anttasks/src/com/android/ant/DependencyGraph.java - platform/sdk - Git at Google

 /*
  * Copyright (C) 2011 The Android Open Source Project
  *
  * 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.android.ant;

 import com.google.common.base.Charsets;
 import com.google.common.io.Files;

 import org.apache.tools.ant.BuildException;

 import java.io.File;
 import java.io.IOException;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;

 /**
  *  This class takes care of dependency tracking for all targets and prerequisites listed in
  *  a single dependency file. A dependency graph always has a dependency file associated with it
  *  for the duration of its lifetime
  */
 public class DependencyGraph {

     private final static boolean DEBUG = false;

     private static enum DependencyStatus {
         NONE, NEW_FILE, UPDATED_FILE, MISSING_FILE, ERROR;
     }

     // Files that we know about from the dependency file
     private Set<File> mTargets = Collections.emptySet();
     private Set<File> mPrereqs = mTargets;
     private File mFirstPrereq = null;
     private boolean mMissingDepFile = false;
     private long mDepFileLastModified;
     private final List<InputPath> mNewInputs;

     public DependencyGraph(String dependencyFilePath, List<InputPath> newInputPaths) {
         mNewInputs = newInputPaths;
         parseDependencyFile(dependencyFilePath);
     }

     /**
      * Check all the dependencies to see if anything has changed.
      *
      * @param printStatus will print to {@link System#out} the dependencies status.
      * @return true if new prerequisites have appeared, target files are missing or if
      *         prerequisite files have been modified since the last target generation.
      */
     public boolean dependenciesHaveChanged(boolean printStatus) {
         // If no dependency file has been set up, then we'll just return true
         // if we have a dependency file, we'll check to see what's been changed
         if (mMissingDepFile) {
             System.out.println("No Dependency File Found");
             return true;
         }

         // check for missing output first
         if (missingTargetFile()) {
             if (printStatus) {
                 System.out.println("Found Deleted Target File");
             }
             return true;
         }

         // get the time stamp of the oldest target.
         long oldestTarget = getOutputLastModified();

         // first look through the input folders and look for new files or modified files.
         DependencyStatus status = checkInputs(oldestTarget);

         // this can't find missing files. This is done later.
         switch (status) {
             case ERROR:
                 throw new BuildException();
             case NEW_FILE:
                 if (printStatus) {
                     System.out.println("Found new input file");
                 }
                 return true;
             case UPDATED_FILE:
                 if (printStatus) {
                     System.out.println("Found modified input file");
                 }
                 return true;
         }

         // now do a full check on the remaining files.
         status = checkPrereqFiles(oldestTarget);
         // this can't find new input files. This is done above.
         switch (status) {
             case ERROR:
                 throw new BuildException();
             case MISSING_FILE:
                 if (printStatus) {
                     System.out.println("Found deleted input file");
                 }
                 return true;
             case UPDATED_FILE:
                 if (printStatus) {
                     System.out.println("Found modified input file");
                 }
                 return true;
         }

         return false;
     }

     public Set<File> getTargets() {
         return Collections.unmodifiableSet(mTargets);
     }

     public File getFirstPrereq() {
         return mFirstPrereq;
     }

     /**
      * Parses the given dependency file and stores the file paths
      *
      * @param dependencyFilePath the dependency file
      */
     private void parseDependencyFile(String dependencyFilePath) {
         // first check if the dependency file is here.
         File depFile = new File(dependencyFilePath);
         if (depFile.isFile() == false) {
             mMissingDepFile = true;
             return;
         }

         // get the modification time of the dep file as we may need it later
         mDepFileLastModified = depFile.lastModified();

         // Read in our dependency file
         List<String> content = readFile(depFile);
         if (content == null) {
             System.err.println("ERROR: Couldn't read " + dependencyFilePath);
             return;
         }

         // The format is something like:
         // output1 output2 [...]: dep1 dep2 [...]
         // expect it's likely split on several lines. So let's move it back on a single line
         // first
         StringBuilder sb = new StringBuilder();
         for (String line : content) {
             line = line.trim();
             if (line.endsWith("\\")) {
                 line = line.substring(0, line.length() - 1);
             }
             sb.append(line);
         }

         // split the left and right part
         String[] files = sb.toString().split(":");

         // get the target files:
         String[] targets = files[0].trim().split(" ");

         String[] prereqs = {};
         // Check to make sure our dependency file is okay
         if (files.length < 1) {
             System.err.println(
                     "Warning! Dependency file does not list any prerequisites after ':' ");
         } else {
             // and the prerequisite files:
             prereqs = files[1].trim().split(" ");
         }

         mTargets = new HashSet<File>(targets.length);
         for (String path : targets) {
             if (path.length() > 0) {
                 mTargets.add(new File(path));
             }
         }

         mPrereqs = new HashSet<File>(prereqs.length);
         for (String path : prereqs) {
             if (path.length() > 0) {
                 if (DEBUG) {
                     System.out.println("PREREQ: " + path);
                 }
                 File f = new File(path);
                 if (mFirstPrereq == null) {
                     mFirstPrereq = f;
                 }
                 mPrereqs.add(f);
             }
         }
     }

     /**
      * Check all the input files and folders to see if there have been new
      * files added to them or if any of the existing files have been modified.
      *
      * This looks at the input paths, not at the list of known prereq. Therefore this
      * will not find missing files. It will however remove processed files from the
      * prereq file list so that we can process those in a 2nd step.
      *
      * This should be followed by a call to {@link #checkPrereqFiles(long)} which
      * will process the remaining files in the prereq list.
      *
      * If a change is found, this will return immediately with either
      * {@link DependencyStatus#NEW_FILE} or {@link DependencyStatus#UPDATED_FILE}.
      *
      * @param oldestTarget the timestamp of the oldest output file to compare against.
      *
      * @return the status of the file in the watched folders.
      *
      */
     private DependencyStatus checkInputs(long oldestTarget) {
         if (mNewInputs != null) {
             for (InputPath input : mNewInputs) {
                 File file = input.getFile();
                 if (file.isDirectory()) {
                     DependencyStatus status = checkInputFolder(file, input, oldestTarget);
                     if (status != DependencyStatus.NONE) {
                         return status;
                     }
                 } else if (file.isFile()) {
                     DependencyStatus status = checkInputFile(file, input, oldestTarget);
                     if (status != DependencyStatus.NONE) {
                         return status;
                     }
                 }
             }
         }

         // If we make it all the way through our directories we're good.
         return DependencyStatus.NONE;
     }

     /**
      * Check all the files in the tree under root and check to see if the files are
      * listed under the dependencies, or if they have been modified. Recurses into subdirs.
      *
      * @param folder the folder to search through.
      * @param inputFolder the root level inputFolder
      * @param oldestTarget the time stamp of the oldest output file to compare against.
      *
      * @return the status of the file in the folder.
      */
     private DependencyStatus checkInputFolder(File folder, InputPath inputFolder,
             long oldestTarget) {
         if (inputFolder.ignores(folder)) {
             return DependencyStatus.NONE;
         }

         File[] files = folder.listFiles();
         if (files == null) {
             System.err.println("ERROR " + folder.toString() + " is not a dir or can't be read");
             return DependencyStatus.ERROR;
         }
         // Loop through files in this folder
         for (File file : files) {
             // If this is a directory, recurse into it
             if (file.isDirectory()) {
                 DependencyStatus status = checkInputFolder(file, inputFolder, oldestTarget);
                 if (status != DependencyStatus.NONE) {
                     return status;
                 }
             } else if (file.isFile()) {
                 DependencyStatus status = checkInputFile(file, inputFolder, oldestTarget);
                 if (status != DependencyStatus.NONE) {
                     return status;
                 }
             }
         }
         // If we got to here then we didn't find anything interesting
         return DependencyStatus.NONE;
     }

     private DependencyStatus checkInputFile(File file, InputPath inputFolder,
             long oldestTarget) {
         if (inputFolder.ignores(file)) {
             return DependencyStatus.NONE;
         }

         // if it's a file, remove it from the list of prereqs.
         // This way if files in this folder don't trigger a build we'll have less
         // files to go through manually
         if (mPrereqs.remove(file) == false) {
             // turns out this is a new file!

             if (DEBUG) {
                 System.out.println("NEW FILE: " + file.getAbsolutePath());
             }
             return DependencyStatus.NEW_FILE;
         } else {
             // check the time stamp on this file if it's a file we care about based what the
             // input folder decides.
             if (inputFolder.checksForModification(file)) {
                 if (file.lastModified() > oldestTarget) {
                     if (DEBUG) {
                         System.out.println("UPDATED FILE: " + file.getAbsolutePath());
                     }
                     return DependencyStatus.UPDATED_FILE;
                 }
             }
         }

         return DependencyStatus.NONE;
     }

     /**
      * Check all the prereq files we know about to make sure they're still there, or that they
      * haven't been modified since the last build.
      *
      * @param oldestTarget the time stamp of the oldest output file to compare against.
      *
      * @return the status of the files
      */
     private DependencyStatus checkPrereqFiles(long oldestTarget) {
         // TODO: Optimize for the case of a specific file as inputPath.
         //       We should have a map of filepath to inputpath to quickly search through them?

         // Loop through our prereq files and make sure they still exist
         for (File prereq : mPrereqs) {
             if (prereq.exists() == false) {
                 if (DEBUG) {
                     System.out.println("MISSING FILE: " + prereq.getAbsolutePath());
                 }
                 return DependencyStatus.MISSING_FILE;
             }

             // check the time stamp on this file if it's a file we care about.
             // To know if we care about the file we have to find the matching input.
             if (mNewInputs != null) {
                 String filePath = prereq.getAbsolutePath();
                 for (InputPath input : mNewInputs) {
                     File inputFile = input.getFile();
                     // if the input path is a directory, check if the prereq file is in it,
                     // otherwise check if the prereq file match exactly the input path.
                     if (inputFile.isDirectory()) {
                         if (filePath.startsWith(inputFile.getAbsolutePath())) {
                             // ok file is inside a directory type input folder.
                             // check if we need to check this type of file, and if yes, check it.
                             if (input.checksForModification(prereq)) {
                                 if (prereq.lastModified() > oldestTarget) {
                                     if (DEBUG) {
                                         System.out.println(
                                                 "UPDATED FILE: " + prereq.getAbsolutePath());
                                     }
                                     return DependencyStatus.UPDATED_FILE;
                                 }
                             }
                         }
                     } else {
                         // this is a file input path, we must check if the match is exact.
                         if (prereq.equals(inputFile)) {
                             if (input.checksForModification(prereq)) {
                                 if (prereq.lastModified() > oldestTarget) {
                                     if (DEBUG) {
                                         System.out.println(
                                                 "UPDATED FILE: " + prereq.getAbsolutePath());
                                     }
                                     return DependencyStatus.UPDATED_FILE;
                                 }
                             }
                         }
                     }
                 }
             } else {
                 // no input? we consider all files.
                 if (prereq.lastModified() > oldestTarget) {
                     if (DEBUG) {
                         System.out.println("UPDATED FILE: " + prereq.getAbsolutePath());
                     }
                     return DependencyStatus.UPDATED_FILE;
                 }
             }
         }

         // If we get this far, then all our prereq are okay
         return DependencyStatus.NONE;
     }

     /**
      * Check all the target files we know about to make sure they're still there
      * @return true if any of the target files are missing.
      */
     private boolean missingTargetFile() {
         // Loop through our target files and make sure they still exist
         for (File target : mTargets) {
             if (target.exists() == false) {
                 return true;
             }
         }
         // If we get this far, then all our targets are okay
         return false;
     }

     /**
      * Returns the earliest modification time stamp from all the output targets. If there
      * are no known target, the dependency file time stamp is returned.
      */
     private long getOutputLastModified() {
         // Find the oldest target
         long oldestTarget = Long.MAX_VALUE;
         // if there's no output, then compare to the time of the dependency file.
         if (mTargets.size() == 0) {
             oldestTarget = mDepFileLastModified;
         } else {
             for (File target : mTargets) {
                 if (target.lastModified() < oldestTarget) {
                     oldestTarget = target.lastModified();
                 }
             }
         }

         return oldestTarget;
     }

     /**
      * Reads and returns the content of a text file.
      * @param file the file to read
      * @return null if the file could not be read
      */
     private static List<String> readFile(File file) {
         try {
             return Files.readLines(file, Charsets.UTF_8);
         } catch (IOException e) {
             // return null below
         }

         return null;
     }
 }
	/*
	* Copyright (C) 2011 The Android Open Source Project
	*
	* 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.android.ant;

	import com.google.common.base.Charsets;
	import com.google.common.io.Files;

	import org.apache.tools.ant.BuildException;

	import java.io.File;
	import java.io.IOException;
	import java.util.Collections;
	import java.util.HashSet;
	import java.util.List;
	import java.util.Set;

	/**
	* This class takes care of dependency tracking for all targets and prerequisites listed in
	* a single dependency file. A dependency graph always has a dependency file associated with it
	* for the duration of its lifetime
	*/
	public class DependencyGraph {

	private final static boolean DEBUG = false;

	private static enum DependencyStatus {
	NONE, NEW_FILE, UPDATED_FILE, MISSING_FILE, ERROR;
	}

	// Files that we know about from the dependency file
	private Set<File> mTargets = Collections.emptySet();
	private Set<File> mPrereqs = mTargets;
	private File mFirstPrereq = null;
	private boolean mMissingDepFile = false;
	private long mDepFileLastModified;
	private final List<InputPath> mNewInputs;

	public DependencyGraph(String dependencyFilePath, List<InputPath> newInputPaths) {
	mNewInputs = newInputPaths;
	parseDependencyFile(dependencyFilePath);
	}

	/**
	* Check all the dependencies to see if anything has changed.
	*
	* @param printStatus will print to {@link System#out} the dependencies status.
	* @return true if new prerequisites have appeared, target files are missing or if
	* prerequisite files have been modified since the last target generation.
	*/
	public boolean dependenciesHaveChanged(boolean printStatus) {
	// If no dependency file has been set up, then we'll just return true
	// if we have a dependency file, we'll check to see what's been changed
	if (mMissingDepFile) {
	System.out.println("No Dependency File Found");
	return true;
	}

	// check for missing output first
	if (missingTargetFile()) {
	if (printStatus) {
	System.out.println("Found Deleted Target File");
	}
	return true;
	}

	// get the time stamp of the oldest target.
	long oldestTarget = getOutputLastModified();

	// first look through the input folders and look for new files or modified files.
	DependencyStatus status = checkInputs(oldestTarget);

	// this can't find missing files. This is done later.
	switch (status) {
	case ERROR:
	throw new BuildException();
	case NEW_FILE:
	if (printStatus) {
	System.out.println("Found new input file");
	}
	return true;
	case UPDATED_FILE:
	if (printStatus) {
	System.out.println("Found modified input file");
	}
	return true;
	}

	// now do a full check on the remaining files.
	status = checkPrereqFiles(oldestTarget);
	// this can't find new input files. This is done above.
	switch (status) {
	case ERROR:
	throw new BuildException();
	case MISSING_FILE:
	if (printStatus) {
	System.out.println("Found deleted input file");
	}
	return true;
	case UPDATED_FILE:
	if (printStatus) {
	System.out.println("Found modified input file");
	}
	return true;
	}

	return false;
	}

	public Set<File> getTargets() {
	return Collections.unmodifiableSet(mTargets);
	}

	public File getFirstPrereq() {
	return mFirstPrereq;
	}

	/**
	* Parses the given dependency file and stores the file paths
	*
	* @param dependencyFilePath the dependency file
	*/
	private void parseDependencyFile(String dependencyFilePath) {
	// first check if the dependency file is here.
	File depFile = new File(dependencyFilePath);
	if (depFile.isFile() == false) {
	mMissingDepFile = true;
	return;
	}

	// get the modification time of the dep file as we may need it later
	mDepFileLastModified = depFile.lastModified();

	// Read in our dependency file
	List<String> content = readFile(depFile);
	if (content == null) {
	System.err.println("ERROR: Couldn't read " + dependencyFilePath);
	return;
	}

	// The format is something like:
	// output1 output2 [...]: dep1 dep2 [...]
	// expect it's likely split on several lines. So let's move it back on a single line
	// first
	StringBuilder sb = new StringBuilder();
	for (String line : content) {
	line = line.trim();
	if (line.endsWith("\\")) {
	line = line.substring(0, line.length() - 1);
	}
	sb.append(line);
	}

	// split the left and right part
	String[] files = sb.toString().split(":");

	// get the target files:
	String[] targets = files[0].trim().split(" ");

	String[] prereqs = {};
	// Check to make sure our dependency file is okay
	if (files.length < 1) {
	System.err.println(
	"Warning! Dependency file does not list any prerequisites after ':' ");
	} else {
	// and the prerequisite files:
	prereqs = files[1].trim().split(" ");
	}

	mTargets = new HashSet<File>(targets.length);
	for (String path : targets) {
	if (path.length() > 0) {
	mTargets.add(new File(path));
	}
	}

	mPrereqs = new HashSet<File>(prereqs.length);
	for (String path : prereqs) {
	if (path.length() > 0) {
	if (DEBUG) {
	System.out.println("PREREQ: " + path);
	}
	File f = new File(path);
	if (mFirstPrereq == null) {
	mFirstPrereq = f;
	}
	mPrereqs.add(f);
	}
	}
	}

	/**
	* Check all the input files and folders to see if there have been new
	* files added to them or if any of the existing files have been modified.
	*
	* This looks at the input paths, not at the list of known prereq. Therefore this
	* will not find missing files. It will however remove processed files from the
	* prereq file list so that we can process those in a 2nd step.
	*
	* This should be followed by a call to {@link #checkPrereqFiles(long)} which
	* will process the remaining files in the prereq list.
	*
	* If a change is found, this will return immediately with either
	* {@link DependencyStatus#NEW_FILE} or {@link DependencyStatus#UPDATED_FILE}.
	*
	* @param oldestTarget the timestamp of the oldest output file to compare against.
	*
	* @return the status of the file in the watched folders.
	*
	*/
	private DependencyStatus checkInputs(long oldestTarget) {
	if (mNewInputs != null) {
	for (InputPath input : mNewInputs) {
	File file = input.getFile();
	if (file.isDirectory()) {
	DependencyStatus status = checkInputFolder(file, input, oldestTarget);
	if (status != DependencyStatus.NONE) {
	return status;
	}
	} else if (file.isFile()) {
	DependencyStatus status = checkInputFile(file, input, oldestTarget);
	if (status != DependencyStatus.NONE) {
	return status;
	}
	}
	}
	}

	// If we make it all the way through our directories we're good.
	return DependencyStatus.NONE;
	}

	/**
	* Check all the files in the tree under root and check to see if the files are
	* listed under the dependencies, or if they have been modified. Recurses into subdirs.
	*
	* @param folder the folder to search through.
	* @param inputFolder the root level inputFolder
	* @param oldestTarget the time stamp of the oldest output file to compare against.
	*
	* @return the status of the file in the folder.
	*/
	private DependencyStatus checkInputFolder(File folder, InputPath inputFolder,
	long oldestTarget) {
	if (inputFolder.ignores(folder)) {
	return DependencyStatus.NONE;
	}

	File[] files = folder.listFiles();
	if (files == null) {
	System.err.println("ERROR " + folder.toString() + " is not a dir or can't be read");
	return DependencyStatus.ERROR;
	}
	// Loop through files in this folder
	for (File file : files) {
	// If this is a directory, recurse into it
	if (file.isDirectory()) {
	DependencyStatus status = checkInputFolder(file, inputFolder, oldestTarget);
	if (status != DependencyStatus.NONE) {
	return status;
	}
	} else if (file.isFile()) {
	DependencyStatus status = checkInputFile(file, inputFolder, oldestTarget);
	if (status != DependencyStatus.NONE) {
	return status;
	}
	}
	}
	// If we got to here then we didn't find anything interesting
	return DependencyStatus.NONE;
	}

	private DependencyStatus checkInputFile(File file, InputPath inputFolder,
	long oldestTarget) {
	if (inputFolder.ignores(file)) {
	return DependencyStatus.NONE;
	}

	// if it's a file, remove it from the list of prereqs.
	// This way if files in this folder don't trigger a build we'll have less
	// files to go through manually
	if (mPrereqs.remove(file) == false) {
	// turns out this is a new file!

	if (DEBUG) {
	System.out.println("NEW FILE: " + file.getAbsolutePath());
	}
	return DependencyStatus.NEW_FILE;
	} else {
	// check the time stamp on this file if it's a file we care about based what the
	// input folder decides.
	if (inputFolder.checksForModification(file)) {
	if (file.lastModified() > oldestTarget) {
	if (DEBUG) {
	System.out.println("UPDATED FILE: " + file.getAbsolutePath());
	}
	return DependencyStatus.UPDATED_FILE;
	}
	}
	}

	return DependencyStatus.NONE;
	}

	/**
	* Check all the prereq files we know about to make sure they're still there, or that they
	* haven't been modified since the last build.
	*
	* @param oldestTarget the time stamp of the oldest output file to compare against.
	*
	* @return the status of the files
	*/
	private DependencyStatus checkPrereqFiles(long oldestTarget) {
	// TODO: Optimize for the case of a specific file as inputPath.
	// We should have a map of filepath to inputpath to quickly search through them?

	// Loop through our prereq files and make sure they still exist
	for (File prereq : mPrereqs) {
	if (prereq.exists() == false) {
	if (DEBUG) {
	System.out.println("MISSING FILE: " + prereq.getAbsolutePath());
	}
	return DependencyStatus.MISSING_FILE;
	}

	// check the time stamp on this file if it's a file we care about.
	// To know if we care about the file we have to find the matching input.
	if (mNewInputs != null) {
	String filePath = prereq.getAbsolutePath();
	for (InputPath input : mNewInputs) {
	File inputFile = input.getFile();
	// if the input path is a directory, check if the prereq file is in it,
	// otherwise check if the prereq file match exactly the input path.
	if (inputFile.isDirectory()) {
	if (filePath.startsWith(inputFile.getAbsolutePath())) {
	// ok file is inside a directory type input folder.
	// check if we need to check this type of file, and if yes, check it.
	if (input.checksForModification(prereq)) {
	if (prereq.lastModified() > oldestTarget) {
	if (DEBUG) {
	System.out.println(
	"UPDATED FILE: " + prereq.getAbsolutePath());
	}
	return DependencyStatus.UPDATED_FILE;
	}
	}
	}
	} else {
	// this is a file input path, we must check if the match is exact.
	if (prereq.equals(inputFile)) {
	if (input.checksForModification(prereq)) {
	if (prereq.lastModified() > oldestTarget) {
	if (DEBUG) {
	System.out.println(
	"UPDATED FILE: " + prereq.getAbsolutePath());
	}
	return DependencyStatus.UPDATED_FILE;
	}
	}
	}
	}
	}
	} else {
	// no input? we consider all files.
	if (prereq.lastModified() > oldestTarget) {
	if (DEBUG) {
	System.out.println("UPDATED FILE: " + prereq.getAbsolutePath());
	}
	return DependencyStatus.UPDATED_FILE;
	}
	}
	}

	// If we get this far, then all our prereq are okay
	return DependencyStatus.NONE;
	}

	/**
	* Check all the target files we know about to make sure they're still there
	* @return true if any of the target files are missing.
	*/
	private boolean missingTargetFile() {
	// Loop through our target files and make sure they still exist
	for (File target : mTargets) {
	if (target.exists() == false) {
	return true;
	}
	}
	// If we get this far, then all our targets are okay
	return false;
	}

	/**
	* Returns the earliest modification time stamp from all the output targets. If there
	* are no known target, the dependency file time stamp is returned.
	*/
	private long getOutputLastModified() {
	// Find the oldest target
	long oldestTarget = Long.MAX_VALUE;
	// if there's no output, then compare to the time of the dependency file.
	if (mTargets.size() == 0) {
	oldestTarget = mDepFileLastModified;
	} else {
	for (File target : mTargets) {
	if (target.lastModified() < oldestTarget) {
	oldestTarget = target.lastModified();
	}
	}
	}

	return oldestTarget;
	}

	/**
	* Reads and returns the content of a text file.
	* @param file the file to read
	* @return null if the file could not be read
	*/
	private static List<String> readFile(File file) {
	try {
	return Files.readLines(file, Charsets.UTF_8);
	} catch (IOException e) {
	// return null below
	}

	return null;
	}
	}