ThreadLocal内存泄漏问题实践（三）非静态threadlocal

结论：

1 借助非静态threadlocal对象，实现多数据源的事务控制，threadlocal<Connection>与session对象绑定，每个数据源一个session对象注入ioc

2 threadlocal对象，没有equals方法，直接用==判断是否map同一个key

 

在mybatis Guice 事务源码解析中，我们观察到SqlSessionManager有个threadlocalsession，设置为非静态threadlocal对象

引起注意，做个实验：

/**
 * https://www.cnblogs.com/silyvin/articles/13455224.html
 * Created by joyce on 2020/8/7.
 */
public class TestThreadLocalStatic {

    private static ThreadLocal<Integer> staticThreadLocal = new ThreadLocal<>();
    private ThreadLocal<Integer> nonStaticThreadLocal = new ThreadLocal<>();

    private Integer integer;

    public TestThreadLocalStatic(Integer integer) {
        this.integer = integer;
    }

    public static void main(String f[]) {
        new TestThreadLocalStatic(2).setThreadLocal();
        new TestThreadLocalStatic(3).getThreadLocal();
    }

    public void setThreadLocal() {
        staticThreadLocal.set(this.integer);
        System.out.println(staticThreadLocal.hashCode());
        nonStaticThreadLocal.set(this.integer);
        System.out.println(nonStaticThreadLocal.hashCode());
    }

    public void getThreadLocal() {
        System.out.println(staticThreadLocal.get());
        System.out.println(staticThreadLocal.hashCode());
        System.out.println(nonStaticThreadLocal.get());
        System.out.println(nonStaticThreadLocal.hashCode());
    }

    /**
     * 第1个对象的nonStaticThreadLocal hash为在线程threadlocalmap上x位
     * 第2个对象的nonStaticThreadLocal hash为在线程threadlocalmap上y位
     * 适用于希望一个类的不同对象有不同的threadlocal可以用，有不同的线程内值，比如多数据源
     */

}

输出：

1023892928
558638686
2
1023892928
null
1149319664

threadlocal作为key，同一个类的非静态threadlocal为不同对象，自然在map上不同槽位

静态threadlocal所有对象都共享一个，自然在map上相同位置

 

同时注意到，ThreadLocal类没有equels方法，那么怎么判断同一对象？

ThreadLocal.ThreadlocalMap:

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); ThreadLocal内存泄漏问题实践（二）中清理key为null（因为弱引用）的value
            return;
        }
    }

 

首先取hash值，static threadlocal 是同一个对象，hash一样，non static threadlocal不是同一个对象，按Threadlocal hash规则肯定也不相同

可以看到直接用==判断，key引用对象相同直接覆盖