| // Copyright 2014 The Bazel Authors. All rights reserved. |
| // |
| // 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 |
| // |
| // http://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 com.google.devtools.build.lib.analysis; |
| |
| import com.google.devtools.build.lib.actions.ActionAnalysisMetadata; |
| import com.google.devtools.build.lib.packages.RuleClass; |
| |
| /** |
| * A shortcut class to the appropriate specialization of {@code RuleClass.ConfiguredTargetFactory}. |
| * |
| * <p>Here follows an overview of how loading and analysis works in Bazel: |
| * |
| * <p>Actions (i.e. commands that are run during the build) are created by configured targets |
| * (see {@link ConfiguredTarget}), which are a pair of a target (e.g. <code>//src:bazel</code>) and |
| * a {@link com.google.devtools.build.lib.analysis.config.BuildConfiguration}, which is a blob of |
| * data that contains extra information about how the target should be built (for example, for which |
| * platform or with which C++ preprocessor definitions). Accordingly, a target can give rise to |
| * multiple configured targets, for example, if it needs to be built both for the host and the |
| * target configuration. |
| * |
| * <p>The process of creating the appropriate {@link com.google.devtools.build.lib.actions.Action}s |
| * for a configured target is called "analysis". The analysis of a configured target is composed of |
| * the following steps (which process is orchestrated by |
| * {@link com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction}): |
| * <ol> |
| * <li>The corresponding {@link com.google.devtools.build.lib.packages.Target} is loaded, i.e. |
| * the BUILD file is parsed.</li> |
| * <li>Its direct dependencies are analyzed, during which in turn indirect dependencies are |
| * also analyzed.</li> |
| * <li>Aspects specified by the configured target are analyzed. These can be thought of as |
| * visitations of the transitive dependencies of the target. For more information, see |
| * {@link com.google.devtools.build.lib.packages.AspectClass}. |
| * <li>The configured target and the actions it generates are created based on the data from the |
| * previous two steps.</li> |
| * </ol> |
| * |
| * Targets can be of three main kinds (plus a few special ones which are not important for |
| * understanding the big picture): |
| * <p> |
| * <li>Input and output files. These represent either a file that is in the source tree or a |
| * file produced by during the build. Not every file produced during the build has a |
| * corresponding output file target.</li> |
| * <li>Rules. These describe things a build actually does. Each rule has a class (e.g. |
| * <code>cc_binary</code>). Rule classes can be defined either in Skylark using the |
| * <code>rule()</code> function or in Java code by implementing |
| * {@link com.google.devtools.build.lib.analysis.RuleDefinition}.</li> |
| * </p> |
| * <p>During the analysis of a configured target, the following pieces of data are available: |
| * <ul> |
| * <li>The corresponding target itself. This is necessary so that the analysis has access to e.g. |
| * the attributes a rule has in the BUILD file.</li> |
| * <li>The {@link com.google.devtools.build.lib.analysis.TransitiveInfoCollection}s of direct |
| * dependencies. They are used to gather information from the transitive closure, for example, |
| * the include path entries for C++ compilation or all the object files that need to be |
| * compiled into a C++ binary.</li> |
| * <li>The configuration, which is used to determine which compiler to use and to get access |
| * to some command line options of Bazel that influence analysis.</li> |
| * <li>Skyframe, for requesting arbitrary Skyframe nodes. This is an escape hatch that should be |
| * used when other mechanisms provided are not suitable and allows one to e.g. read arbitrary |
| * files. With great power... |
| * </ul> |
| * |
| * <p>Analysis of non-rule configured targets is special-cased and is not covered here. |
| * |
| * <p>The analysis of a rule itself is done by implementations {@link RuleConfiguredTargetFactory} |
| * (there should be one for each rule class). The data above is available using the |
| * {@link RuleContext} argument passed into its create() method. It should |
| * result in three things: |
| * <ul> |
| * <li>A set of actions. These should be passed to |
| * {@link RuleContext#registerAction(ActionAnalysisMetadata...)}, although for more |
| * common cases (e.g. {@link com.google.devtools.build.lib.analysis.actions.SpawnAction}), |
| * shortcuts are provided.</li> |
| * <li>A set of artifacts (files produced by actions). These should be created using methods of |
| * {@link RuleContext}. Each artifact thus created must have a generating action.</li> |
| * <li>A set of {@link com.google.devtools.build.lib.analysis.TransitiveInfoProvider}s that are |
| * passed on to direct dependencies. These must be registered using |
| * {@link com.google.devtools.build.lib.analysis.RuleConfiguredTargetBuilder#add( |
| * Class, com.google.devtools.build.lib.analysis.TransitiveInfoProvider)} |
| * </li> |
| * </ul> |
| * |
| * <p>Configured targets are currently allowed to create artifacts at any exec path. It would be |
| * better if they could be constrained to a subtree based on the label of the configured target, |
| * but this is currently not feasible because multiple rules violate this constraint and the |
| * output format is part of its interface. |
| * |
| * <p>In principle, multiple configured targets should not create actions with conflicting |
| * outputs. There are still a few exceptions to this rule that are slated to be eventually |
| * removed, we have provisions to handle this case (Action instances that share at least one |
| * output file are required to be exactly the same), but this does put some pressure on the design |
| * and we are eventually planning to eliminate this option. |
| * |
| * <p>These restrictions together make it possible to: |
| * <ul> |
| * <li>Correctly cache the analysis phase; by tightly constraining what a configured target is |
| * allowed to access and what it is not, we can know when it needs to invalidate a particular |
| * one and when it can reuse an already existing one. |
| * <li>Serialize / deserialize individual configured targets at will, making it possible for |
| * example to swap out part of the analysis state if there is memory pressure or to move them in |
| * persistent storage so that the state can be reconstructed at a different time or in a |
| * different process. The stretch goal is to eventually facilitate cross-user caching of this |
| * information. |
| * </ul> |
| * |
| */ |
| public interface RuleConfiguredTargetFactory |
| extends RuleClass.ConfiguredTargetFactory<ConfiguredTarget, RuleContext> { |
| } |
