1 /* ThreadLocal -- a variable with a unique value per thread
2 Copyright (C) 2000, 2002, 2003, 2006 Free Software Foundation, Inc.
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21 Linking this library statically or dynamically with other modules is
22 making a combined work based on this library. Thus, the terms and
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30 terms of your choice, provided that you also meet, for each linked
31 independent module, the terms and conditions of the license of that
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36 exception statement from your version. */
44 * ThreadLocal objects have a different state associated with every
45 * Thread that accesses them. Every access to the ThreadLocal object
46 * (through the <code>get()</code> and <code>set()</code> methods)
47 * only affects the state of the object as seen by the currently
50 * <p>The first time a ThreadLocal object is accessed on a particular
51 * Thread, the state for that Thread's copy of the local variable is set by
52 * executing the method <code>initialValue()</code>.
55 * <p>An example how you can use this:
61 * private static ThreadLocal owner = new ThreadLocal()
63 * public Object initialValue()
72 * <p>Now all instances of connection can see who the owner of the currently
73 * executing Thread is by calling <code>owner.get()</code>. By default any
74 * Thread would be associated with 'nobody'. But the Connection object could
75 * offer a method that changes the owner associated with the Thread on
76 * which the method was called by calling <code>owner.put("somebody")</code>.
77 * (Such an owner changing method should then be guarded by security checks.)
80 * <p>When a Thread is garbage collected all references to values of
81 * the ThreadLocal objects associated with that Thread are removed.
84 * @author Mark Wielaard (mark@klomp.org)
85 * @author Eric Blake (ebb9@email.byu.edu)
87 * @status updated to 1.5
89 public class ThreadLocal<T>
92 * Placeholder to distinguish between uninitialized and null set by the
93 * user. Do not expose this to the public. Package visible for use by
94 * InheritableThreadLocal
96 static final Object sentinel = new Object();
99 * Creates a ThreadLocal object without associating any value to it yet.
107 * Called once per thread on the first invocation of get(), if set() was
108 * not already called. The default implementation returns <code>null</code>.
109 * Often, this method is overridden to create the appropriate initial object
110 * for the current thread's view of the ThreadLocal.
112 * @return the initial value of the variable in this thread
114 protected T initialValue()
120 * Gets the value associated with the ThreadLocal object for the currently
121 * executing Thread. If this is the first time the current thread has called
122 * get(), and it has not already called set(), the value is obtained by
123 * <code>initialValue()</code>.
125 * @return the value of the variable in this thread
127 public native T get();
129 private final Object internalGet()
131 Map<ThreadLocal<T>,T> map = (Map<ThreadLocal<T>,T>) Thread.getThreadLocals();
132 // Note that we don't have to synchronize, as only this thread will
133 // ever modify the map.
134 T value = map.get(this);
137 value = initialValue();
138 map.put(this, (T) (value == null ? sentinel : value));
140 return value == (T) sentinel ? null : value;
144 * Sets the value associated with the ThreadLocal object for the currently
145 * executing Thread. This overrides any existing value associated with the
146 * current Thread and prevents <code>initialValue()</code> from being
147 * called if this is the first access to this ThreadLocal in this Thread.
149 * @param value the value to set this thread's view of the variable to
151 public native void set(T value);
153 private final void internalSet(Object value)
155 Map map = Thread.getThreadLocals();
156 // Note that we don't have to synchronize, as only this thread will
157 // ever modify the map.
158 map.put(this, value == null ? sentinel : value);
162 * Removes the value associated with the ThreadLocal object for the
163 * currently executing Thread.
166 public native void remove();
168 private final void internalRemove()
170 Map map = Thread.getThreadLocals();
174 protected native void finalize () throws Throwable;
176 private native void constructNative();
178 private gnu.gcj.RawData TLSPointer;