線程安全:
當多個線程訪問某一個類(對象或方法)時,這個對象始終都能表現出正確的行爲,那麼這個類(對象或方法)就是線程安全的
synchronized:
可以在任意對象及方法上加鎖,而加鎖的這段代碼稱爲"互斥區"或"臨界區"
例子:
public class MyThread extends Thread{
private int count = 5 ;
//synchronized加鎖
public void run(){
count--;
System.out.println(this.currentThread().getName() + " count = "+ count);
}
public static void main(String[] args) {
MyThread myThread = new MyThread();
Thread t1 = new Thread(myThread,"t1");
Thread t2 = new Thread(myThread,"t2");
Thread t3 = new Thread(myThread,"t3");
Thread t4 = new Thread(myThread,"t4");
Thread t5 = new Thread(myThread,"t5");
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
}
}
輸出結果:t2 count = 3
t1 count = 3
t4 count = 2
t3 count = 1
t5 count = 0
count值並沒有按照順序去減
下面 將run方法加上synchronized
public class MyThread extends Thread{
private int count = 5 ;
//synchronized加鎖
public synchronized void run(){
count--;
System.out.println(this.currentThread().getName() + " count = "+ count);
}
public static void main(String[] args) {
MyThread myThread = new MyThread();
Thread t1 = new Thread(myThread,"t1");
Thread t2 = new Thread(myThread,"t2");
Thread t3 = new Thread(myThread,"t3");
Thread t4 = new Thread(myThread,"t4");
Thread t5 = new Thread(myThread,"t5");
t1.start();
t2.start();
t3.start();
t4.start();
t5.start();
}
}
輸出結果:t1 count = 4
t3 count = 3
t4 count = 2
t2 count = 1
t5 count = 0
結論分析:
當多個線程訪問myThread的run方法時,以排隊的方式進行處理(這裏排對是按照CPU分配的先後順序而定的),一個線程想要執行synchronized修飾的方法裏的代碼:
1 嘗試獲得鎖
2 如果拿到鎖,執行synchronized代碼體內容;拿不到鎖,這個線程就會不斷的嘗試獲得這把鎖,直到拿到爲止,
而且是多個線程同時去競爭這把鎖。(也就是會有鎖競爭的問題)
對象鎖和類級別鎖
關鍵字synchronized取得的鎖都是對象鎖,而不是把一段代碼(方法)當做鎖,所以代碼中哪個線程先執行synchronized關鍵字的方法,哪個線程就持有該方法所屬對象的鎖(Lock)
在靜態方法上加synchronized關鍵字,表示鎖定.class類,類一級別的鎖(獨佔.class類)。
列子
public class MultiThread {
private int num = 0;
/** static */
public synchronized void printNum(String tag){
try {
if(tag.equals("a")){
num = 100;
System.out.println("tag a, set num over!");
Thread.sleep(1000);
} else {
num = 200;
System.out.println("tag b, set num over!");
}
System.out.println("tag " + tag + ", num = " + num);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
//注意觀察run方法輸出順序
public static void main(String[] args) {
//倆個不同的對象
final MultiThread m1 = new MultiThread();
final MultiThread m2 = new MultiThread();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
m1.printNum("a");
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
m2.printNum("b");
}
});
t1.start();
t2.start();
}
結果
tag a, set num over!
tag b, set num over!
tag b, num = 200
tag a, num = 100
結果分析: 線程m1 m2都執行了run方法,如果m1線程調到run方法時,獲得了對象鎖,m2等待m1執行完才執行,結果應該是 :
tag a, set num over!
tag a, num = 100
tag b, set num over!
tag b, num = 200
其實不然,m1 m2 是2個對象,有2把鎖,沒有衝突,所以都可以執行,可以使用類級別鎖,輸出結果會按照順序
public class MultiThread {
private static int num = 0;
/** static */
public static synchronized void printNum(String tag){
try {
if(tag.equals("a")){
num = 100;
System.out.println("tag a, set num over!");
Thread.sleep(1000);
} else {
num = 200;
System.out.println("tag b, set num over!");
}
System.out.println("tag " + tag + ", num = " + num);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
//注意觀察run方法輸出順序
public static void main(String[] args) {
//倆個不同的對象
final MultiThread m1 = new MultiThread();
final MultiThread m2 = new MultiThread();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
m1.printNum("a");
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
m2.printNum("b");
}
});
t1.start();
t2.start();
}
結果:
tag a, set num over!
tag a, num = 100
tag b, set num over!
tag b, num = 200
結果分析:在靜態方法上加synchronized關鍵字,表示鎖定.class類,類一級別的鎖(獨佔.class類),無論創建幾個對象,調用printNum()方法,獲取的都是類級別鎖
結論:
關鍵字synchronized取得的鎖都是對象鎖,而不是把一段代碼(方法)當做鎖,所以代碼中哪個線程先執行synchronized關鍵字的方法,哪個線程就持有該方法所屬對象的鎖(Lock),
在靜態方法上加synchronized關鍵字,表示鎖定.class類,類一級別的鎖(獨佔.class類)。
對象鎖的同步和異步
同步: synchronized
同步的概念就是共享,我們要牢牢記住共享這2個字,如果不是“共享”資源,就沒必要進行同步
異步:
異步的概念就是獨立,相互之間不受任何制約,就好像學習http的時候,在頁面發起ajax請求,還可以繼續瀏覽和操作頁面的內容,二者之間沒有任何關係
public class MyObject {
//同步方法
public synchronized void method1(){
try {
System.out.println(Thread.currentThread().getName());
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/** 異步方法,沒有任何鎖的機制就是異步方法*/
public void method2(){
System.out.println(Thread.currentThread().getName());
}
}
區別:
method1異步方法多個線程需要排隊的方法,而method2同步方法多個線程可以併發的訪問
例子
public class MyObject {
public synchronized void method1(){
try {
System.out.println(Thread.currentThread().getName());
Thread.sleep(4000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/** synchronized */
public void method2(){
System.out.println(Thread.currentThread().getName());
}
public static void main(String[] args) {
final MyObject mo = new MyObject();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
mo.method1();
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
mo.method2();
}
},"t2");
t1.start();
t2.start();
}
}
結果:
同時輸出 t1 t2,沒有等待4秒輸出t2
分析:
t1線程先持有object對象的Lock鎖,t2線程可以以異步的方式調用對象中的非synchronized修飾的方法
t1線程先持有object對象的Lock鎖,t2線程如果在這個時候調用對象中的同步(synchronized)方法則需等待,也就是同步