Java 并发 - ThreadLocal详解

class ConnectionManager {
    private static Connection connect = null;

    public static Connection openConnection() {
        if (connect == null) {
            connect = DriverManager.getConnection();
        }
        return connect;
    }

    public static void closeConnection() {
        if (connect != null)
            connect.close();
    }
}

class ConnectionManager {
    private Connection connect = null;

    public Connection openConnection() {
        if (connect == null) {
            connect = DriverManager.getConnection();
        }
        return connect;
    }

    public void closeConnection() {
        if (connect != null)
            connect.close();
    }
}

class Dao {
    public void insert() {
        ConnectionManager connectionManager = new ConnectionManager();
        Connection connection = connectionManager.openConnection();

        // 使用connection进行操作

        connectionManager.closeConnection();
    }
}

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class ConnectionManager {

    private static final ThreadLocal<Connection> dbConnectionLocal = new ThreadLocal<Connection>() {
        @Override
        protected Connection initialValue() {
            try {
                return DriverManager.getConnection("", "", "");
            } catch (SQLException e) {
                e.printStackTrace();
            }
            return null;
        }
    };

    public Connection getConnection() {
        return dbConnectionLocal.get();
    }
}

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap threadLocals = getMap(t);
    if (threadLocals != null) {
        ThreadLocalMap.Entry e = threadLocals.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}

public Connection getConnection() {
    Connection connection = dbConnectionLocal.get();
    if (connection == null) {
        try {
            connection = DriverManager.getConnection("", "", "");
            dbConnectionLocal.set(connection);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
    return connection;
}

private void set(ThreadLocal<?> key, Object value) {

    // We don't use a fast path as with get() because it is at
    // least as common to use set() to create new entries as
    // it is to replace existing ones, in which case, a fast
    // path would fail more often than not.

    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);

    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        ThreadLocal<?> k = e.get();

        if (k == key) {
            e.value = value;
            return;
        }

        if (k == null) {
            replaceStaleEntry(key, value, i);
            return;
        }
    }

    tab[i] = new Entry(key, value);
    int sz = ++size;
    if (!cleanSomeSlots(i, sz) && sz >= threshold)
        rehash();
}

private Entry getEntry(ThreadLocal<?> key) {
    int i = key.threadLocalHashCode & (table.length - 1);
    Entry e = table[i];
    if (e != null && e.get() == key)
        return e;
    else
        return getEntryAfterMiss(key, i, e);
}

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class ThreadLocalDemo {
    static class LocalVariable {
        private Long[] a = new Long[1024 * 1024];
    }

    // (1)
    final static ThreadPoolExecutor poolExecutor = new ThreadPoolExecutor(5, 5, 1, TimeUnit.MINUTES,
            new LinkedBlockingQueue<>());
    // (2)
    final static ThreadLocal<LocalVariable> localVariable = new ThreadLocal<LocalVariable>();

    public static void main(String[] args) throws InterruptedException {
        // (3)
        Thread.sleep(5000 * 4);
        for (int i = 0; i < 50; ++i) {
            poolExecutor.execute(new Runnable() {
                public void run() {
                    // (4)
                    localVariable.set(new LocalVariable());
                    // (5)
                    System.out.println("use local varaible" + localVariable.get());
                    localVariable.remove();
                }
            });
        }
        // (6)
        System.out.println("pool execute over");
    }
}

private void remove(ThreadLocal<?> key) {
    Entry[] tab = table;
    int len = tab.length;
    int i = key.threadLocalHashCode & (len-1);
    for (Entry e = tab[i];
         e != null;
         e = tab[i = nextIndex(i, len)]) {
        if (e.get() == key) {
            e.clear();
            expungeStaleEntry(i);
            return;
        }
    }
}

public class SerialNum {
    // The next serial number to be assigned
    private static int nextSerialNum = 0;

    private static ThreadLocal serialNum = new ThreadLocal() {
        protected synchronized Object initialValue() {
            return new Integer(nextSerialNum++);
        }
    };

    public static int get() {
        return ((Integer) (serialNum.get())).intValue();
    }
}

private static final ThreadLocal threadSession = new ThreadLocal();  
  
public static Session getSession() throws InfrastructureException {  
    Session s = (Session) threadSession.get();  
    try {  
        if (s == null) {  
            s = getSessionFactory().openSession();  
            threadSession.set(s);  
        }  
    } catch (HibernateException ex) {  
        throw new InfrastructureException(ex);  
    }  
    return s;  
}  

public class ThreadLocalTest implements Runnable{
    
    ThreadLocal<Student> StudentThreadLocal = new ThreadLocal<Student>();

    @Override
    public void run() {
        String currentThreadName = Thread.currentThread().getName();
        System.out.println(currentThreadName + " is running...");
        Random random = new Random();
        int age = random.nextInt(100);
        System.out.println(currentThreadName + " is set age: "  + age);
        Student Student = getStudentt(); //通过这个方法，为每个线程都独立的new一个Studentt对象，每个线程的的Studentt对象都可以设置不同的值
        Student.setAge(age);
        System.out.println(currentThreadName + " is first get age: " + Student.getAge());
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println( currentThreadName + " is second get age: " + Student.getAge());
        
    }
    
    private Student getStudentt() {
        Student Student = StudentThreadLocal.get();
        if (null == Student) {
            Student = new Student();
            StudentThreadLocal.set(Student);
        }
        return Student;
    }

    public static void main(String[] args) {
        ThreadLocalTest t = new ThreadLocalTest();
        Thread t1 = new Thread(t,"Thread A");
        Thread t2 = new Thread(t,"Thread B");
        t1.start();
        t2.start();
    }
    
}

class Student{
    int age;
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    
}

import java.text.DateFormat;
import java.text.SimpleDateFormat;
 
public class DateUtils {
    public static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>(){
        @Override
        protected DateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd");
        }
    };
}

DateUtils.df.get().format(new Date());