src/share/classes/java/lang/ThreadLocal.java
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rev 6191 : [mq]: threadlocal
*** 23,45 ****
* questions.
*/
package java.lang;
import java.lang.ref.*;
import java.util.concurrent.atomic.AtomicInteger;
/**
* This class provides thread-local variables. These variables differ from
* their normal counterparts in that each thread that accesses one (via its
! * <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
! * copy of the variable. <tt>ThreadLocal</tt> instances are typically private
* static fields in classes that wish to associate state with a thread (e.g.,
* a user ID or Transaction ID).
*
* <p>For example, the class below generates unique identifiers local to each
* thread.
! * A thread's id is assigned the first time it invokes <tt>ThreadId.get()</tt>
* and remains unchanged on subsequent calls.
* <pre>
* import java.util.concurrent.atomic.AtomicInteger;
*
* public class ThreadId {
--- 23,47 ----
* questions.
*/
package java.lang;
import java.lang.ref.*;
+ import java.util.Objects;
import java.util.concurrent.atomic.AtomicInteger;
+ import java.util.function.Supplier;
/**
* This class provides thread-local variables. These variables differ from
* their normal counterparts in that each thread that accesses one (via its
! * {@code get} or {@code set} method) has its own, independently initialized
! * copy of the variable. {@code ThreadLocal} instances are typically private
* static fields in classes that wish to associate state with a thread (e.g.,
* a user ID or Transaction ID).
*
* <p>For example, the class below generates unique identifiers local to each
* thread.
! * A thread's id is assigned the first time it invokes {@code ThreadId.get()}
* and remains unchanged on subsequent calls.
* <pre>
* import java.util.concurrent.atomic.AtomicInteger;
*
* public class ThreadId {
*** 59,69 ****
* return threadId.get();
* }
* }
* </pre>
* <p>Each thread holds an implicit reference to its copy of a thread-local
! * variable as long as the thread is alive and the <tt>ThreadLocal</tt>
* instance is accessible; after a thread goes away, all of its copies of
* thread-local instances are subject to garbage collection (unless other
* references to these copies exist).
*
* @author Josh Bloch and Doug Lea
--- 61,71 ----
* return threadId.get();
* }
* }
* </pre>
* <p>Each thread holds an implicit reference to its copy of a thread-local
! * variable as long as the thread is alive and the {@code ThreadLocal}
* instance is accessible; after a thread goes away, all of its copies of
* thread-local instances are subject to garbage collection (unless other
* references to these copies exist).
*
* @author Josh Bloch and Doug Lea
*** 106,134 ****
/**
* Returns the current thread's "initial value" for this
* thread-local variable. This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
! * method, in which case the <tt>initialValue</tt> method will not
* be invoked for the thread. Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
! * <p>This implementation simply returns <tt>null</tt>; if the
* programmer desires thread-local variables to have an initial
! * value other than <tt>null</tt>, <tt>ThreadLocal</tt> must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
return null;
}
/**
* Creates a thread local variable.
*/
public ThreadLocal() {
}
/**
--- 108,149 ----
/**
* Returns the current thread's "initial value" for this
* thread-local variable. This method will be invoked the first
* time a thread accesses the variable with the {@link #get}
* method, unless the thread previously invoked the {@link #set}
! * method, in which case the {@code initialValue} method will not
* be invoked for the thread. Normally, this method is invoked at
* most once per thread, but it may be invoked again in case of
* subsequent invocations of {@link #remove} followed by {@link #get}.
*
! * <p>This implementation simply returns {@code null}; if the
* programmer desires thread-local variables to have an initial
! * value other than {@code null}, {@code ThreadLocal} must be
* subclassed, and this method overridden. Typically, an
* anonymous inner class will be used.
*
* @return the initial value for this thread-local
*/
protected T initialValue() {
return null;
}
/**
+ * Creates a thread local variable. The initial value of the variable is
+ * determined by invoking the {@code get} method on the {@code Supplier}.
+ *
+ * @param supplier the supplier to be used to determine the initial value
+ * @return a new thread local variable
+ * @since 1.8
+ */
+ public static <T> ThreadLocal<T> withInitial(Supplier<? extends T> supplier) {
+ return new SuppliedThreadLocal<>(supplier);
+ }
+
+ /**
* Creates a thread local variable.
+ * @see #withInitial(java.util.function.Supplier)
*/
public ThreadLocal() {
}
/**
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* variable. If this thread-local variable is subsequently
* {@linkplain #get read} by the current thread, its value will be
* reinitialized by invoking its {@link #initialValue} method,
* unless its value is {@linkplain #set set} by the current thread
* in the interim. This may result in multiple invocations of the
! * <tt>initialValue</tt> method in the current thread.
*
* @since 1.5
*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
--- 208,218 ----
* variable. If this thread-local variable is subsequently
* {@linkplain #get read} by the current thread, its value will be
* reinitialized by invoking its {@link #initialValue} method,
* unless its value is {@linkplain #set set} by the current thread
* in the interim. This may result in multiple invocations of the
! * {@code initialValue} method in the current thread.
*
* @since 1.5
*/
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
*** 249,258 ****
--- 264,291 ----
T childValue(T parentValue) {
throw new UnsupportedOperationException();
}
/**
+ * An extension of ThreadLocal that obtains its initial value from
+ * the specified {@code Supplier}.
+ */
+ static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
+
+ private final Supplier<? extends T> supplier;
+
+ SuppliedThreadLocal(Supplier<? extends T> supplier) {
+ this.supplier = Objects.requireNonNull(supplier);
+ }
+
+ @Override
+ protected T initialValue() {
+ return supplier.get();
+ }
+ }
+
+ /**
* ThreadLocalMap is a customized hash map suitable only for
* maintaining thread local values. No operations are exported
* outside of the ThreadLocal class. The class is package private to
* allow declaration of fields in class Thread. To help deal with
* very large and long-lived usages, the hash table entries use
*** 597,609 ****
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
! * @param n scan control: <tt>log2(n)</tt> cells are scanned,
* unless a stale entry is found, in which case
! * <tt>log2(table.length)-1</tt> additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
--- 630,642 ----
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
! * @param n scan control: {@code log2(n)} cells are scanned,
* unless a stale entry is found, in which case
! * {@code log2(table.length)-1} additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and