Java tutorial
/* * Copyright 2002-2018 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * https://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.springframework.util; import org.springframework.lang.Nullable; /** * Helper class for resolving placeholders in texts. Usually applied to file paths. * * <p>A text may contain {@code ${...}} placeholders, to be resolved as system properties: * e.g. {@code ${user.dir}}. Default values can be supplied using the ":" separator * between key and value. * * @author Juergen Hoeller * @author Rob Harrop * @author Dave Syer * @since 1.2.5 * @see #PLACEHOLDER_PREFIX * @see #PLACEHOLDER_SUFFIX * @see System#getProperty(String) */ public abstract class SystemPropertyUtils { /** Prefix for system property placeholders: "${". */ public static final String PLACEHOLDER_PREFIX = "${"; /** Suffix for system property placeholders: "}". */ public static final String PLACEHOLDER_SUFFIX = "}"; /** Value separator for system property placeholders: ":". */ public static final String VALUE_SEPARATOR = ":"; private static final PropertyPlaceholderHelper strictHelper = new PropertyPlaceholderHelper(PLACEHOLDER_PREFIX, PLACEHOLDER_SUFFIX, VALUE_SEPARATOR, false); private static final PropertyPlaceholderHelper nonStrictHelper = new PropertyPlaceholderHelper( PLACEHOLDER_PREFIX, PLACEHOLDER_SUFFIX, VALUE_SEPARATOR, true); /** * Resolve {@code ${...}} placeholders in the given text, replacing them with * corresponding system property values. * @param text the String to resolve * @return the resolved String * @throws IllegalArgumentException if there is an unresolvable placeholder * @see #PLACEHOLDER_PREFIX * @see #PLACEHOLDER_SUFFIX */ public static String resolvePlaceholders(String text) { return resolvePlaceholders(text, false); } /** * Resolve {@code ${...}} placeholders in the given text, replacing them with * corresponding system property values. Unresolvable placeholders with no default * value are ignored and passed through unchanged if the flag is set to {@code true}. * @param text the String to resolve * @param ignoreUnresolvablePlaceholders whether unresolved placeholders are to be ignored * @return the resolved String * @throws IllegalArgumentException if there is an unresolvable placeholder * @see #PLACEHOLDER_PREFIX * @see #PLACEHOLDER_SUFFIX * and the "ignoreUnresolvablePlaceholders" flag is {@code false} */ public static String resolvePlaceholders(String text, boolean ignoreUnresolvablePlaceholders) { PropertyPlaceholderHelper helper = (ignoreUnresolvablePlaceholders ? nonStrictHelper : strictHelper); return helper.replacePlaceholders(text, new SystemPropertyPlaceholderResolver(text)); } /** * PlaceholderResolver implementation that resolves against system properties * and system environment variables. */ private static class SystemPropertyPlaceholderResolver implements PropertyPlaceholderHelper.PlaceholderResolver { private final String text; public SystemPropertyPlaceholderResolver(String text) { this.text = text; } @Override @Nullable public String resolvePlaceholder(String placeholderName) { try { String propVal = System.getProperty(placeholderName); if (propVal == null) { // Fall back to searching the system environment. propVal = System.getenv(placeholderName); } return propVal; } catch (Throwable ex) { System.err.println("Could not resolve placeholder '" + placeholderName + "' in [" + this.text + "] as system property: " + ex); return null; } } } }