/**
* - void set(Object value)设置当前线程的线程局部变量的值。
* - public Object get()该方法返回当前线程所对应的线程局部变量。
* - public void remove()将当前线程局部变量的值删除，目的是为了减少内存的占用，该方法是JDK 5.0新增的方法。
* 需要指出的是，当线程结束后，对应该线程的局部变量将自动被垃圾回收，所以显式调用该方法清除线程的局部变量并不是必须的操作，但它可以加快内存回收的速度。
* - protected Object initialValue()返回该线程局部变量的初始值，该方法是一个protected的方法，显然是为了让子类覆盖而设计的。
* 这个方法是一个延迟调用方法，在线程第1次调用get()或set(Object)时才执行，并且仅执行1次。ThreadLocal中的缺省实现直接返回一个null。
*/
public class ThreadLocalDemo extends Thread{
private Res res;
public ThreadLocalDemo(Res res) {
this.res = res;
}
@Override
public void run() {
for (int i = 0; i < 3; i++) {
System.out.println(Thread.currentThread().getName()+"-i:"+i+"-num:"+res.getNum());
}
}
public static void main(String[] args){
Res res = new Res();
new ThreadLocalDemo(res).start();
new ThreadLocalDemo(res).start();
new ThreadLocalDemo(res).start();
}
//Thread-0-i:0-num:1
//Thread-0-i:1-num:2
//Thread-0-i:2-num:3
//Thread-1-i:0-num:1
//Thread-1-i:1-num:2
//Thread-1-i:2-num:3
//Thread-2-i:0-num:1
//Thread-2-i:1-num:2
//Thread-2-i:2-num:3
}
class Res{
public static Integer count = 0;
public static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>(){
@Override
protected Integer initialValue() {
return 0;
}
};
public Integer getNum(){
int count = threadLocal.get() + 1;
threadLocal.set(count);
return count;
}
}
/**
* 如果你创建了 一个 ThreadLocal 变量，那么访问这个变量的每个线程都会有这个变量的一个本地副本。
* 当多个线程操作这个变量时，实际操作的是自己本地内存里面的变量，从而避免了线程安全问题
*/
public class ThreadLocalDemo {
static ThreadLocal<String> local = new ThreadLocal<>();
static void print(String s){
System.out.println(s+":"+local.get());
local.remove();
}
public static void main(String[] args){
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
local.set("threadA");
print("threadA");
System.out.println("threadA remove after:"+ local.get());
}
});
Thread threadB = new Thread(new Runnable() {
@Override
public void run() {
local.set("threadB");
print("threadB");
System.out.println("threadB remove after:"+ local.get());
}
});
threadA.start();
threadB.start();
//threadB:threadB
//threadA:threadA
//threadB remove after:threadB
//threadA remove after:threadA
//执行local.remove();
//threadA:threadA
//threadB:threadB
//threadA remove after:null
//threadB remove after:null
}
}
public class ThreadLocalDemo2 {
static ThreadLocal<String> local = new ThreadLocal<>();
public static void main(String[] args){
local.set("hello");
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("thread:"+local.get());
}
});
threadA.start();
System.out.println("main:"+local.get());
//thread:null
//main:hello
//同一个 ThreadLocal 变量在父线程中被设置值后， 在子线程中是获取不到的。
//因为在子线程 threadA 里面调用 get 方法时当前线程 为 threadA 线程，而这里调用 set 方法设置线程变量的是 main 线程，
// 两者是不同的线程，自然子线程访问时返回 null
}
}
/**
* InheritableThreadLocal
* 子线程获取父线程的threadlocal变量
*/
public class ThreadLocalDemo3 {
static ThreadLocal<String> local = new InheritableThreadLocal<>();
public static void main(String[] args){
local.set("hello");
Thread threadA = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("thread:"+local.get());
}
});
threadA.start();
System.out.println("main:"+local.get());
//main:hello
//thread:hello
//当父线程创建子线程时，构造函数会把父线程中 inheritableThreadLocals 变量里面的本地变量复制一份保存到子线程的 inheritableThreadLocals 变量里面
}
}
简单使用
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ParseDate{
private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
public static class Parse implements Runnable{
int i = 0;
public Parse(int i) {
this.i = i;
}
@Override
public void run() {
try {
Date t = sdf.parse("2019-01-10 19:12:" + i % 60);
System.out.println(i+":"+t);
}catch (ParseException e){
e.printStackTrace();
}
}
}
public static void main(String[] args){
ExecutorService executorService = Executors.newFixedThreadPool(10);
for (int i = 0; i < 1000; i++) {
executorService.execute(new Parse(i));
}
}
//Exception in thread "pool-1-thread-121" Exception in thread "pool-1-thread-123" Exception in thread "pool-1-thread-120" java.lang.NumberFormatException: multiple points
// at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1890)
// at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
// at java.lang.Double.parseDouble(Double.java:538)
//SimpleDateFormat.parse()方法并不是线程安全的，因此在线程池中共享这个对象必然导致错误
}
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* 为每一个线程分配不同的对象，需要在应用层面保证，ThreadLocal只是起到了简单的容器作用
*/
public class ParseDate2 {
static ThreadLocal<SimpleDateFormat> t1 = new ThreadLocal<>();
public static class Parse implements Runnable{
int i = 0;
public Parse(int i) {
this.i = i;
}
@Override
public void run() {
try {
if (t1.get()==null){
t1.set(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
}
Date t = t1.get().parse("2019-01-10 19:12:" + i % 60);
System.out.println(i+":"+t);
}catch (ParseException e){
e.printStackTrace();
}
}
}
public static void main(String[] args){
ExecutorService executorService = Executors.newFixedThreadPool(10);
for (int i = 0; i < 1000; i++) {
executorService.execute(new Parse(i));
}
}
}
