called Mark Word, which is the key to implementing the lock. In 32-bit systems, Mark Word is a 32-bit data, in 64-bit systems, it occupies 64-bit. It is a multi-functional data area, which can store the hash value of the object, the age of the object, the pointer of the lock and other information. Whether an object occupies a lock and which lock it occupies is recorded in this Mark Word.

Taking a 32-bit system as an example, the object headers of ordinary objects are as follows:

Hash: 25-> | age: 4 biased_lock: 1 lock: 2

It indicates that there are 25 bits in the Mark Word that represent the hash value of the object, 4 bits indicate the age of the object, 1 bit indicates whether it is a biased lock, and 2 bits represent the information of the lock.

1.2 the object head of the bias lock

Biased lock is a lock optimization method proposed by jdk 1.6. The core idea is that if there is no competition in the program, the thread synchronization operation that has previously acquired the lock is cancelled. In other words, after a lock is acquired by the thread, it will enter the leaning mode, and when the thread requests the lock again, there is no need for related synchronization operations, thus saving operation time. If another thread makes a lock request during this period, the lock exits biased mode. In jvm, use-XX:+UseBiasedLocking to set bias locks to be enabled.

For objects that are biased towards locks, it has the following format:

[JavaThread* | epoch | age | 1 | 01]

The first 23 bits represent the thread holding the biased lock, the next 2 bits indicate the epoch of the biased lock, 4 bits indicate the age of the object, and the age is fixed to 1, indicating the biased lock, and the last 2 bits are 01, indicating that they can be biased / unlocked.

Example of a bias lock:

Package jvm.chapter08;import java.util.List;import java.util.Vector;/** * uses a thread to write to Vector, and since access to Vector is internally controlled by synchronous locks, * each add () operation requests a lock on the numberList object. * * @ author chengyan * @ date 2019-11-17 7:41 afternoon * / public class Demo01 {public static List numberList = new Vector (); public static void main (String [] args) throws InterruptedException {long begin = System.currentTimeMillis (); int count = 0; int startnum = 0; while (count < 100000000) {numberList.add (startnum); startnum + = 2; count++ } long end = System.currentTimeMillis (); System.out.println (end-begin);}}

Run with the parameter-XX:+UseBiasedLocking-XX:BiasedLockingStartupDelay=0-client-Xmx512m-Xms512m, and the results are as follows:

two hundred and twenty seven

This shows that the program does all the work in 227 milliseconds. The parameter-XX:BiasedLockingStartupDelay indicates that the virtual machine uses bias locks immediately after startup. If this parameter is not set, the virtual machine will enable bias lock by default 4 seconds after startup. Considering the short running time of the program, this setting is made to enable bias lock as soon as possible.

If biased locks are disabled, simply start the program with the following parameters:

-XX:-UseBiasedLocking-client-Xmx512m-Xms512m

The results are as follows:

3631.3 object head of lightweight lock

When the object is in a lightweight lock, the Mark Word is as follows (00 represents the value of the last two bits):

[ptr | 00] locked

At this point, it points to the real object header of the object stored in the thread stack that acquired the lock.

1.4 object head of heavyweight lock

When the object is in a lightweight lock, the Mark Word is as follows:

[ptr | 10] monitor

At this point, the last two bits are 10, and the entire Mark Word points to the pointer to Minitor.

1.5 the object head of an ordinary object

When an object is processed in a normal unlocked state, the format is as follows:

[header | 0 | 01 |] unlocked

The first 29 bits represent the hash value, age and other information of the object. The penultimate digit is 0, and the last two digits are 01, indicating that it is not locked. It can be found that the values of the last two bits are the same as the bias state, and the virtual machine distinguishes whether it is a bias lock by the third-to-last bit.

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