eclair snapshot

services/java/com/android/server/JournaledFile.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright (C) 2009 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.server;
+
+import java.io.File;
+import java.io.IOException;
+
+public class JournaledFile {
+    File mReal;
+    File mTemp;
+    boolean mWriting;
+
+    public JournaledFile(File real, File temp) {
+        mReal = real;
+        mTemp = temp;
+    }
+
+    /** Returns the file for you to read.
+     * @more
+     * Prefers the real file.  If it doesn't exist, uses the temp one, and then copies
+     * it to the real one.  If there is both a real file and a temp one, assumes that the
+     * temp one isn't fully written and deletes it.
+     */
+    public File chooseForRead() {
+        File result;
+        if (mReal.exists()) {
+            result = mReal;
+            if (mTemp.exists()) {
+                mTemp.delete();
+            }
+        } else if (mTemp.exists()) {
+            result = mTemp;
+            mTemp.renameTo(mReal);
+        } else {
+            return mReal;
+        }
+        return result;
+    }
+
+    /**
+     * Returns a file for you to write.
+     * @more
+     * If a write is already happening, throws.  In other words, you must provide your
+     * own locking.
+     * <p>
+     * Call {@link #commit} to commit the changes, or {@link #rollback} to forget the changes.
+     */
+    public File chooseForWrite() {
+        if (mWriting) {
+            throw new IllegalStateException("uncommitted write already in progress");
+        }
+        if (!mReal.exists()) {
+            // If the real one doesn't exist, it's either because this is the first time
+            // or because something went wrong while copying them.  In this case, we can't
+            // trust anything that's in temp.  In order to have the chooseForRead code not
+            // use the temporary one until it's fully written, create an empty file
+            // for real, which will we'll shortly delete.
+            try {
+                mReal.createNewFile();
+            } catch (IOException e) {
+                // Ignore
+            }
+        }
+
+        if (mTemp.exists()) {
+            mTemp.delete();
+        }
+        mWriting = true;
+        return mTemp;
+    }
+
+    /**
+     * Commit changes.
+     */
+    public void commit() {
+        if (!mWriting) {
+            throw new IllegalStateException("no file to commit");
+        }
+        mWriting = false;
+        mTemp.renameTo(mReal);
+    }
+
+    /**
+     * Roll back changes.
+     */
+    public void rollback() {
+        if (!mWriting) {
+            throw new IllegalStateException("no file to roll back");
+        }
+        mWriting = false;
+        mTemp.delete();
+    }
+}