Showing the changes to java/pkg/com/sun/jmx/snmp/ThreadContext.java (1 of 18352 files). Show all
Unified · Split
| @@ -0,0 +1,326 @@ | |||
| 1 | /* | ||
| 2 | * Copyright (c) 2000, 2007, Oracle and/or its affiliates. All rights reserved. | ||
| 3 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | ||
| 4 | * | ||
| 5 | * This code is free software; you can redistribute it and/or modify it | ||
| 6 | * under the terms of the GNU General Public License version 2 only, as | ||
| 7 | * published by the Free Software Foundation. Oracle designates this | ||
| 8 | * particular file as subject to the "Classpath" exception as provided | ||
| 9 | * by Oracle in the LICENSE file that accompanied this code. | ||
| 10 | * | ||
| 11 | * This code is distributed in the hope that it will be useful, but WITHOUT | ||
| 12 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | ||
| 13 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | ||
| 14 | * version 2 for more details (a copy is included in the LICENSE file that | ||
| 15 | * accompanied this code). | ||
| 16 | * | ||
| 17 | * You should have received a copy of the GNU General Public License version | ||
| 18 | * 2 along with this work; if not, write to the Free Software Foundation, | ||
| 19 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | ||
| 20 | * | ||
| 21 | * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | ||
| 22 | * or visit www.oracle.com if you need additional information or have any | ||
| 23 | * questions. | ||
| 24 | */ | ||
| 25 | |||
| 26 | |||
| 27 | package com.sun.jmx.snmp; | ||
| 28 | |||
| 29 | import java.util.Stack; | ||
| 30 | import java.util.EmptyStackException; | ||
| 31 | |||
| 32 | /** | ||
| 33 | * <p><b>Warning: The interface of this class is subject to change. | ||
| 34 | * Use at your own risk.</b></p> | ||
| 35 | * | ||
| 36 | * <p>This class associates a context with each thread that | ||
| 37 | * references it. The context is a set of mappings between Strings | ||
| 38 | * and Objects. It is managed as a stack, typically with code like | ||
| 39 | * this:</p> | ||
| 40 | * | ||
| 41 | * <pre> | ||
| 42 | * ThreadContext oldContext = ThreadContext.push(myKey, myObject); | ||
| 43 | * // plus possibly further calls to ThreadContext.push... | ||
| 44 | * try { | ||
| 45 | * doSomeOperation(); | ||
| 46 | * } finally { | ||
| 47 | * ThreadContext.restore(oldContext); | ||
| 48 | * } | ||
| 49 | * </pre> | ||
| 50 | * | ||
| 51 | * <p>The <code>try</code>...<code>finally</code> block ensures that | ||
| 52 | * the <code>restore</code> is done even if | ||
| 53 | * <code>doSomeOperation</code> terminates abnormally (with an | ||
| 54 | * exception).</p> | ||
| 55 | * | ||
| 56 | * <p>A thread can consult its own context using | ||
| 57 | * <code>ThreadContext.get(myKey)</code>. The result is the | ||
| 58 | * value that was most recently pushed with the given key.</p> | ||
| 59 | * | ||
| 60 | * <p>A thread cannot read or modify the context of another thread.</p> | ||
| 61 | * | ||
| 62 | * <p><b>This API is a Sun Microsystems internal API and is subject | ||
| 63 | * to change without notice.</b></p> | ||
| 64 | */ | ||
| 65 | public class ThreadContext implements Cloneable { | ||
| 66 | |||
| 67 | /* The context of a thread is stored as a linked list. At the | ||
| 68 | head of the list is the value returned by localContext.get(). | ||
| 69 | At the tail of the list is a sentinel ThreadContext value with | ||
| 70 | "previous" and "key" both null. There is a different sentinel | ||
| 71 | object for each thread. | ||
| 72 | |||
| 73 | Because a null key indicates the sentinel, we reject attempts to | ||
| 74 | push context entries with a null key. | ||
| 75 | |||
| 76 | The reason for using a sentinel rather than just terminating | ||
| 77 | the list with a null reference is to protect against incorrect | ||
| 78 | or even malicious code. If you have a reference to the | ||
| 79 | sentinel value, you can erase the context stack. Only the | ||
| 80 | caller of the first "push" that put something on the stack can | ||
| 81 | get such a reference, so if that caller does not give this | ||
| 82 | reference away, no one else can erase the stack. | ||
| 83 | |||
| 84 | If the restore method took a null reference to mean an empty | ||
| 85 | stack, anyone could erase the stack, since anyone can make a | ||
| 86 | null reference. | ||
| 87 | |||
| 88 | When the stack is empty, we discard the sentinel object and | ||
| 89 | have localContext.get() return null. Then we recreate the | ||
| 90 | sentinel object on the first subsequent push. | ||
| 91 | |||
| 92 | ThreadContext objects are immutable. As a consequence, you can | ||
| 93 | give a ThreadContext object to setInitialContext that is no | ||
| 94 | longer current. But the interface says this can be rejected, | ||
| 95 | in case we remove immutability later. */ | ||
| 96 | |||
| 97 | /* We have to comment out "final" here because of a bug in the JDK1.1 | ||
| 98 | compiler. Uncomment it when we discard 1.1 compatibility. */ | ||
| 99 | private /*final*/ ThreadContext previous; | ||
| 100 | private /*final*/ String key; | ||
| 101 | private /*final*/ Object value; | ||
| 102 | |||
| 103 | private ThreadContext(ThreadContext previous, String key, Object value) { | ||
| 104 | this.previous = previous; | ||
| 105 | this.key = key; | ||
| 106 | this.value = value; | ||
| 107 | } | ||
| 108 | |||
| 109 | /** | ||
| 110 | * <p>Get the Object that was most recently pushed with the given key.</p> | ||
| 111 | * | ||
| 112 | * @param key the key of interest. | ||
| 113 | * | ||
| 114 | * @return the last Object that was pushed (using | ||
| 115 | * <code>push</code>) with that key and not subsequently canceled | ||
| 116 | * by a <code>restore</code>; or null if there is no such object. | ||
| 117 | * A null return value may also indicate that the last Object | ||
| 118 | * pushed was the value <code>null</code>. Use the | ||
| 119 | * <code>contains</code> method to distinguish this case from the | ||
| 120 | * case where there is no Object. | ||
| 121 | * | ||
| 122 | * @exception IllegalArgumentException if <code>key</code> is null. | ||
| 123 | */ | ||
| 124 | public static Object get(String key) throws IllegalArgumentException { | ||
| 125 | ThreadContext context = contextContaining(key); | ||
| 126 | if (context == null) | ||
| 127 | return null; | ||
| 128 | else | ||
| 129 | return context.value; | ||
| 130 | } | ||
| 131 | |||
| 132 | /** | ||
| 133 | * <p>Check whether a value with the given key exists in the stack. | ||
| 134 | * This means that the <code>push</code> method was called with | ||
| 135 | * this key and it was not cancelled by a subsequent | ||
| 136 | * <code>restore</code>. This method is useful when the | ||
| 137 | * <code>get</code> method returns null, to distinguish between | ||
| 138 | * the case where the key exists in the stack but is associated | ||
| 139 | * with a null value, and the case where the key does not exist in | ||
| 140 | * the stack.</p> | ||
| 141 | * | ||
| 142 | * @return true if the key exists in the stack. | ||
| 143 | * | ||
| 144 | * @exception IllegalArgumentException if <code>key</code> is null. | ||
| 145 | */ | ||
| 146 | public static boolean contains(String key) | ||
| 147 | throws IllegalArgumentException { | ||
| 148 | return (contextContaining(key) != null); | ||
| 149 | } | ||
| 150 | |||
| 151 | /** | ||
| 152 | * <p>Find the ThreadContext in the stack that contains the given key, | ||
| 153 | * or return null if there is none.</p> | ||
| 154 | * | ||
| 155 | * @exception IllegalArgumentException if <code>key</code> is null. | ||
| 156 | */ | ||
| 157 | private static ThreadContext contextContaining(String key) | ||
| 158 | throws IllegalArgumentException { | ||
| 159 | if (key == null) | ||
| 160 | throw new IllegalArgumentException("null key"); | ||
| 161 | for (ThreadContext context = getContext(); | ||
| 162 | context != null; | ||
| 163 | context = context.previous) { | ||
| 164 | if (key.equals(context.key)) | ||
| 165 | return context; | ||
| 166 | /* Note that "context.key" may be null if "context" is the | ||
| 167 | sentinel, so don't write "if (context.key.equals(key))"! */ | ||
| 168 | } | ||
| 169 | return null; | ||
| 170 | } | ||
| 171 | |||
| 172 | // /** | ||
| 173 | // * Change the value that was most recently associated with the given key | ||
| 174 | // * in a <code>push</code> operation not cancelled by a subsequent | ||
| 175 | // * <code>restore</code>. If there is no such association, nothing happens | ||
| 176 | // * and the return value is null. | ||
| 177 | // * | ||
| 178 | // * @param key the key of interest. | ||
| 179 | // * @param value the new value to associate with that key. | ||
| 180 | // * | ||
| 181 | // * @return the value that was previously associated with the key, or null | ||
| 182 | // * if the key does not exist in the stack. | ||
| 183 | // * | ||
| 184 | // * @exception IllegalArgumentException if <code>key</code> is null. | ||
| 185 | // */ | ||
| 186 | // public static Object set(String key, Object value) | ||
| 187 | // throws IllegalArgumentException { | ||
| 188 | // ThreadContext context = contextContaining(key); | ||
| 189 | // if (context == null) | ||
| 190 | // return null; | ||
| 191 | // Object old = context.value; | ||
| 192 | // context.value = value; | ||
| 193 | // return old; | ||
| 194 | // } | ||
| 195 | |||
| 196 | /** | ||
| 197 | * <p>Push an object on the context stack with the given key. | ||
| 198 | * This operation can subsequently be undone by calling | ||
| 199 | * <code>restore</code> with the ThreadContext value returned | ||
| 200 | * here.</p> | ||
| 201 | * | ||
| 202 | * @param key the key that will be used to find the object while it is | ||
| 203 | * on the stack. | ||
| 204 | * @param value the value to be associated with that key. It may be null. | ||
| 205 | * | ||
| 206 | * @return a ThreadContext that can be given to <code>restore</code> to | ||
| 207 | * restore the stack to its state before the <code>push</code>. | ||
| 208 | * | ||
| 209 | * @exception IllegalArgumentException if <code>key</code> is null. | ||
| 210 | */ | ||
| 211 | public static ThreadContext push(String key, Object value) | ||
| 212 | throws IllegalArgumentException { | ||
| 213 | if (key == null) | ||
| 214 | throw new IllegalArgumentException("null key"); | ||
| 215 | |||
| 216 | ThreadContext oldContext = getContext(); | ||
| 217 | if (oldContext == null) | ||
| 218 | oldContext = new ThreadContext(null, null, null); // make sentinel | ||
| 219 | ThreadContext newContext = new ThreadContext(oldContext, key, value); | ||
| 220 | setContext(newContext); | ||
| 221 | return oldContext; | ||
| 222 | } | ||
| 223 | |||
| 224 | /** | ||
| 225 | * <p>Return an object that can later be supplied to <code>restore</code> | ||
| 226 | * to restore the context stack to its current state. The object can | ||
| 227 | * also be given to <code>setInitialContext</code>.</p> | ||
| 228 | * | ||
| 229 | * @return a ThreadContext that represents the current context stack. | ||
| 230 | */ | ||
| 231 | public static ThreadContext getThreadContext() { | ||
| 232 | return getContext(); | ||
| 233 | } | ||
| 234 | |||
| 235 | /** | ||
| 236 | * <p>Restore the context stack to an earlier state. This typically | ||
| 237 | * undoes the effect of one or more <code>push</code> calls.</p> | ||
| 238 | * | ||
| 239 | * @param oldContext the state to return. This is usually the return | ||
| 240 | * value of an earlier <code>push</code> operation. | ||
| 241 | * | ||
| 242 | * @exception NullPointerException if <code>oldContext</code> is null. | ||
| 243 | * @exception IllegalArgumentException if <code>oldContext</code> | ||
| 244 | * does not represent a context from this thread, or if that | ||
| 245 | * context was undone by an earlier <code>restore</code>. | ||
| 246 | */ | ||
| 247 | public static void restore(ThreadContext oldContext) | ||
| 248 | throws NullPointerException, IllegalArgumentException { | ||
| 249 | /* The following test is not strictly necessary in the code as it | ||
| 250 | stands today, since the reference to "oldContext.key" would | ||
| 251 | generate a NullPointerException anyway. But if someone | ||
| 252 | didn't notice that during subsequent changes, they could | ||
| 253 | accidentally permit restore(null) with the semantics of | ||
| 254 | trashing the context stack. */ | ||
| 255 | if (oldContext == null) | ||
| 256 | throw new NullPointerException(); | ||
| 257 | |||
| 258 | /* Check that the restored context is in the stack. */ | ||
| 259 | for (ThreadContext context = getContext(); | ||
| 260 | context != oldContext; | ||
| 261 | context = context.previous) { | ||
| 262 | if (context == null) { | ||
| 263 | throw new IllegalArgumentException("Restored context is not " + | ||
| 264 | "contained in current " + | ||
| 265 | "context"); | ||
| 266 | } | ||
| 267 | } | ||
| 268 | |||
| 269 | /* Discard the sentinel if the stack is empty. This means that it | ||
| 270 | is an error to call "restore" a second time with the | ||
| 271 | ThreadContext value that means an empty stack. That's why we | ||
| 272 | don't say that it is all right to restore the stack to the | ||
| 273 | state it was already in. */ | ||
| 274 | if (oldContext.key == null) | ||
| 275 | oldContext = null; | ||
| 276 | |||
| 277 | setContext(oldContext); | ||
| 278 | } | ||
| 279 | |||
| 280 | /** | ||
| 281 | * <p>Set the initial context of the calling thread to a context obtained | ||
| 282 | * from another thread. After this call, the calling thread will see | ||
| 283 | * the same results from the <code>get</code> method as the thread | ||
| 284 | * from which the <code>context</code> argument was obtained, at the | ||
| 285 | * time it was obtained.</p> | ||
| 286 | * | ||
| 287 | * <p>The <code>context</code> argument must be the result of an earlier | ||
| 288 | * <code>push</code> or <code>getThreadContext</code> call. It is an | ||
| 289 | * error (which may or may not be detected) if this context has been | ||
| 290 | * undone by a <code>restore</code>.</p> | ||
| 291 | * | ||
| 292 | * <p>The context stack of the calling thread must be empty before this | ||
| 293 | * call, i.e., there must not have been a <code>push</code> not undone | ||
| 294 | * by a subsequent <code>restore</code>.</p> | ||
| 295 | * | ||
| 296 | * @exception IllegalArgumentException if the context stack was | ||
| 297 | * not empty before the call. An implementation may also throw this | ||
| 298 | * exception if <code>context</code> is no longer current in the | ||
| 299 | * thread from which it was obtained. | ||
| 300 | */ | ||
| 301 | /* We rely on the fact that ThreadContext objects are immutable. | ||
| 302 | This means that we don't have to check that the "context" | ||
| 303 | argument is valid. It necessarily represents the head of a | ||
| 304 | valid chain of ThreadContext objects, even if the thread from | ||
| 305 | which it was obtained has subsequently been set to a point | ||
| 306 | later in that chain using "restore". */ | ||
| 307 | public void setInitialContext(ThreadContext context) | ||
| 308 | throws IllegalArgumentException { | ||
| 309 | /* The following test assumes that we discard sentinels when the | ||
| 310 | stack is empty. */ | ||
| 311 | if (getContext() != null) | ||
| 312 | throw new IllegalArgumentException("previous context not empty"); | ||
| 313 | setContext(context); | ||
| 314 | } | ||
| 315 | |||
| 316 | private static ThreadContext getContext() { | ||
| 317 | return localContext.get(); | ||
| 318 | } | ||
| 319 | |||
| 320 | private static void setContext(ThreadContext context) { | ||
| 321 | localContext.set(context); | ||
| 322 | } | ||
| 323 | |||
| 324 | private static ThreadLocal<ThreadContext> localContext = | ||
| 325 | new ThreadLocal<ThreadContext>(); | ||
| 326 | } | ||