Index: android/java/src/org/chromium/base/README_logging.md |
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-## Logging ## |
- |
-Logging used to be done using Android's [android.util.Log] |
-(http://developer.android.com/reference/android/util/Log.html). |
- |
-A wrapper on that is now available: org.chromium.base.Log. It is designed to write logs as |
-belonging to logical groups going beyond single classes, and to make it easy to switch logging on |
-or off for individual groups. |
- |
-Usage: |
- |
- private static final String TAG = "cr.YourModuleTag"; |
- ... |
- Log.i(TAG, "Logged INFO message."); |
- Log.d(TAG, "Some DEBUG info: %s", data); |
- |
-Output: |
- |
- I/cr.YourModuleTag: ( 999): Logged INFO message |
- D/cr.YourModuleTag: ( 999): [MyClass.java:42] Some DEBUG info: data's toString output |
- |
-Here, **TAG** will be a feature or package name, "MediaRemote" or "NFC" for example. In most |
-cases, the class name is not needed. |
- |
-**Caveat:** Property keys are limited to 23 characters. If the tag is too long, `Log#isLoggable` |
-throws a RuntimeException. |
- |
-### Verbose and Debug logs have special handling ### |
- |
-* `Log.v` and `Log.d` Calls made using `org.chromium.base.Log` are stripped |
- out of production binaries using Proguard. There is no way to get those logs |
- in release builds. |
- |
-* The file name and line number will be prepended to the log message. |
- For higher priority logs, those are not added for performance concerns. |
- |
-* By default, Verbose and Debug logs are not enabled, see guarding: |
- |
-### Log calls are guarded: Tag groups can be enabled or disabled using ADB ### |
- |
- adb shell setprop log.tag.cr.YourModuleTag <LEVEL> |
- |
-Level here is either `VERBOSE`, `DEBUG`, `INFO`, `WARN`, `ERROR`, `ASSERT`, or `SUPPRESS` |
-By default, the level for all tags is `INFO`. |
- |
-### An exception trace is printed when the exception is the last parameter ### |
- |
-As with `java.util.Log`, putting a throwable as last parameter will dump the corresponding stack |
-trace: |
- |
- Log.i(TAG, "An error happened: %s", e) |
- |
- I/cr.YourModuleTag: ( 999): An error happened: This is the exception's message |
- I/cr.YourModuleTag: ( 999): java.lang.Exception: This is the exception's message |
- I/cr.YourModuleTag: ( 999): at foo.bar.MyClass.test(MyClass.java:42) |
- I/cr.YourModuleTag: ( 999): ... |
- |
-Having the exception as last parameter doesn't prevent it from being used for string formatting. |
- |
-### Logging Best Practices |
- |
-#### Rule #1: Never log PII (Personal Identification Information): |
- |
-This is a huge concern, because other applications can access the log and extract a lot of data |
-from your own by doing so. Even if JellyBean restricted this, people are going to run your |
-application on rooted devices and allow some apps to access it. Also anyone with USB access to the |
-device can use ADB to get the full logcat and get the same data right now. |
- |
-If you really need to print something , print a series of Xs instead (e.g. "XXXXXX"), or print a |
-truncated hash of the PII instead. Truncation is required to make it harder for an attacker to |
-recover the full data through rainbow tables and similar methods. |
- |
-Similarly, avoid dumping API keys, cookies, etc... |
- |
-#### Rule #2: Do not write debug logs in production code: |
- |
-The kernel log buffer is global and of limited size. Any extra debug log you add to your activity |
-or service makes it more difficult to diagnose problems on other parts of the system, because they |
-tend to push the interesting bit out of the buffer too soon. This is a recurring problem on |
-Android, so avoid participating into it. |
- |
-Logs can be disabled using system properties. Because log messages might not be |
-written, the cost of creating them should also be avoided. This can be done using three |
-complementary ways: |
- |
-- Use string formatting instead of concatenations |
- |
- // BAD |
- Log.d(TAG, "I " + preference + " writing logs."); |
- |
- // BETTER |
- Log.d(TAG, "I %s writing logs.", preference); |
- |
- If logging is disabled, the function's arguments will still have to be computed and provided |
- as input. The first call above will always lead to the creation of a `StringBuilder` and a few |
- concatenations, while the second just passes the arguments and won't need that. |
- |
-- Guard expensive calls |
- |
- Sometimes the values to log aren't readily available and need to be computed specially. This |
- should be avoided when logging is disabled. |
- |
- Using `Log#isLoggable` will return whether logging for a specific tag is allowed or not. It is |
- the call used inside the log functions and using allows to know when running the expensive |
- functions is needed. |
- |
- if (Log.isLoggable(TAG, Log.DEBUG) { |
- Log.d(TAG, "Something happened: %s", dumpDom(tab)); |
- } |
- |
- For more info, See the [android framework documentation] |
- (http://developer.android.com/tools/debugging/debugging-log.html). |
- |
- Using a debug constant is a less flexible, but more perfomance oriented alternative. |
- |
- static private final boolean DEBUG = false; // set to 'true' to enable debug |
- ... |
- if (DEBUG) { |
- Log.i(TAG, createThatExpensiveLogMessage(activity)) |
- } |
- |
- Because the variable is a `static final` that can be evaluated at compile time, the Java |
- compiler will optimize out all guarded calls from the generated `.class` file. Changing it |
- however requires editing each of the files for which debug should be enabled and recompiling, |
- while the previous method can enable or disable debugging for a whole feature without changing |
- any source file. |
- |
-- Annotate debug functions with the `@RemovableInRelease` annotation. |
- |
- That annotation tells Proguard to assume that a given function has no side effects, and is |
- called only for its returned value. If this value is unused, the call will be removed. If the |
- function is not called at all, it will also be removed. Since Proguard is already used to |
- strip debug and verbose calls out of release builds, this annotation allows it to have a |
- deeper action by removing also function calls used to generate the log call's arguments. |
- |
- /* If that function is only used in Log.d calls, proguard should completely remove it from |
- * the release builds. */ |
- @RemovableInRelease |
- private static String getSomeDebugLogString(Thing[] things) { |
- /* Still needs to be guarded to avoid impacting debug builds, or in case it's used for |
- * some other log levels. But at least it is done only once, inside the function. */ |
- if (!Log.isLoggable(TAG, Log.DEBUG)) return null; |
- |
- StringBuilder sb = new StringBuilder("Reporting " + thing.length + " things:"); |
- for (Thing thing : things) { |
- sb.append('\n').append(thing.id).append(' ').append(report.foo); |
- } |
- return sb.toString(); |
- } |
- |
- public void bar() { |
- ... |
- Log.d(TAG, getSomeDebugLogString(things)); /* In debug builds, the function does nothing |
- * is debug is disabled, and the entire line |
- * is removed in release builds. */ |
- } |
- |
- Again, this is useful only if the input to that function are variables already available in |
- the scope. The idea is to move computations, concatenations, etc. to a place where that can be |
- removed when not needed, without invading the main function's logic. |
- |
-#### Rule #3: Favor small log messages |
- |
-This is still related to the global fixed-sized kernel buffer used to keep all logs. Try to make |
-your log information as terse as possible. This reduces the risk of pushing interesting log data |
-out of the buffer when something really nasty happens. It's really better to have a single-line |
-log message, than several ones. I.e. don't use: |
- |
- Log.GROUP.d(TAG, "field1 = %s", value1); |
- Log.GROUP.d(TAG, "field2 = %s", value2); |
- Log.GROUP.d(TAG, "field3 = %s", value3); |
- |
-Instead, write this as: |
- |
- Log.d(TAG, "field1 = %s, field2 = %s, field3 = %s", value1, value2, value3); |
- |
-That doesn't seem to be much different if you count overall character counts, but each independent |
-log entry also implies a small, but non-trivial header, in the kernel log buffer. |
-And since every byte count, you can also try something even shorter, as in: |
- |
- Log.d(TAG, "fields [%s,%s,%s]", value1, value2, value3); |