查看transaction事务代码的时候 发现TransactionSynchronizationManager类中存在一些这样的属性，于是看看ThreadLocal这个类
public abstract class TransactionSynchronizationManager {
private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal("Transactional resources");
private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = new NamedThreadLocal("Transaction synchronizations");
private static final ThreadLocal<String> currentTransactionName = new NamedThreadLocal("Current transaction name");
private static final ThreadLocal<Boolean> currentTransactionReadOnly = new NamedThreadLocal("Current transaction read-only status");
private static final ThreadLocal<Integer> currentTransactionIsolationLevel = new NamedThreadLocal("Current transaction isolation level");
private static final ThreadLocal<Boolean> actualTransactionActive = new NamedThreadLocal("Actual transaction active");
}
1.ThreadLocal是什么
顾名思义：thread是线程，local是本地的意思。也就是线程本地的变量，ThreadLocal是属于当前线程的变量，线程之间是隔离的。因为不是线程之间共享的变量，所以是线程安全的。
Thread中有这样一个属性threadLocals，用来存放ThreadLocal和相应的value
public class Thread implements Runnable {
ThreadLocal.ThreadLocalMap threadLocals = null;
......
}
//ThreadLocalMap为ThreadLocal的静态内部类
public class ThreadLocal<T> {
static class ThreadLocalMap {
//Entry为ThreadLocalMap的静态内部类，真正存放ThreadLocal和value的地方
static class Entry extends WeakReference<ThreadLocal<?>> {
/** The value associated with this ThreadLocal. */
Object value;
Entry(ThreadLocal<?> k, Object v) {
super(k);
value = v;
}
}
}
private static final int INITIAL_CAPACITY = 16;
//ThreadLocalMap存储Entry数组
private Entry[] table;
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
//数组初始大小为16
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
//之后用来扩容
setThreshold(INITIAL_CAPACITY);
}
}
2.ThreadLocal中的方法
1.ThreadLocal中的set()方法
-
1.取当前线程中threadLocals属性，即存放值的ThreadLocalMap
-
2.1 线程中的map非null，存储当前ThreadLocal的引用和对应的值
-
2.2 线程中的map为null，创建map，再存值
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
//ThreadLocalMap中的set方法
private void set(ThreadLocal<?> key, Object value) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
//用来找存放位置
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
//如果当前位置的key和要插入的相等，则覆盖原值
if (k == key) {
e.value = value;
return;
}
if (k == null) {
//1.查找是否存在当前key
//2.1存在则覆盖该值，并且和在他前边key为null的entry交换位置
//2.2不存在，则将key为null的entry进行覆盖
//3.清除key为null的entry
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
//ThreadLocal.set()->createMap() set()方法中的createMap()方法
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
2.ThreadLocal中的get()方法
-
1.取当前线程中threadLocals属性，即存放值的ThreadLocalMap
-
2.1 线程中的map非null，获取map中的entry
entry非null，取值返回
-
2.2 否则，返回null
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
//存在key为null的entry置为null
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
//get()->getEntry() get()方法中引用的getEntry()方法
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);
}
//getEntry()->getEntryAfterMiss() get()方法中引用的getEntryAfterMiss()方法
private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal<?> k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
//get()->setInitialValue() get()方法中引用的setInitialValue()方法
//在map中存放(this,null)的entry
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;
}
3.ThreadLocal中的remove()方法
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}
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) {
//将entry中的key置为null
e.clear();
expungeStaleEntry(i);
return;
}
}
}
4.上面几个方法中都引用的方法expungeStaleEntry()
- 1 将staleSlot位置的entry和其value置为null
- 2.1 从下一个节点开始遍历，如果key为null，则将其entry和其value置为null
- 2.2 如果key不为null，则对当前的key取hash值，判断当前的位置和hash出来的值是否一样
- 2.2.1 如果值一样，则继续遍历下一个entry不为null的节点
- 2.2.2 如果值不一样，从hash出来的位置开始遍历，放到第一个为null的entry中
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// 将key为null的entry置为null
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
Entry e;
int i;
//从下一个结点开始找key为null的entry
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
//key为null，将entry置为null
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
//如果key不为null，将当前位置的key进行hash查找位置。如果得出来得位置和当前位置不同，则代表
//是从最开始得节点进行hash碰撞(向下一节点延申)得到得位置，则将当前entry的值放到从h开始第一个为null的entry位置
int h = k.threadLocalHashCode & (len - 1);
if (h != i) {
tab[i] = null;
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
3.ThreadLocal中的set()方法中存在key为null的情况
ThreadLocalMap中entry中的key为ThreadLocal的弱引用，将ThreadLocal设置为null时，只存在key对ThreadLocal对象存在弱引用，会在gc进行回收，使得key为null。这些key为null的值不会被使用，造成内存泄漏，因此在set()，get()，remove()方法中会将部分key为null的entry释放掉。
4.子线程会继承父线程的变量：InheritableThreadLocal
在Thread中存在inheritableThreadLocals属性
public class Thread implements Runnable {
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
private void init(ThreadGroup g, Runnable target, String name, long stackSize) {
init(g, target, name, stackSize, null, true);
}
private void init(ThreadGroup g, Runnable target, String name, long stackSize, AccessControlContext acc, boolean inheritThreadLocals) {
......省略部分代码
Thread parent = currentThread();
//当前线程的inheritableThreadLocals不为null，当前线程中的值拷贝一份给子线程
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
}
}
子线程可以获得和父线程中的值
public class TestInheritableThreadLocal {
private ThreadLocal<String> threadLocal = new ThreadLocal<>();
private InheritableThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<>();
@Test
void test(){
threadLocal.set("hello threadLocal");
inheritableThreadLocal.set("hello inheritableThreadLocal");
new Thread(()->{
System.out.println(threadLocal.get());
System.out.println(inheritableThreadLocal.get());
}).start();
}
}
