1 /* |
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2 * Copyright (c) 1999, 2012, Oracle and/or its affiliates. All rights reserved. |
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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4 * |
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5 * This code is free software; you can redistribute it and/or modify it |
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6 * under the terms of the GNU General Public License version 2 only, as |
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7 * published by the Free Software Foundation. Oracle designates this |
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8 * particular file as subject to the "Classpath" exception as provided |
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9 * by Oracle in the LICENSE file that accompanied this code. |
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10 * |
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11 * This code is distributed in the hope that it will be useful, but WITHOUT |
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12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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14 * version 2 for more details (a copy is included in the LICENSE file that |
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15 * accompanied this code). |
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16 * |
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17 * You should have received a copy of the GNU General Public License version |
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18 * 2 along with this work; if not, write to the Free Software Foundation, |
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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20 * |
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21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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22 * or visit www.oracle.com if you need additional information or have any |
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23 * questions. |
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24 */ |
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25 |
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26 package sun.misc; |
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27 |
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28 import java.io.BufferedReader; |
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29 import java.io.IOException; |
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30 import java.io.InputStream; |
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31 import java.io.InputStreamReader; |
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32 import java.net.URL; |
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33 import java.util.ArrayList; |
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34 import java.util.Enumeration; |
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35 import java.util.Iterator; |
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36 import java.util.List; |
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37 import java.util.NoSuchElementException; |
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38 import java.util.Set; |
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39 import java.util.TreeSet; |
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40 |
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41 |
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42 /** |
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43 * A simple service-provider lookup mechanism. A <i>service</i> is a |
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44 * well-known set of interfaces and (usually abstract) classes. A <i>service |
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45 * provider</i> is a specific implementation of a service. The classes in a |
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46 * provider typically implement the interfaces and subclass the classes defined |
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47 * in the service itself. Service providers may be installed in an |
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48 * implementation of the Java platform in the form of extensions, that is, jar |
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49 * files placed into any of the usual extension directories. Providers may |
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50 * also be made available by adding them to the applet or application class |
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51 * path or by some other platform-specific means. |
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52 * |
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53 * <p> In this lookup mechanism a service is represented by an interface or an |
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54 * abstract class. (A concrete class may be used, but this is not |
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55 * recommended.) A provider of a given service contains one or more concrete |
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56 * classes that extend this <i>service class</i> with data and code specific to |
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57 * the provider. This <i>provider class</i> will typically not be the entire |
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58 * provider itself but rather a proxy that contains enough information to |
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59 * decide whether the provider is able to satisfy a particular request together |
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60 * with code that can create the actual provider on demand. The details of |
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61 * provider classes tend to be highly service-specific; no single class or |
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62 * interface could possibly unify them, so no such class has been defined. The |
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63 * only requirement enforced here is that provider classes must have a |
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64 * zero-argument constructor so that they may be instantiated during lookup. |
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65 * |
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66 * <p> A service provider identifies itself by placing a provider-configuration |
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67 * file in the resource directory <tt>META-INF/services</tt>. The file's name |
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68 * should consist of the fully-qualified name of the abstract service class. |
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69 * The file should contain a list of fully-qualified concrete provider-class |
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70 * names, one per line. Space and tab characters surrounding each name, as |
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71 * well as blank lines, are ignored. The comment character is <tt>'#'</tt> |
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72 * (<tt>0x23</tt>); on each line all characters following the first comment |
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73 * character are ignored. The file must be encoded in UTF-8. |
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74 * |
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75 * <p> If a particular concrete provider class is named in more than one |
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76 * configuration file, or is named in the same configuration file more than |
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77 * once, then the duplicates will be ignored. The configuration file naming a |
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78 * particular provider need not be in the same jar file or other distribution |
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79 * unit as the provider itself. The provider must be accessible from the same |
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80 * class loader that was initially queried to locate the configuration file; |
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81 * note that this is not necessarily the class loader that found the file. |
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82 * |
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83 * <p> <b>Example:</b> Suppose we have a service class named |
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84 * <tt>java.io.spi.CharCodec</tt>. It has two abstract methods: |
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85 * |
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86 * <pre> |
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87 * public abstract CharEncoder getEncoder(String encodingName); |
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88 * public abstract CharDecoder getDecoder(String encodingName); |
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89 * </pre> |
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90 * |
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91 * Each method returns an appropriate object or <tt>null</tt> if it cannot |
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92 * translate the given encoding. Typical <tt>CharCodec</tt> providers will |
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93 * support more than one encoding. |
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94 * |
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95 * <p> If <tt>sun.io.StandardCodec</tt> is a provider of the <tt>CharCodec</tt> |
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96 * service then its jar file would contain the file |
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97 * <tt>META-INF/services/java.io.spi.CharCodec</tt>. This file would contain |
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98 * the single line: |
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99 * |
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100 * <pre> |
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101 * sun.io.StandardCodec # Standard codecs for the platform |
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102 * </pre> |
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103 * |
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104 * To locate an encoder for a given encoding name, the internal I/O code would |
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105 * do something like this: |
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106 * |
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107 * <pre> |
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108 * CharEncoder getEncoder(String encodingName) { |
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109 * Iterator ps = Service.providers(CharCodec.class); |
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110 * while (ps.hasNext()) { |
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111 * CharCodec cc = (CharCodec)ps.next(); |
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112 * CharEncoder ce = cc.getEncoder(encodingName); |
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113 * if (ce != null) |
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114 * return ce; |
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115 * } |
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116 * return null; |
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117 * } |
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118 * </pre> |
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119 * |
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120 * The provider-lookup mechanism always executes in the security context of the |
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121 * caller. Trusted system code should typically invoke the methods in this |
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122 * class from within a privileged security context. |
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123 * |
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124 * @author Mark Reinhold |
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125 * @since 1.3 |
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126 */ |
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127 |
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128 public final class Service<S> { |
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129 |
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130 private static final String prefix = "META-INF/services/"; |
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131 |
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132 private Service() { } |
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133 |
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134 private static void fail(Class<?> service, String msg, Throwable cause) |
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135 throws ServiceConfigurationError |
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136 { |
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137 ServiceConfigurationError sce |
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138 = new ServiceConfigurationError(service.getName() + ": " + msg); |
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139 sce.initCause(cause); |
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140 throw sce; |
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141 } |
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142 |
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143 private static void fail(Class<?> service, String msg) |
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144 throws ServiceConfigurationError |
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145 { |
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146 throw new ServiceConfigurationError(service.getName() + ": " + msg); |
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147 } |
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148 |
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149 private static void fail(Class<?> service, URL u, int line, String msg) |
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150 throws ServiceConfigurationError |
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151 { |
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152 fail(service, u + ":" + line + ": " + msg); |
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153 } |
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154 |
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155 /** |
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156 * Parse a single line from the given configuration file, adding the name |
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157 * on the line to both the names list and the returned set iff the name is |
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158 * not already a member of the returned set. |
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159 */ |
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160 private static int parseLine(Class<?> service, URL u, BufferedReader r, int lc, |
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161 List<String> names, Set<String> returned) |
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162 throws IOException, ServiceConfigurationError |
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163 { |
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164 String ln = r.readLine(); |
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165 if (ln == null) { |
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166 return -1; |
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167 } |
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168 int ci = ln.indexOf('#'); |
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169 if (ci >= 0) ln = ln.substring(0, ci); |
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170 ln = ln.trim(); |
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171 int n = ln.length(); |
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172 if (n != 0) { |
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173 if ((ln.indexOf(' ') >= 0) || (ln.indexOf('\t') >= 0)) |
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174 fail(service, u, lc, "Illegal configuration-file syntax"); |
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175 int cp = ln.codePointAt(0); |
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176 if (!Character.isJavaIdentifierStart(cp)) |
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177 fail(service, u, lc, "Illegal provider-class name: " + ln); |
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178 for (int i = Character.charCount(cp); i < n; i += Character.charCount(cp)) { |
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179 cp = ln.codePointAt(i); |
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180 if (!Character.isJavaIdentifierPart(cp) && (cp != '.')) |
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181 fail(service, u, lc, "Illegal provider-class name: " + ln); |
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182 } |
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183 if (!returned.contains(ln)) { |
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184 names.add(ln); |
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185 returned.add(ln); |
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186 } |
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187 } |
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188 return lc + 1; |
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189 } |
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190 |
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191 /** |
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192 * Parse the content of the given URL as a provider-configuration file. |
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193 * |
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194 * @param service |
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195 * The service class for which providers are being sought; |
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196 * used to construct error detail strings |
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197 * |
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198 * @param url |
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199 * The URL naming the configuration file to be parsed |
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200 * |
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201 * @param returned |
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202 * A Set containing the names of provider classes that have already |
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203 * been returned. This set will be updated to contain the names |
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204 * that will be yielded from the returned <tt>Iterator</tt>. |
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205 * |
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206 * @return A (possibly empty) <tt>Iterator</tt> that will yield the |
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207 * provider-class names in the given configuration file that are |
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208 * not yet members of the returned set |
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209 * |
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210 * @throws ServiceConfigurationError |
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211 * If an I/O error occurs while reading from the given URL, or |
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212 * if a configuration-file format error is detected |
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213 */ |
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214 private static Iterator<String> parse(Class<?> service, URL u, Set<String> returned) |
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215 throws ServiceConfigurationError |
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216 { |
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217 InputStream in = null; |
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218 BufferedReader r = null; |
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219 ArrayList<String> names = new ArrayList<>(); |
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220 try { |
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221 in = u.openStream(); |
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222 r = new BufferedReader(new InputStreamReader(in, "utf-8")); |
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223 int lc = 1; |
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224 while ((lc = parseLine(service, u, r, lc, names, returned)) >= 0); |
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225 } catch (IOException x) { |
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226 fail(service, ": " + x); |
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227 } finally { |
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228 try { |
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229 if (r != null) r.close(); |
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230 if (in != null) in.close(); |
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231 } catch (IOException y) { |
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232 fail(service, ": " + y); |
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233 } |
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234 } |
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235 return names.iterator(); |
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236 } |
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237 |
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238 |
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239 /** |
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240 * Private inner class implementing fully-lazy provider lookup |
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241 */ |
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242 private static class LazyIterator<S> implements Iterator<S> { |
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243 |
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244 Class<S> service; |
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245 ClassLoader loader; |
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246 Enumeration<URL> configs = null; |
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247 Iterator<String> pending = null; |
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248 Set<String> returned = new TreeSet<>(); |
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249 String nextName = null; |
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250 |
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251 private LazyIterator(Class<S> service, ClassLoader loader) { |
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252 this.service = service; |
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253 this.loader = loader; |
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254 } |
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255 |
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256 public boolean hasNext() throws ServiceConfigurationError { |
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257 if (nextName != null) { |
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258 return true; |
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259 } |
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260 if (configs == null) { |
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261 try { |
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262 String fullName = prefix + service.getName(); |
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263 if (loader == null) |
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264 configs = ClassLoader.getSystemResources(fullName); |
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265 else |
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266 configs = loader.getResources(fullName); |
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267 } catch (IOException x) { |
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268 fail(service, ": " + x); |
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269 } |
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270 } |
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271 while ((pending == null) || !pending.hasNext()) { |
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272 if (!configs.hasMoreElements()) { |
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273 return false; |
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274 } |
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275 pending = parse(service, configs.nextElement(), returned); |
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276 } |
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277 nextName = pending.next(); |
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278 return true; |
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279 } |
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280 |
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281 public S next() throws ServiceConfigurationError { |
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282 if (!hasNext()) { |
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283 throw new NoSuchElementException(); |
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284 } |
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285 String cn = nextName; |
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286 nextName = null; |
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287 Class<?> c = null; |
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288 try { |
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289 c = Class.forName(cn, false, loader); |
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290 } catch (ClassNotFoundException x) { |
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291 fail(service, |
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292 "Provider " + cn + " not found"); |
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293 } |
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294 if (!service.isAssignableFrom(c)) { |
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295 fail(service, |
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296 "Provider " + cn + " not a subtype"); |
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297 } |
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298 try { |
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299 return service.cast(c.newInstance()); |
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300 } catch (Throwable x) { |
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301 fail(service, |
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302 "Provider " + cn + " could not be instantiated: " + x, |
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303 x); |
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304 } |
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305 return null; /* This cannot happen */ |
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306 } |
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307 |
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308 public void remove() { |
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309 throw new UnsupportedOperationException(); |
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310 } |
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311 |
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312 } |
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313 |
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314 |
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315 /** |
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316 * Locates and incrementally instantiates the available providers of a |
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317 * given service using the given class loader. |
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318 * |
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319 * <p> This method transforms the name of the given service class into a |
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320 * provider-configuration filename as described above and then uses the |
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321 * <tt>getResources</tt> method of the given class loader to find all |
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322 * available files with that name. These files are then read and parsed to |
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323 * produce a list of provider-class names. The iterator that is returned |
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324 * uses the given class loader to lookup and then instantiate each element |
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325 * of the list. |
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326 * |
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327 * <p> Because it is possible for extensions to be installed into a running |
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328 * Java virtual machine, this method may return different results each time |
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329 * it is invoked. <p> |
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330 * |
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331 * @param service |
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332 * The service's abstract service class |
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333 * |
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334 * @param loader |
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335 * The class loader to be used to load provider-configuration files |
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336 * and instantiate provider classes, or <tt>null</tt> if the system |
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337 * class loader (or, failing that the bootstrap class loader) is to |
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338 * be used |
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339 * |
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340 * @return An <tt>Iterator</tt> that yields provider objects for the given |
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341 * service, in some arbitrary order. The iterator will throw a |
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342 * <tt>ServiceConfigurationError</tt> if a provider-configuration |
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343 * file violates the specified format or if a provider class cannot |
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344 * be found and instantiated. |
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345 * |
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346 * @throws ServiceConfigurationError |
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347 * If a provider-configuration file violates the specified format |
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348 * or names a provider class that cannot be found and instantiated |
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349 * |
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350 * @see #providers(java.lang.Class) |
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351 * @see #installedProviders(java.lang.Class) |
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352 */ |
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353 public static <S> Iterator<S> providers(Class<S> service, ClassLoader loader) |
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354 throws ServiceConfigurationError |
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355 { |
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356 return new LazyIterator<S>(service, loader); |
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357 } |
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358 |
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359 |
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360 /** |
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361 * Locates and incrementally instantiates the available providers of a |
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362 * given service using the context class loader. This convenience method |
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363 * is equivalent to |
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364 * |
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365 * <pre> |
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366 * ClassLoader cl = Thread.currentThread().getContextClassLoader(); |
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367 * return Service.providers(service, cl); |
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368 * </pre> |
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369 * |
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370 * @param service |
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371 * The service's abstract service class |
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372 * |
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373 * @return An <tt>Iterator</tt> that yields provider objects for the given |
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374 * service, in some arbitrary order. The iterator will throw a |
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375 * <tt>ServiceConfigurationError</tt> if a provider-configuration |
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376 * file violates the specified format or if a provider class cannot |
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377 * be found and instantiated. |
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378 * |
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379 * @throws ServiceConfigurationError |
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380 * If a provider-configuration file violates the specified format |
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381 * or names a provider class that cannot be found and instantiated |
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382 * |
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383 * @see #providers(java.lang.Class, java.lang.ClassLoader) |
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384 */ |
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385 public static <S> Iterator<S> providers(Class<S> service) |
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386 throws ServiceConfigurationError |
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387 { |
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388 ClassLoader cl = Thread.currentThread().getContextClassLoader(); |
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389 return Service.providers(service, cl); |
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390 } |
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391 |
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392 |
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393 /** |
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394 * Locates and incrementally instantiates the available providers of a |
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395 * given service using the extension class loader. This convenience method |
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396 * simply locates the extension class loader, call it |
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397 * <tt>extClassLoader</tt>, and then does |
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398 * |
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399 * <pre> |
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400 * return Service.providers(service, extClassLoader); |
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401 * </pre> |
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402 * |
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403 * If the extension class loader cannot be found then the system class |
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404 * loader is used; if there is no system class loader then the bootstrap |
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405 * class loader is used. |
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406 * |
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407 * @param service |
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408 * The service's abstract service class |
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409 * |
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410 * @return An <tt>Iterator</tt> that yields provider objects for the given |
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411 * service, in some arbitrary order. The iterator will throw a |
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412 * <tt>ServiceConfigurationError</tt> if a provider-configuration |
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413 * file violates the specified format or if a provider class cannot |
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414 * be found and instantiated. |
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415 * |
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416 * @throws ServiceConfigurationError |
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417 * If a provider-configuration file violates the specified format |
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418 * or names a provider class that cannot be found and instantiated |
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419 * |
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420 * @see #providers(java.lang.Class, java.lang.ClassLoader) |
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421 */ |
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422 public static <S> Iterator<S> installedProviders(Class<S> service) |
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423 throws ServiceConfigurationError |
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424 { |
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425 ClassLoader cl = ClassLoader.getSystemClassLoader(); |
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426 ClassLoader prev = null; |
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427 while (cl != null) { |
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428 prev = cl; |
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429 cl = cl.getParent(); |
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430 } |
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431 return Service.providers(service, prev); |
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432 } |
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433 |
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434 } |
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