FileSnapshot.java
/*
* Copyright (C) 2010, Google Inc.
* and other copyright owners as documented in the project's IP log.
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* This program and the accompanying materials are made available
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package org.eclipse.jgit.internal.storage.file;
import java.io.File;
import java.io.IOException;
import java.nio.file.attribute.BasicFileAttributes;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import org.eclipse.jgit.util.FS;
/**
* Caches when a file was last read, making it possible to detect future edits.
* <p>
* This object tracks the last modified time of a file. Later during an
* invocation of {@link #isModified(File)} the object will return true if the
* file may have been modified and should be re-read from disk.
* <p>
* A snapshot does not "live update" when the underlying filesystem changes.
* Callers must poll for updates by periodically invoking
* {@link #isModified(File)}.
* <p>
* To work around the "racy git" problem (where a file may be modified multiple
* times within the granularity of the filesystem modification clock) this class
* may return true from isModified(File) if the last modification time of the
* file is less than 3 seconds ago.
*/
public class FileSnapshot {
/**
* An unknown file size.
*
* This value is used when a comparison needs to happen purely on the lastUpdate.
*/
public static final long UNKNOWN_SIZE = -1;
/**
* A FileSnapshot that is considered to always be modified.
* <p>
* This instance is useful for application code that wants to lazily read a
* file, but only after {@link #isModified(File)} gets invoked. The returned
* snapshot contains only invalid status information.
*/
public static final FileSnapshot DIRTY = new FileSnapshot(-1, -1, UNKNOWN_SIZE);
/**
* A FileSnapshot that is clean if the file does not exist.
* <p>
* This instance is useful if the application wants to consider a missing
* file to be clean. {@link #isModified(File)} will return false if the file
* path does not exist.
*/
public static final FileSnapshot MISSING_FILE = new FileSnapshot(0, 0, 0) {
@Override
public boolean isModified(File path) {
return FS.DETECTED.exists(path);
}
};
/**
* Record a snapshot for a specific file path.
* <p>
* This method should be invoked before the file is accessed.
*
* @param path
* the path to later remember. The path's current status
* information is saved.
* @return the snapshot.
*/
public static FileSnapshot save(File path) {
long read = System.currentTimeMillis();
long modified;
long size;
try {
BasicFileAttributes fileAttributes = FS.DETECTED.fileAttributes(path);
modified = fileAttributes.lastModifiedTime().toMillis();
size = fileAttributes.size();
} catch (IOException e) {
modified = path.lastModified();
size = path.length();
}
return new FileSnapshot(read, modified, size);
}
/**
* Record a snapshot for a file for which the last modification time is
* already known.
* <p>
* This method should be invoked before the file is accessed.
*
* @param modified
* the last modification time of the file
* @return the snapshot.
*/
public static FileSnapshot save(long modified) {
final long read = System.currentTimeMillis();
return new FileSnapshot(read, modified, -1);
}
/** Last observed modification time of the path. */
private final long lastModified;
/** Last wall-clock time the path was read. */
private volatile long lastRead;
/** True once {@link #lastRead} is far later than {@link #lastModified}. */
private boolean cannotBeRacilyClean;
/** Underlying file-system size in bytes.
*
* When set to {@link #UNKNOWN_SIZE} the size is not considered for modification checks. */
private final long size;
private FileSnapshot(long read, long modified, long size) {
this.lastRead = read;
this.lastModified = modified;
this.cannotBeRacilyClean = notRacyClean(read);
this.size = size;
}
/**
* Get time of last snapshot update
*
* @return time of last snapshot update
*/
public long lastModified() {
return lastModified;
}
/**
* @return file size in bytes of last snapshot update
*/
public long size() {
return size;
}
/**
* Check if the path may have been modified since the snapshot was saved.
*
* @param path
* the path the snapshot describes.
* @return true if the path needs to be read again.
*/
public boolean isModified(File path) {
long currLastModified;
long currSize;
try {
BasicFileAttributes fileAttributes = FS.DETECTED.fileAttributes(path);
currLastModified = fileAttributes.lastModifiedTime().toMillis();
currSize = fileAttributes.size();
} catch (IOException e) {
currLastModified = path.lastModified();
currSize = path.length();
}
return (currSize != UNKNOWN_SIZE && currSize != size) || isModified(currLastModified);
}
/**
* Update this snapshot when the content hasn't changed.
* <p>
* If the caller gets true from {@link #isModified(File)}, re-reads the
* content, discovers the content is identical, and
* {@link #equals(FileSnapshot)} is true, it can use
* {@link #setClean(FileSnapshot)} to make a future
* {@link #isModified(File)} return false. The logic goes something like
* this:
*
* <pre>
* if (snapshot.isModified(path)) {
* FileSnapshot other = FileSnapshot.save(path);
* Content newContent = ...;
* if (oldContent.equals(newContent) && snapshot.equals(other))
* snapshot.setClean(other);
* }
* </pre>
*
* @param other
* the other snapshot.
*/
public void setClean(FileSnapshot other) {
final long now = other.lastRead;
if (notRacyClean(now))
cannotBeRacilyClean = true;
lastRead = now;
}
/**
* Compare two snapshots to see if they cache the same information.
*
* @param other
* the other snapshot.
* @return true if the two snapshots share the same information.
*/
public boolean equals(FileSnapshot other) {
return lastModified == other.lastModified;
}
/** {@inheritDoc} */
@Override
public boolean equals(Object other) {
if (other instanceof FileSnapshot)
return equals((FileSnapshot) other);
return false;
}
/** {@inheritDoc} */
@Override
public int hashCode() {
// This is pretty pointless, but override hashCode to ensure that
// x.hashCode() == y.hashCode() when x.equals(y) is true.
//
return (int) lastModified;
}
/** {@inheritDoc} */
@Override
public String toString() {
if (this == DIRTY)
return "DIRTY"; //$NON-NLS-1$
if (this == MISSING_FILE)
return "MISSING_FILE"; //$NON-NLS-1$
DateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS", //$NON-NLS-1$
Locale.US);
return "FileSnapshot[modified: " + f.format(new Date(lastModified)) //$NON-NLS-1$
+ ", read: " + f.format(new Date(lastRead)) + "]"; //$NON-NLS-1$ //$NON-NLS-2$
}
private boolean notRacyClean(long read) {
// The last modified time granularity of FAT filesystems is 2 seconds.
// Using 2.5 seconds here provides a reasonably high assurance that
// a modification was not missed.
//
return read - lastModified > 2500;
}
private boolean isModified(long currLastModified) {
// Any difference indicates the path was modified.
//
if (lastModified != currLastModified)
return true;
// We have already determined the last read was far enough
// after the last modification that any new modifications
// are certain to change the last modified time.
//
if (cannotBeRacilyClean)
return false;
if (notRacyClean(lastRead)) {
// Our last read should have marked cannotBeRacilyClean,
// but this thread may not have seen the change. The read
// of the volatile field lastRead should have fixed that.
//
return false;
}
// We last read this path too close to its last observed
// modification time. We may have missed a modification.
// Scan again, to ensure we still see the same state.
//
return true;
}
}