New in CodeNarc 0.13
Busy waiting (forcing a Thread.sleep() while waiting on a condition) should be avoided. Prefer using the gate and barrier objects in the java.util.concurrent package.
Example of violations:
while (x) { Thread.sleep(1000) } while (x) { Thread.sleep(1000) { /* interruption handler */} } for (int x = 10; x; x--) { sleep(1000) // sleep is added to Object in Groovy } // here is the proper way to wait: countDownLatch.await() // this is weird code to write, but does not cause a violation for (def x : collections) { sleep(1000) } while (x) { // you should use a lock here, but technically you are // not just busy waiting because you are doing other work doSomething() sleep(1000) }
New in CodeNarc 0.13
This rule detects double checked locking, where a 'lock hint' is tested for null before initializing an object within a synchronized block. Double checked locking does not guarantee correctness and is an anti-pattern.
A full explanation of why double checked locking is broken in Java is available on Wikipedia:
Example of violations:
if (object == null) { synchronized(this) { if (object == null) { // createObject() could be called twice depending // on the Thread Scheduler. object = createObject() } } } // there are several idioms to fix this problem. def result = object; if (result == null) { synchronized(this) { result = object; if (result == null) object = result = createObject() } } // and a better solution for a singleton: class myClass { private static class ObjectHolder { public static Object object = createObject() } public static Object getObject() { return ObjectHolder.object; } }
New in CodeNarc 0.13
Class contains similarly-named get and set methods where one method of the pair is marked either @WithReadLock or @WithWriteLock and the other is not locked at all. This may result in incorrect behavior at runtime, as callers of the get and set methods will not necessarily lock correctly and my see an inconsistent state for the object. The get and set method should both be guarded by @WithReadLock/@WithWriteLock or neither should be guarded.
Example of violations:
class Person { String name Date birthday boolean deceased boolean parent @WithWriteLock setName(String name) { this.name = name } // violation, get method should be locked String getName() { name } // violation, set method should be locked void setBirthday(Date birthday) { this.birthday = birthday } @WithReadLock String getBirthday() { birthday } // violation, set method should be locked void setDeceased(boolean deceased) { this.deceased = deceased } @WithReadLock boolean isDeceased() { deceased } @WithWriteLock void setParent(boolean parent) { this.parent = parent } // violation, get method should be locked boolean isParent() { parent } }
New in CodeNarc 0.13
Class contains similarly-named get and set methods where the set method is synchronized and the get method is not, or the get method is synchronized and the set method is not. This may result in incorrect behavior at runtime, as callers of the get and set methods will not necessarily see a consistent state for the object. The get and set method should both be synchronized or neither should be synchronized.
Example of violations:
class Person { String name Date birthday boolean deceased boolean parent synchronized setName(String name) { this.name = name } // violation, get method should be synchronized String getName() { name } // violation, set method should be synchronized void setBirthday(Date birthday) { this.birthday = birthday } synchronized String getBirthday() { birthday } // violation, set method should be synchronized void setDeceased(boolean deceased) { this.deceased = deceased } synchronized boolean isDeceased() { deceased } synchronized void setParent(boolean parent) { this.parent = parent } // violation, get method should be synchronized boolean isParent() { parent } }
This rule reports occurrences of nested synchronized statements.
Nested synchronized statements should be avoided. Nested synchronized statements are either useless (if the lock objects are identical) or prone to deadlock.
Note that a closure or an anonymous inner class carries its own context (scope). A synchronized statement within a closure or an anonymous inner class defined within an outer synchronized statement does not cause a violation (though nested synchronized statements within either of those will).
Here is an example of code that produces a violation:
def myMethod() { synchronized(this) { // do something ... synchronized(this) { // do something else ... } } }
New in CodeNarc 0.13
Calendar objects should not be used as static fields. Calendars are inherently unsafe for multithreaded use. Sharing a single instance across thread boundaries without proper synchronization will result in erratic behavior of the application. Under 1.4 problems seem to surface less often than under Java 5 where you will probably see random ArrayIndexOutOfBoundsExceptions or IndexOutOfBoundsExceptions in sun.util.calendar.BaseCalendar.getCalendarDateFromFixedDate(). You may also experience serialization problems. Using an instance field or a ThreadLocal is recommended.
For more information on this see Sun Bug #6231579 and Sun Bug #6178997.
Examples:
// two violations class MyClass { static Calendar calendar1 static java.util.Calendar calendar2 } // these usages are OK class MyCorrectClass { private final Calendar calendar1 static ThreadLocal<Calendar> calendar2 }
New in CodeNarc 0.13
DateFormat objects should not be used as static fields. DateFormat are inherently unsafe for multithreaded use. Sharing a single instance across thread boundaries without proper synchronization will result in erratic behavior of the application. Under 1.4 problems seem to surface less often than under Java 5 where you will probably see random ArrayIndexOutOfBoundsExceptions or IndexOutOfBoundsExceptions in sun.util.calendar.BaseCalendar.getCalendarDateFromFixedDate(). You may also experience serialization problems. Using an instance field or a ThreadLocal is recommended.
For more information on this see Sun Bug #6231579 and Sun Bug #6178997.
Examples:
// two violations class MyClass { static DateFormat dateFormat1 static java.text.DateFormat dateFormat2 } // these usages are OK class MyCorrectClass { private DateFormat calendar1 static ThreadLocal<DateFormat> calendar2 }
New in CodeNarc 0.13
Matcher objects should not be used as static fields. Calendars are inherently unsafe for multithreaded use. Sharing a single instance across thread boundaries without proper synchronization will result in erratic behavior of the application.
Example of violations:
// two violations class MyClass { static Matcher matcher1 static java.util.regex.Matcher matcher2 } // these usages are OK class MyCorrectClass { private Matcher matcher1 static ThreadLocal<Matcher> matcher2 }
This rule reports uses of the synchronized keyword on methods. Synchronized methods are the same as synchronizing on 'this', which effectively make your synchronization policy public and modifiable by other objects. To avoid possibilities of deadlock, it is better to synchronize on internal objects.
Here is an example of code that produces a violation:
synchronized def myMethod() { // do stuff ... }
Since CodeNarc 0.11
Checks for synchronization on getClass() rather than class literal. This instance method synchronizes on this.getClass(). If this class is subclassed, subclasses will synchronize on the class object for the subclass, which isn't likely what was intended.
New in CodeNarc 0.13
The code synchronizes on a boxed primitive constant, such as an Integer. Since Integer objects can be cached and shared, this code could be synchronizing on the same object as other, unrelated code, leading to unresponsiveness and possible deadlock.
Example of violations:
class MyClass { Byte byte1 = 100 Short short1 = 1 Double double1 = 1 Integer integer1 = 1 Long long1 = 1 Float float1 = 1 Character char1 = 1 byte byte2 = getValue() short short2 = getValue() double double2 = getValue() int integer2 = getValue() long long2 = getValue() float float2 = getValue() char char2 = getValue() def byte3 = new Byte((byte)100) def short3 = new Short((short)1) def double3 = new Double((double)1) def integer3 = new Integer(1) def long3 = new Long(1) def float3 = new Float(1) def char3 = new Character((char)'1') def byte4 = 1 as byte def short4 = 1 as short def double4 = 1 as double def integer4 = 1 as int def long4 = 1 as long def float4 = 1 as float def char4 = 1 as char def byte5 = 1 as Byte def short5 = 1 as Short def double5 = 1 as Double def integer5 = 1 as Integer def long5 = 1 as Long def float5 = 1 as Float def char5 = 1 as Character def byte6 = (byte)1 def short6 = (short)1 def double6 = (double)1 def integer6 = (int)1 def long6 = (long)1 def float6 = (float)1 def char6 = (char)1 def method() { // all of these synchronization blocks produce violations synchronized(byte1) {} synchronized(short1) {} synchronized(double1) {} synchronized(integer1) {} synchronized(long1) {} synchronized(float1) {} synchronized(char1) {} synchronized(byte2) {} synchronized(short2) {} synchronized(double2) {} synchronized(integer2) {} synchronized(long2) {} synchronized(float2) {} synchronized(char2) {} synchronized(byte3) {} synchronized(short3) {} synchronized(double3) {} synchronized(integer3) {} synchronized(long3) {} synchronized(float3) {} synchronized(char3) {} synchronized(byte4) {} synchronized(short4) {} synchronized(double4) {} synchronized(integer4) {} synchronized(long4) {} synchronized(float4) {} synchronized(char4) {} synchronized(byte5) {} synchronized(short5) {} synchronized(double5) {} synchronized(integer5) {} synchronized(long5) {} synchronized(float5) {} synchronized(char5) {} synchronized(byte6) {} synchronized(short6) {} synchronized(double6) {} synchronized(integer6) {} synchronized(long6) {} synchronized(float6) {} synchronized(char6) {} } }
And here is an in-depth example of how it works within inner classes and such:
class MyClass { final String lock = false def method() { // violation synchronized(lock) { } } } class MyClass { final String lock = false class MyInnerClass { def method() { // violation synchronized(lock) { } } } } class MyClass { // implicit typing final def lock = true def method() { // violation synchronized(lock) { } } } class MyClass { // implicit typing final def lock = new Object[0] // correct idiom def method() { return new Runnable() { final def lock = false // shadows parent from inner class public void run() { // violation synchronized(stringLock) { } } } } } class MyClass { // implicit typing final def lock = new Object[0] // correct idiom class MyInnerClass { final def lock = true // shadows parent from inner class def method() { // violation synchronized(stringLock) { } } } }
New in CodeNarc 0.13
Synchronization on a String field can lead to deadlock. Constant Strings are interned and shared across all other classes loaded by the JVM. Thus, this could is locking on something that other code might also be locking. This could result in very strange and hard to diagnose blocking and deadlock behavior.
See JETTY-352.
Examples:
class MyClass { final String stringLock = "stringLock" def method() { // violation synchronized(stringLock) { } } } class MyClass { final String stringLock = "stringLock" class MyInnerClass { def method() { synchronized(stringLock) { } } } } class MyClass { // implicit typing final def stringLock = "stringLock" def method() { // violation synchronized(stringLock) { } } } class MyClass { // implicit typing final def lock = new Object[0] // correct idiom def method() { return new Runnable() { final def lock = "" // shadows parent from inner class public void run() { // violation synchronized(stringLock) { } } } } } class MyClass { // implicit typing final def lock = new Object[0] // correct idiom class MyInnerClass { final def lock = "" // shadows parent from inner class def method() { // violation synchronized(stringLock) { } } } }
This rule reports uses of the synchronized blocks where the synchronization reference is 'this'. Doing this effectively makes your synchronization policy public and modifiable by other objects. To avoid possibilities of deadlock, it is better to synchronize on internal objects.
Here is an example of code that produces a violation:
def method3() { synchronized(this) { // do stuff ... } }
New in CodeNarc 0.13
Catches Serializable classes that define a synchronized readObject method. By definition, an object created by deserialization is only reachable by one thread, and thus there is no need for readObject() to be synchronized. If the readObject() method itself is causing the object to become visible to another thread, that is an example of very dubious coding style.
Examples:
class MyClass implements Serializable { private synchronized void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException { // violation, no need to synchronized } } class MyClass implements Serializable { private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException { synchronized(lock) { // violation, no need to synchronized } } } // OK, class not Serializable class MyClass { private synchronized void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException { } } // OK, class not Serializable class MyClass { private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException { synchronized(lock) { } } } class MyClass implements Serializable { private void readObject(ObjectInputStream input) throws IOException, ClassNotFoundException { // OK, this block is more than just a simple sync statement synchronized(lock) { } doSomething() } }
New in CodeNarc 0.13
Synchronizing on a ReentrantLock field is almost never the intended usage. A ReentrantLock should be obtained using the lock() method and released in a finally block using the unlock() method.
This rule take from Alex Miller's Java Concurrency in Practice slides.
Here is the proper usage of ReentrantLock:
import java.util.concurrent.locks.ReentrantLock; final lock = new ReentrantLock(); def method() { //Trying to enter the critical section lock.lock(); // will wait until this thread gets the lock try { // critical section } finally { //releasing the lock so that other threads can get notifies lock.unlock(); } }
Example of violations:
class MyClass { final ReentrantLock lock = new ReentrantLock() def method() { // violation synchronized(lock) { } } } class MyClass { final ReentrantLock lock = new ReentrantLock() class MyInnerClass { def method() { synchronized(lock) { } } } } class MyClass { // implicit typing final def lock = new ReentrantLock() def method() { // violation synchronized(lock) { } } } class MyClass { // implicit typing final def lock = new Object[0] // correct idiom def method() { return new Runnable() { final def lock = new ReentrantLock() // shadows parent from inner class public void run() { // violation synchronized(lock) { } } } } } class MyClass { // implicit typing final def lock = new Object[0] // correct idiom class MyInnerClass { final def lock = new ReentrantLock() // shadows parent from inner class def method() { // violation synchronized(lock) { } } } }
This rule reports uses of the System.runFinalizersOnExit() method.
Method calls to System.runFinalizersOnExit() should not be allowed. This method is inherently non-thread-safe, may result in data corruption, deadlock, and may affect parts of the program far removed from it's call point. It is deprecated, and it's use strongly discouraged.
Here is an example of code that produces a violation:
def method() { System.runFinalizersOnExit(true) }
New in CodeNarc 0.13
Avoid using ThreadGroup; although it is intended to be used in a threaded environment it contains methods that are not thread safe.
Here is an example of code that produces a violation:
new ThreadGroup("...") new ThreadGroup(tg, "my thread group") Thread.currentThread().getThreadGroup() System.getSecurityManager().getThreadGroup()
This rule reports definition of the ThreadLocal fields that are not static and final.
ThreadLocal fields should be static and final. In the most common case a java.lang.ThreadLocal instance associates state with a thread. A non-static non-final java.lang.ThreadLocal field associates state with an instance-thread combination. This is seldom necessary and often a bug which can cause memory leaks and possibly incorrect behavior.
Here is an example of code that produces a violation:
private static ThreadLocal local1 = new ThreadLocal() private final ThreadLocal local2 = new ThreadLocal() protected ThreadLocal local3 = new ThreadLocal() ThreadLocal local4 = new ThreadLocal()
This rule reports uses of the Thread.yield() method.
Method calls to Thread.yield() should not be allowed. This method has no useful guaranteed semantics, and is often used by inexperienced programmers to mask race conditions.
Here is an example of code that produces a violation:
def method() { Thread.yield() }
Since CodeNarc 0.11
Checks for code that calls notify() rather than notifyAll(). Java monitors are often used for multiple conditions. Calling notify() only wakes up one thread, meaning that the awakened thread might not be the one waiting for the condition that the caller just satisfied.
Also see Java_Concurrency_in_Practice, Brian Goetz, p 303.
New in CodeNarc 0.13
Volatile array fields are unsafe because the contents of the array are not treated as volatile. Changing the entire array reference is visible to other threads, but changing an array element is not.
This rule take from Alex Miller's Java Concurrency in Practice slides, available at http://www.slideshare.net/alexmiller/java-concurrency-gotchas-3666977.
Example of violations:
class MyClass { private volatile Object[] field1 = value() volatile field2 = value as Object[] volatile field3 = (Object[])foo }
This rule reports on long or double fields that are declared volatile.
Long or double fields should not be declared as volatile. Java specifies that reads and writes from such fields are atomic, but many JVM's have violated this specification. Unless you are certain of your JVM, it is better to synchronize access to such fields rather than declare them volatile. This rule flags fields marked volatile when their type is double or long or the name of their type is "Double" or "Long".
Here is an example of code that produces a violation:
def method() { private volatile double d private volatile long f }
Since CodeNarc 0.13
Calls to Object.wait() must be within a while loop. This ensures that the awaited condition has not already been satisfied by another thread before the wait() is invoked. It also ensures that the proper thread was resumed and guards against incorrect notification. See [1] and [3].
As a more modern and flexible alternative, consider using the Java concurrency utilities instead of wait() and notify(). See discussion in Effective Java [2].
Example of violation:
class MyClass { private data void processData() synchronized(data) { if (!data.isReady()) { data.wait() } data.calculateStatistics() } } }
Example of correct usage:
class MyClass { private data void processData() synchronized(data) { while (!data.isReady()) { data.wait() } data.calculateStatistics() } } }