1、方法内的变量是安全的
方法内定义的变量,每个变量对应单独的内存变量地址,多个线程之间相互不影响。多个线程之间的变量根本没有一毛钱关系
public class ThreadFuncVarSafe {
public static void main(String[] args) {
Runnable r = () -> {
String tmp ;
String currentThreadName = Thread.currentThread().getName();
if("A".equals(currentThreadName)){
tmp = "I am A";
}else {
tmp = "I am B";
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread Name:" + currentThreadName + ", func_var is:" + tmp);
};
Thread threadA = new Thread(r,"A");
Thread threadB = new Thread(r,"B");
threadA.start();
threadB.start();
}
}
2、实体变量线程不安全
/**
* @ClassName ThreadsClassVarNotSafe
* @projectName: object1
* @author: Zhangmingda
* @description: XXX
* date: 2021/4/22.
*/
public class ThreadsClassVarNotSafe {
private static class MyRunnable implements Runnable{
private String tmp;
@Override
public void run() {
String currentThreadName = Thread.currentThread().getName();
if("A".equals(currentThreadName)){
tmp = "I am A";
}else {
tmp = "I am B";
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread Name:" + currentThreadName + ", func_var is:" + tmp);
}
} public static void main(String[] args) {
Runnable r = new MyRunnable();
Thread threadA = new Thread(r,"A"); //同一个实体对象,最后输出被后执行的覆盖两个线程最后输出的结果一样都为A,或者都为B
Thread threadB = new Thread(r,"B"); //同一个实体对象,最后输出被后执行的覆盖两个线程最后输出的结果一样都为A,或者都为B
// Thread threadA = new Thread(new MyRunnable(),"A");//这种是不同的实体对象,相互无影响
// Thread threadB = new Thread(new MyRunnable(),"B");//这种是不同的实体对象,相互无影响
threadA.start();
threadB.start();
}
}
…方法安全…
3、方法用synchronized关键字修饰方法(方法安全),多线程遇到同一个对象的该方法变串行
同一个实体对象,对实体的方法加锁,多个线程调用同一个实体对象的 安全方法
/**
* @ClassName ThreadsClassVarNotSafe
* @projectName: object1
* @author: Zhangmingda
* @description: XXX
* date: 2021/4/22.
*/
public class ThreadsFuncSafe {
private static class MyRunnable implements Runnable{
private String tmp; @Override
public synchronized void run() {
String currentThreadName = Thread.currentThread().getName();
if("A".equals(currentThreadName)){
tmp = "I am A";
}else {
tmp = "I am B";
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread Name:" + currentThreadName + ", func_var is:" + tmp);
}
} public static void main(String[] args) {
Runnable r = new MyRunnable();
Thread threadA = new Thread(r,"A"); //同一个实体对象,MyRunnable类run方法被synchronized修饰,只能一个线程持有run()方法执行权限,两个线程运行到run()变串行
Thread threadB = new Thread(r,"B"); //同一个实体对象,MyRunnable类run方法被synchronized修饰,只能一个线程持有run()方法执行权限,两个线程运行到run()变串行
threadA.start();
threadB.start();
}
}
4、多个实例多个锁,每个实例独立变量对象互不影响;实例外变量则有影响
示例:1、每个实例独立变量对象互不影响
/**
* @ClassName ThreadsClassVarNotSafe
* @projectName: object1
* @author: Zhangmingda
* @description: XXX
* date: 2021/4/22.
*/
public class ThreadsMultiObjectVarSafe {
private static class MyRunnable implements Runnable{
private String tmp; @Override
public void run() {
String currentThreadName = Thread.currentThread().getName();
if("A".equals(currentThreadName)){
tmp = "I am A";
}else {
tmp = "I am B";
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Thread Name:" + currentThreadName + ", func_var is:" + tmp);
}
} public static void main(String[] args) {
Thread threadA = new Thread(new MyRunnable(),"A"); //不同实体对象,方法无synchronized,变量独立互不影响
Thread threadB = new Thread(new MyRunnable(),"B"); //不同实体对象,方法无synchronized,变量独立互不影响
threadA.start();
threadB.start();
}
}
示例:2、多个线程实例修改实例外变量,不安全
import java.util.HashSet;
import java.util.Set;/**
* @ClassName ThreadsMultiObjectOutofVarNotSafe
* @projectName: object1
* @author: Zhangmingda
* @description: XXX
* date: 2021/4/22.
*/
public class ThreadsMultiObjectOutofVarNotSafe {
private static int num = 0;
private static class MyThread extends Thread{
@Override
public synchronized void run() {
for (int i=0; i<100000; i++){
num ++;
}
System.out.println(getName() + "num:" + num);
}
} public static void main(String[] args) {
Set<MyThread> threads = new HashSet<>();
for (int i=0; i<5; i++){
threads.add(new MyThread());
}
threads.forEach(thread ->thread.start());
/**
* 等待子线程结束再从main中获取num看值多少
*/
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("main 线程中 sum最终为:" + num); //不足50万
}
}
运行结果,我们就算加上了synchronized关键字之后,结果依然可能会出现不正确,那是因为我们创建了5个实例,我们的synchronized会锁住当前的对象实例,而我们现在有5个实例,所以,我们多个线程的run方法不会被synchronized相互锁上,所以结果不正确。
5、同步synchronized静态static方法多实例之间安全
示例:多个线程对象调用同一个类实现的对象 的 synchronized修饰的静态static方法。被锁住的对象变为当前的类,而非对象,所以多个线程遇到此类方法顺序进行。
import java.util.HashSet;
import java.util.Set;/**
* @ClassName ThreadsMultiObjectOutofVarNotSafe
* @projectName: object1
* @author: Zhangmingda
* @description: XXX
* date: 2021/4/22.
*/
public class ThreadsMultiObjectStaticFuncSyncSafe {
private static int num = 0;
private static class MyThread extends Thread{
private static synchronized void safeRun(){
for (int i=0; i<100000; i++){
num ++;
}
System.out.println(Thread.currentThread().getName() + " num:" + num);
}
@Override
public void run() {
safeRun();
}
} public static void main(String[] args) {
Set<MyThread> threads = new HashSet<>();
for (int i=0; i<5; i++){
threads.add(new MyThread());
}
threads.forEach(thread ->thread.start());
/**
* 等待子线程结束再从main中获取num看值多少
*/
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("main 线程中 sum最终为:" + num); //不足50万
}
}
每次的结果都是正确的,是500000,这个是为什么?
因为,我们在static的方法上面加上synchronized之后,锁的钥匙就变成了当前对象的Class属性(MimlThread.class)。class属性在java里面是一个元数据,元数据在JVM中只保存了一份,所以我们对这个class上锁之后,不同的线程调用,会被相互排斥。
6,异常发生的时候,锁会自动释放
如上代码safeRun()方法中num++上抛出异常,不影响其他线程运行。
if(i >9){throw new RuntimeException();} //测试抛出异常后,是否影响其他线程,结果为 main 线程中 sum最终为:50
num ++;