synchronized keyword

_posts/技术/2024-08-02-常见问题总结.md

@@ -231,6 +231,12 @@ https://tuonioooo-notebook.gitbook.io/performance-optimization/sqlyou-hua-pian/m
 
 
 
+## 多线程
+
+### 公平锁和非公平锁
+
+https://www.cnblogs.com/vipstone/p/16248006.html
+
 
 
 ## JAVA
@@ -241,3 +247,6 @@ https://tuonioooo-notebook.gitbook.io/performance-optimization/sqlyou-hua-pian/m
 https://www.cnblogs.com/lidong422339/p/17498611.html
 ```
 
+
+1. mybatis在一个对象中拼接集合
+2. thread如何给分配到另一个线程

_posts/技术/java/2024-09-23-describe-synchronized-contnet.md

@@ -0,0 +1,100 @@
+---
+title: "Describe the Synchronized Keyword in Java"
+subtitle: "Describe the Synchronized Keyword in Java"
+layout: post
+author: "bulingfeng"
+header-style: text
+tags:
+- Java-English
+---
+
+## Overview
+
+In this article, we'll learn using synchronized block in Java.
+
+**Simple put,** in a multi-thread environment, a race condition occurs when two or more threads attempt to update **mutable** share data at the same time. Jave Offers a **mechanism** to void race conditions by synchronizing thread access to shared data.
+
+A piece of logic marked with synchronized becomes a synchronized block, allowing only one thread to execute at any given time.
+
+## The Synchronized Keywork Usage
+
+We can use the synchronized keyword on differnt leveles:
+
+- Instance methods
+- Static methods
+- Code blocks
+
+When we use a synchronized bock, Java internally uses a monitor, also known as monitor lock or **intrinsic** lock, to provide synchronization. These monitors are bound to a object;
+
+Therefore, all synchronized blocks of the same object can have only one thread executing them at the same time.
+
+### Synchronized Instance Method
+
+We can add the synchronized keyword in the method declaration to make the method synchronized:
+
+```java
+public synchronized void sum(){
+        count++;
+}
+```
+
+Instance methods are synchroinzed over the instance of the class owning the method, **which means only one thread per instance of the class can execute this method.**
+
+```java
+public static void main(String[] args) throws InterruptedException {
+        SynchronizedDemo synchronizedDemo1=new SynchronizedDemo();
+        SynchronizedDemo synchronizedDemo2=new SynchronizedDemo();
+
+        Thread t1=new Thread(()->{
+            for (int i = 0; i < 100000; i++) {
+                synchronizedDemo1.sum();
+            }
+        });
+
+
+        Thread t2=new Thread(()->{
+            for (int i = 0; i < 100000; i++) {
+                synchronizedDemo2.sum();
+            }
+        });
+
+        t1.start();
+        t2.start();
+
+        t1.join();
+        t2.join();
+
+        System.out.println("count:"+count);
+    }
+```
+
+If there is two or more instances of class executing the synchronized method, synchronized keyword doesn't guarantee the method synchronizing, just like above code.
+
+### Synchronized Static Method
+
+Static methods are synchronized just like below:
+
+```java
+public static synchronized void sum(){
+        count++;
+}
+```
+
+These methods are synchronized on the Class object associated with the class. Since only one Class object exists  per JVM per class, **only on thread can execute inside a static synchronized method person class, irrespective of the number of instances it has.**
+
+## Synchronized Blocks Within Methods
+
+Sometimes we don't want to synchronize the entire method, only some instuctions within it.
+
+```java
+public  synchronized void sum(){
+        synchronized (this){
+            count++;
+        }
+}
+```
+
+## Reentrancy
+
+Thre lock behind the synchronized methods and blocks is a reentrant. This means the current thread can acquire the same synchronized lock over and over again while holding it.
+