I read this question and thought that would easily be solved (not that it isn't solvable without) if one could write:
#Override
public String toString() {
return super.super.toString();
}
I'm not sure if it is useful in many cases, but I wonder why it isn't and if something like this exists in other languages.
What do you guys think?
EDIT:
To clarify: yes I know, that's impossible in Java and I don't really miss it. This is nothing I expected to work and was surprised getting a compiler error. I just had the idea and like to discuss it.
It violates encapsulation. You shouldn't be able to bypass the parent class's behaviour. It makes sense to sometimes be able to bypass your own class's behaviour (particularly from within the same method) but not your parent's. For example, suppose we have a base "collection of items", a subclass representing "a collection of red items" and a subclass of that representing "a collection of big red items". It makes sense to have:
public class Items
{
public void add(Item item) { ... }
}
public class RedItems extends Items
{
#Override
public void add(Item item)
{
if (!item.isRed())
{
throw new NotRedItemException();
}
super.add(item);
}
}
public class BigRedItems extends RedItems
{
#Override
public void add(Item item)
{
if (!item.isBig())
{
throw new NotBigItemException();
}
super.add(item);
}
}
That's fine - RedItems can always be confident that the items it contains are all red. Now suppose we were able to call super.super.add():
public class NaughtyItems extends RedItems
{
#Override
public void add(Item item)
{
// I don't care if it's red or not. Take that, RedItems!
super.super.add(item);
}
}
Now we could add whatever we like, and the invariant in RedItems is broken.
Does that make sense?
I think Jon Skeet has the correct answer. I'd just like to add that you can access shadowed variables from superclasses of superclasses by casting this:
interface I { int x = 0; }
class T1 implements I { int x = 1; }
class T2 extends T1 { int x = 2; }
class T3 extends T2 {
int x = 3;
void test() {
System.out.println("x=\t\t" + x);
System.out.println("super.x=\t\t" + super.x);
System.out.println("((T2)this).x=\t" + ((T2)this).x);
System.out.println("((T1)this).x=\t" + ((T1)this).x);
System.out.println("((I)this).x=\t" + ((I)this).x);
}
}
class Test {
public static void main(String[] args) {
new T3().test();
}
}
which produces the output:
x= 3
super.x= 2
((T2)this).x= 2
((T1)this).x= 1
((I)this).x= 0
(example from the JLS)
However, this doesn't work for method calls because method calls are determined based on the runtime type of the object.
I think the following code allow to use super.super...super.method() in most case.
(even if it's uggly to do that)
In short
create temporary instance of ancestor type
copy values of fields from original object to temporary one
invoke target method on temporary object
copy modified values back to original object
Usage :
public class A {
public void doThat() { ... }
}
public class B extends A {
public void doThat() { /* don't call super.doThat() */ }
}
public class C extends B {
public void doThat() {
Magic.exec(A.class, this, "doThat");
}
}
public class Magic {
public static <Type, ChieldType extends Type> void exec(Class<Type> oneSuperType, ChieldType instance,
String methodOfParentToExec) {
try {
Type type = oneSuperType.newInstance();
shareVars(oneSuperType, instance, type);
oneSuperType.getMethod(methodOfParentToExec).invoke(type);
shareVars(oneSuperType, type, instance);
} catch (Exception e) {
throw new RuntimeException(e);
}
}
private static <Type, SourceType extends Type, TargetType extends Type> void shareVars(Class<Type> clazz,
SourceType source, TargetType target) throws IllegalArgumentException, IllegalAccessException {
Class<?> loop = clazz;
do {
for (Field f : loop.getDeclaredFields()) {
if (!f.isAccessible()) {
f.setAccessible(true);
}
f.set(target, f.get(source));
}
loop = loop.getSuperclass();
} while (loop != Object.class);
}
}
I don't have enough reputation to comment so I will add this to the other answers.
Jon Skeet answers excellently, with a beautiful example. Matt B has a point: not all superclasses have supers. Your code would break if you called a super of a super that had no super.
Object oriented programming (which Java is) is all about objects, not functions. If you want task oriented programming, choose C++ or something else. If your object doesn't fit in it's super class, then you need to add it to the "grandparent class", create a new class, or find another super it does fit into.
Personally, I have found this limitation to be one of Java's greatest strengths. Code is somewhat rigid compared to other languages I've used, but I always know what to expect. This helps with the "simple and familiar" goal of Java. In my mind, calling super.super is not simple or familiar. Perhaps the developers felt the same?
There's some good reasons to do this. You might have a subclass which has a method which is implemented incorrectly, but the parent method is implemented correctly. Because it belongs to a third party library, you might be unable/unwilling to change the source. In this case, you want to create a subclass but override one method to call the super.super method.
As shown by some other posters, it is possible to do this through reflection, but it should be possible to do something like
(SuperSuperClass this).theMethod();
I'm dealing with this problem right now - the quick fix is to copy and paste the superclass method into the subsubclass method :)
In addition to the very good points that others have made, I think there's another reason: what if the superclass does not have a superclass?
Since every class naturally extends (at least) Object, super.whatever() will always refer to a method in the superclass. But what if your class only extends Object - what would super.super refer to then? How should that behavior be handled - a compiler error, a NullPointer, etc?
I think the primary reason why this is not allowed is that it violates encapsulation, but this might be a small reason too.
I think if you overwrite a method and want to all the super-class version of it (like, say for equals), then you virtually always want to call the direct superclass version first, which one will call its superclass version in turn if it wants.
I think it only makes rarely sense (if at all. i can't think of a case where it does) to call some arbitrary superclass' version of a method. I don't know if that is possible at all in Java. It can be done in C++:
this->ReallyTheBase::foo();
At a guess, because it's not used that often. The only reason I could see using it is if your direct parent has overridden some functionality and you're trying to restore it back to the original.
Which seems to me to be against OO principles, since the class's direct parent should be more closely related to your class than the grandparent is.
Calling of super.super.method() make sense when you can't change code of base class. This often happens when you are extending an existing library.
Ask yourself first, why are you extending that class? If answer is "because I can't change it" then you can create exact package and class in your application, and rewrite naughty method or create delegate:
package com.company.application;
public class OneYouWantExtend extends OneThatContainsDesiredMethod {
// one way is to rewrite method() to call super.method() only or
// to doStuff() and then call super.method()
public void method() {
if (isDoStuff()) {
// do stuff
}
super.method();
}
protected abstract boolean isDoStuff();
// second way is to define methodDelegate() that will call hidden super.method()
public void methodDelegate() {
super.method();
}
...
}
public class OneThatContainsDesiredMethod {
public void method() {...}
...
}
For instance, you can create org.springframework.test.context.junit4.SpringJUnit4ClassRunner class in your application so this class should be loaded before the real one from jar. Then rewrite methods or constructors.
Attention: This is absolute hack, and it is highly NOT recommended to use but it's WORKING! Using of this approach is dangerous because of possible issues with class loaders. Also this may cause issues each time you will update library that contains overwritten class.
#Jon Skeet Nice explanation.
IMO if some one wants to call super.super method then one must be want to ignore the behavior of immediate parent, but want to access the grand parent behavior.
This can be achieved through instance Of. As below code
public class A {
protected void printClass() {
System.out.println("In A Class");
}
}
public class B extends A {
#Override
protected void printClass() {
if (!(this instanceof C)) {
System.out.println("In B Class");
}
super.printClass();
}
}
public class C extends B {
#Override
protected void printClass() {
System.out.println("In C Class");
super.printClass();
}
}
Here is driver class,
public class Driver {
public static void main(String[] args) {
C c = new C();
c.printClass();
}
}
Output of this will be
In C Class
In A Class
Class B printClass behavior will be ignored in this case.
I am not sure about is this a ideal or good practice to achieve super.super, but still it is working.
Look at this Github project, especially the objectHandle variable. This project shows how to actually and accurately call the grandparent method on a grandchild.
Just in case the link gets broken, here is the code:
import lombok.val;
import org.junit.Assert;
import org.junit.Test;
import java.lang.invoke.*;
/*
Your scientists were so preoccupied with whether or not they could, they didn’t stop to think if they should.
Please don't actually do this... :P
*/
public class ImplLookupTest {
private MethodHandles.Lookup getImplLookup() throws NoSuchFieldException, IllegalAccessException {
val field = MethodHandles.Lookup.class.getDeclaredField("IMPL_LOOKUP");
field.setAccessible(true);
return (MethodHandles.Lookup) field.get(null);
}
#Test
public void test() throws Throwable {
val lookup = getImplLookup();
val baseHandle = lookup.findSpecial(Base.class, "toString",
MethodType.methodType(String.class),
Sub.class);
val objectHandle = lookup.findSpecial(Object.class, "toString",
MethodType.methodType(String.class),
// Must use Base.class here for this reference to call Object's toString
Base.class);
val sub = new Sub();
Assert.assertEquals("Sub", sub.toString());
Assert.assertEquals("Base", baseHandle.invoke(sub));
Assert.assertEquals(toString(sub), objectHandle.invoke(sub));
}
private static String toString(Object o) {
return o.getClass().getName() + "#" + Integer.toHexString(o.hashCode());
}
public class Sub extends Base {
#Override
public String toString() {
return "Sub";
}
}
public class Base {
#Override
public String toString() {
return "Base";
}
}
}
Happy Coding!!!!
I would put the super.super method body in another method, if possible
class SuperSuperClass {
public String toString() {
return DescribeMe();
}
protected String DescribeMe() {
return "I am super super";
}
}
class SuperClass extends SuperSuperClass {
public String toString() {
return "I am super";
}
}
class ChildClass extends SuperClass {
public String toString() {
return DescribeMe();
}
}
Or if you cannot change the super-super class, you can try this:
class SuperSuperClass {
public String toString() {
return "I am super super";
}
}
class SuperClass extends SuperSuperClass {
public String toString() {
return DescribeMe(super.toString());
}
protected String DescribeMe(string fromSuper) {
return "I am super";
}
}
class ChildClass extends SuperClass {
protected String DescribeMe(string fromSuper) {
return fromSuper;
}
}
In both cases, the
new ChildClass().toString();
results to "I am super super"
It would seem to be possible to at least get the class of the superclass's superclass, though not necessarily the instance of it, using reflection; if this might be useful, please consider the Javadoc at http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Class.html#getSuperclass()
public class A {
#Override
public String toString() {
return "A";
}
}
public class B extends A {
#Override
public String toString() {
return "B";
}
}
public class C extends B {
#Override
public String toString() {
return "C";
}
}
public class D extends C {
#Override
public String toString() {
String result = "";
try {
result = this.getClass().getSuperclass().getSuperclass().getSuperclass().newInstance().toString();
} catch (InstantiationException ex) {
Logger.getLogger(D.class.getName()).log(Level.SEVERE, null, ex);
} catch (IllegalAccessException ex) {
Logger.getLogger(D.class.getName()).log(Level.SEVERE, null, ex);
}
return result;
}
}
public class Main {
public static void main(String... args) {
D d = new D();
System.out.println(d);
}
}
run:
A
BUILD SUCCESSFUL (total time: 0 seconds)
I have had situations like these when the architecture is to build common functionality in a common CustomBaseClass which implements on behalf of several derived classes.
However, we need to circumvent common logic for specific method for a specific derived class. In such cases, we must use a super.super.methodX implementation.
We achieve this by introducing a boolean member in the CustomBaseClass, which can be used to selectively defer custom implementation and yield to default framework implementation where desirable.
...
FrameworkBaseClass (....) extends...
{
methodA(...){...}
methodB(...){...}
...
methodX(...)
...
methodN(...){...}
}
/* CustomBaseClass overrides default framework functionality for benefit of several derived classes.*/
CustomBaseClass(...) extends FrameworkBaseClass
{
private boolean skipMethodX=false;
/* implement accessors isSkipMethodX() and setSkipMethodX(boolean)*/
methodA(...){...}
methodB(...){...}
...
methodN(...){...}
methodX(...){
if (isSkipMethodX()) {
setSKipMethodX(false);
super.methodX(...);
return;
}
... //common method logic
}
}
DerivedClass1(...) extends CustomBaseClass
DerivedClass2(...) extends CustomBaseClass
...
DerivedClassN(...) extends CustomBaseClass...
DerivedClassX(...) extends CustomBaseClass...
{
methodX(...){
super.setSKipMethodX(true);
super.methodX(...);
}
}
However, with good architecture principles followed in framework as well as app, we could avoid such situations easily, by using hasA approach, instead of isA approach. But at all times it is not very practical to expect well designed architecture in place, and hence the need to get away from solid design principles and introduce hacks like this.
Just my 2 cents...
IMO, it's a clean way to achieve super.super.sayYourName() behavior in Java.
public class GrandMa {
public void sayYourName(){
System.out.println("Grandma Fedora");
}
}
public class Mama extends GrandMa {
public void sayYourName(boolean lie){
if(lie){
super.sayYourName();
}else {
System.out.println("Mama Stephanida");
}
}
}
public class Daughter extends Mama {
public void sayYourName(boolean lie){
if(lie){
super.sayYourName(lie);
}else {
System.out.println("Little girl Masha");
}
}
}
public class TestDaughter {
public static void main(String[] args){
Daughter d = new Daughter();
System.out.print("Request to lie: d.sayYourName(true) returns ");
d.sayYourName(true);
System.out.print("Request not to lie: d.sayYourName(false) returns ");
d.sayYourName(false);
}
}
Output:
Request to lie: d.sayYourName(true) returns Grandma Fedora
Request not to lie: d.sayYourName(false) returns Little girl Masha
I think this is a problem that breaks the inheritance agreement.
By extending a class you obey / agree its behavior, features
Whilst when calling super.super.method(), you want to break your own obedience agreement.
You just cannot cherry pick from the super class.
However, there may happen situations when you feel the need to call super.super.method() - usually a bad design sign, in your code or in the code you inherit !
If the super and super super classes cannot be refactored (some legacy code), then opt for composition over inheritance.
Encapsulation breaking is when you #Override some methods by breaking the encapsulated code.
The methods designed not to be overridden are marked
final.
In C# you can call a method of any ancestor like this:
public class A
internal virtual void foo()
...
public class B : A
public new void foo()
...
public class C : B
public new void foo() {
(this as A).foo();
}
Also you can do this in Delphi:
type
A=class
procedure foo;
...
B=class(A)
procedure foo; override;
...
C=class(B)
procedure foo; override;
...
A(objC).foo();
But in Java you can do such focus only by some gear. One possible way is:
class A {
int y=10;
void foo(Class X) throws Exception {
if(X!=A.class)
throw new Exception("Incorrect parameter of "+this.getClass().getName()+".foo("+X.getName()+")");
y++;
System.out.printf("A.foo(%s): y=%d\n",X.getName(),y);
}
void foo() throws Exception {
System.out.printf("A.foo()\n");
this.foo(this.getClass());
}
}
class B extends A {
int y=20;
#Override
void foo(Class X) throws Exception {
if(X==B.class) {
y++;
System.out.printf("B.foo(%s): y=%d\n",X.getName(),y);
} else {
System.out.printf("B.foo(%s) calls B.super.foo(%s)\n",X.getName(),X.getName());
super.foo(X);
}
}
}
class C extends B {
int y=30;
#Override
void foo(Class X) throws Exception {
if(X==C.class) {
y++;
System.out.printf("C.foo(%s): y=%d\n",X.getName(),y);
} else {
System.out.printf("C.foo(%s) calls C.super.foo(%s)\n",X.getName(),X.getName());
super.foo(X);
}
}
void DoIt() {
try {
System.out.printf("DoIt: foo():\n");
foo();
Show();
System.out.printf("DoIt: foo(B):\n");
foo(B.class);
Show();
System.out.printf("DoIt: foo(A):\n");
foo(A.class);
Show();
} catch(Exception e) {
//...
}
}
void Show() {
System.out.printf("Show: A.y=%d, B.y=%d, C.y=%d\n\n", ((A)this).y, ((B)this).y, ((C)this).y);
}
}
objC.DoIt() result output:
DoIt: foo():
A.foo()
C.foo(C): y=31
Show: A.y=10, B.y=20, C.y=31
DoIt: foo(B):
C.foo(B) calls C.super.foo(B)
B.foo(B): y=21
Show: A.y=10, B.y=21, C.y=31
DoIt: foo(A):
C.foo(A) calls C.super.foo(A)
B.foo(A) calls B.super.foo(A)
A.foo(A): y=11
Show: A.y=11, B.y=21, C.y=31
It is simply easy to do. For instance:
C subclass of B and B subclass of A. Both of three have method methodName() for example.
public abstract class A {
public void methodName() {
System.out.println("Class A");
}
}
public class B extends A {
public void methodName() {
super.methodName();
System.out.println("Class B");
}
// Will call the super methodName
public void hackSuper() {
super.methodName();
}
}
public class C extends B {
public static void main(String[] args) {
A a = new C();
a.methodName();
}
#Override
public void methodName() {
/*super.methodName();*/
hackSuper();
System.out.println("Class C");
}
}
Run class C Output will be:
Class A
Class C
Instead of output:
Class A
Class B
Class C
If you think you are going to be needing the superclass, you could reference it in a variable for that class. For example:
public class Foo
{
public int getNumber()
{
return 0;
}
}
public class SuperFoo extends Foo
{
public static Foo superClass = new Foo();
public int getNumber()
{
return 1;
}
}
public class UltraFoo extends Foo
{
public static void main(String[] args)
{
System.out.println(new UltraFoo.getNumber());
System.out.println(new SuperFoo().getNumber());
System.out.println(new SuperFoo().superClass.getNumber());
}
public int getNumber()
{
return 2;
}
}
Should print out:
2
1
0
public class SubSubClass extends SubClass {
#Override
public void print() {
super.superPrint();
}
public static void main(String[] args) {
new SubSubClass().print();
}
}
class SuperClass {
public void print() {
System.out.println("Printed in the GrandDad");
}
}
class SubClass extends SuperClass {
public void superPrint() {
super.print();
}
}
Output: Printed in the GrandDad
The keyword super is just a way to invoke the method in the superclass.
In the Java tutorial:https://docs.oracle.com/javase/tutorial/java/IandI/super.html
If your method overrides one of its superclass's methods, you can invoke the overridden method through the use of the keyword super.
Don't believe that it's a reference of the super object!!! No, it's just a keyword to invoke methods in the superclass.
Here is an example:
class Animal {
public void doSth() {
System.out.println(this); // It's a Cat! Not an animal!
System.out.println("Animal do sth.");
}
}
class Cat extends Animal {
public void doSth() {
System.out.println(this);
System.out.println("Cat do sth.");
super.doSth();
}
}
When you call cat.doSth(), the method doSth() in class Animal will print this and it is a cat.
I want to get clarity on loading of classes, destruction of objects etc in android because I noticed some weird things happening when using Singleton in My Activity. Best I will describe it using code :
My Singleton class
public class FilterCriteria {
private final String TAG=FilterCriteria.class.getSimpleName();
private static FilterCriteria filterCriteria=new FilterCriteria();
private FilterCriteria()
{
}
public static FilterCriteria getInstance()
{
return filterCriteria;
}
private int rentUpperBound,rentLowerBound;
private int bedrooms,baths;
private float distance;
private ObjectStateListener listener;
public void setFilters(float distance,int baths,int bedrooms,int rentLowerBound,int rentUpperBound) {
this.distance = distance;
this.baths=baths;
this.bedrooms=bedrooms;
this.rentLowerBound=rentLowerBound;
this.rentUpperBound=rentUpperBound;
if(listener!=null)
listener.onObjectStateChanged();
}
public void attachListener(ObjectStateListener listener) {
if (this.listener == null) {
this.listener = listener;
Log.v(TAG, "NO LISTENER PRESENT AS EXPECTED");
} else {
Log.v(TAG, "LISTENER PRESENT!!! BUT THE ACTIVITY WAS STARTED JUST NOW.");
}
}
public void destroy()
{
filterCriteria=null;
}
}
The attachListener(ObjectStateListener listener) function is called only once in the activity. So, when I open my activity the first time, I get this log from attachListener function
NO LISTENER PRESENT AS EXPECTED
Now, I close the activity and then reopen it. But now I get this log
"LISTENER PRESENT!!! BUT THE ACTIVITY WAS STARTED JUST NOW."
So, that means the object still lives on even after the activity (and the application) was closed. Is this normal???
So, I tried to destroy the singleton instance using the destroy() function in the onDestroy() function of Activity.
#Override
protected void onDestroy(){
filterCriteria.destroy();//Trying to destroy the singleton
super.onDestroy();
Log.v(TAG,"Destroying activity");
}
But I got NullPointerException on this line filterCriteria.destroy(). So, that means android has already made object null, whereas when I see in debug mode, other members of the Activity are still alive. Why is only this null?
What is happening!???
When you invoke the method attachListener() you are creating a reference to the linked object (even if it is static): this reference will be binded to the activity lifecycle.
On the other hand, filterCriteria will follow the static field Java-like lifecycle (but you can still remove this reference manually).
I'm working on an Android app and we're investigating memory use.
Looking at a heap dump from hprof, we're seeing nearly 2M (22% of our heap) being used in a static cache in JarURLConnectionImpl:
Looking at the source code for JarURLConnectionImpl, it appears that entries are added to the static jarCache variable, but never removed.
If it's true that they're never removed, that strikes me as a potential memory leak.
Is this a leak? Is there a fix or workaround?
Here's an ugly workaround:
private static HashMap<URL,JarFile> jarCache;
static {
try {
Class<?> jarURLConnectionImplClass = Class.forName("org.apache.harmony.luni.internal.net.www.protocol.jar.JarURLConnectionImpl");
final Field jarCacheField = jarURLConnectionImplClass.getDeclaredField("jarCache");
jarCacheField.setAccessible(true);
//noinspection unchecked
jarCache = (HashMap<URL, JarFile>) jarCacheField.get(null);
} catch(Exception e) {
// ignored
}
}
Then, periodically run the following:
// HACK http://stackoverflow.com/questions/14610350/android-memory-leak-in-apache-harmonys-jarurlconnectionimpl
if( jarCache!=null ) {
try {
for (
final Iterator<Map.Entry<URL, JarFile>> iterator = jarCache.entrySet().iterator(); iterator.hasNext(); ) {
final Map.Entry<URL, JarFile> e = iterator.next();
final URL url = e.getKey();
if (Strings.toString(url).endsWith(".apk")) {
Log.i(TAG,"Removing static hashmap entry for " + url);
try {
final JarFile jarFile = e.getValue();
jarFile.close();
iterator.remove();
} catch( Exception f ) {
Log.e(TAG,"Error removing hashmap entry for "+ url,f);
}
}
}
} catch( Exception e ) {
// ignored
}
}
I run it on activity creation so it gets executed every time one of my activities is created. The ugly hashmap entry doesn't seem to get recreated all that often, but it DOES seem to reappear occasionally, so it's not sufficient to just run this code once.
This is definitely a nasty memory leak. I've opened an issue for it since no one else seems to have reported it.
Thanks for the "ugly workaround" emmby, that was helpful. A safer approach, although potentially with a performance impact, is to disable URLConnection caching altogether. Since the URLConnection.defaultUseCaches flag is static and, as you might guess, is the default for each instance's useCaches flag, you can just set this to false and no more instances will cache their connections. This will affect all implementations of URLConnection, so it may have farther-ranging effects than desired, but I think it's a reasonable trade-off.
You can just create a simple class like this and instantiate it very early in your app's onCreate():
public class URLConnectionNoCache extends URLConnection {
protected URLConnectionNoCache(URL url) {
super(url);
setDefaultUseCaches(false);
}
public void connect() throws IOException {
}
}
The interesting thing is that since this occurs after your app is loaded and run, the system libs should already be cached, and this will only prevent further caching, so this probably gives the best possible trade-off: not caching your apk while allowing the performance benefits of caching the system jars.
Before doing this I did modify emmby's solution a bit to make it a standalone class that creates a background thread to periodically clear the cache. And I restricted it to just clear the app's apk, though that can be relaxed if desired. The thing to worry about here is that you're modifying the objects while they may be in use, which is generally not a good thing. If you do want to go this route you just need to call the start() method with a context, e.g. in your app's onCreate().
package com.example;
import java.lang.reflect.Field;
import java.net.URL;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.jar.JarFile;
import java.util.regex.Pattern;
import android.content.Context;
// hack to remove memory leak in JarURLConnectionImpl
// from http://stackoverflow.com/questions/14610350/android-memory-leak-in-apache-harmonys-jarurlconnectionimpl
public class JarURLMonitor {
private static JarURLMonitor instance;
private Pattern pat;
private Field jarCacheField;
public volatile boolean stop;
private static final long CHECK_INTERVAL = 60 * 1000;
public static synchronized void start(Context context) {
if (instance == null) {
instance = new JarURLMonitor(context);
}
}
public static synchronized void stop() {
if (instance != null) {
instance.stop = true;
}
}
private JarURLMonitor(Context context) {
// get jar cache field
try {
final Class<?> cls = Class.forName("libcore.net.url.JarURLConnectionImpl");
jarCacheField = cls.getDeclaredField("jarCache");
jarCacheField.setAccessible(true);
}
catch (Exception e) {
// log
}
if (jarCacheField != null) {
// create pattern that matches our package: e.g. /data/app/<pkgname>-1.apk
pat = Pattern.compile("^.*/" + context.getPackageName() + "-.*\\.apk$");
// start background thread to check it
new Thread("JarURLMonitor") {
#Override
public void run() {
try {
while (!stop) {
checkJarCache();
Thread.sleep(CHECK_INTERVAL);
}
}
catch (Exception e) {
// log
}
}
}.start();
}
}
private void checkJarCache() throws Exception {
#SuppressWarnings("unchecked")
final HashMap<URL, JarFile> jarCache = (HashMap<URL, JarFile>)jarCacheField.get(null);
final Iterator<Map.Entry<URL, JarFile>> iterator = jarCache.entrySet().iterator();
while (iterator.hasNext()) {
final Map.Entry<URL, JarFile> entry = iterator.next();
final JarFile jarFile = entry.getValue();
final String file = jarFile.getName();
if (pat.matcher(file).matches()) {
try {
jarFile.close();
iterator.remove();
}
catch (Exception e) {
// log
}
}
}
}
}
In an Android app, is there anything wrong with the following approach:
public class MyApp extends android.app.Application {
private static MyApp instance;
public MyApp() {
instance = this;
}
public static Context getContext() {
return instance;
}
}
and pass it everywhere (e.g. SQLiteOpenHelper) where context is required (and not leaking of course)?
There are a couple of potential problems with this approach, though in a lot of circumstances (such as your example) it will work well.
In particular you should be careful when dealing with anything that deals with the GUI that requires a Context. For example, if you pass the application Context into the LayoutInflater you will get an Exception. Generally speaking, your approach is excellent: it's good practice to use an Activity's Context within that Activity, and the Application Context when passing a context beyond the scope of an Activity to avoid memory leaks.
Also, as an alternative to your pattern you can use the shortcut of calling getApplicationContext() on a Context object (such as an Activity) to get the Application Context.
In my experience this approach shouldn't be necessary. If you need the context for anything you can usually get it via a call to View.getContext() and using the Context obtained there you can call Context.getApplicationContext() to get the Application context. If you are trying to get the Application context this from an Activity you can always call Activity.getApplication() which should be able to be passed as the Context needed for a call to SQLiteOpenHelper().
Overall there doesn't seem to be a problem with your approach for this situation, but when dealing with Context just make sure you are not leaking memory anywhere as described on the official Google Android Developers blog.
Some people have asked: how can the singleton return a null pointer?
I'm answering that question. (I cannot answer in a comment because I need to post code.)
It may return null in between two events: (1) the class is loaded, and (2) the object of this class is created. Here's an example:
class X {
static X xinstance;
static Y yinstance = Y.yinstance;
X() {xinstance=this;}
}
class Y {
static X xinstance = X.xinstance;
static Y yinstance;
Y() {yinstance=this;}
}
public class A {
public static void main(String[] p) {
X x = new X();
Y y = new Y();
System.out.println("x:"+X.xinstance+" y:"+Y.yinstance);
System.out.println("x:"+Y.xinstance+" y:"+X.yinstance);
}
}
Let's run the code:
$ javac A.java
$ java A
x:X#a63599 y:Y#9036e
x:null y:null
The second line shows that Y.xinstance and X.yinstance are null; they are null because the variables X.xinstance ans Y.yinstance were read when they were null.
Can this be fixed? Yes,
class X {
static Y y = Y.getInstance();
static X theinstance;
static X getInstance() {if(theinstance==null) {theinstance = new X();} return theinstance;}
}
class Y {
static X x = X.getInstance();
static Y theinstance;
static Y getInstance() {if(theinstance==null) {theinstance = new Y();} return theinstance;}
}
public class A {
public static void main(String[] p) {
System.out.println("x:"+X.getInstance()+" y:"+Y.getInstance());
System.out.println("x:"+Y.x+" y:"+X.y);
}
}
and this code shows no anomaly:
$ javac A.java
$ java A
x:X#1c059f6 y:Y#152506e
x:X#1c059f6 y:Y#152506e
BUT this is not an option for the Android Application object: the programmer does not control the time when it is created.
Once again: the difference between the first example and the second one is that the second example creates an instance if the static pointer is null. But a programmer cannot create the Android application object before the system decides to do it.
UPDATE
One more puzzling example where initialized static fields happen to be null.
Main.java:
enum MyEnum {
FIRST,SECOND;
private static String prefix="<", suffix=">";
String myName;
MyEnum() {
myName = makeMyName();
}
String makeMyName() {
return prefix + name() + suffix;
}
String getMyName() {
return myName;
}
}
public class Main {
public static void main(String args[]) {
System.out.println("first: "+MyEnum.FIRST+" second: "+MyEnum.SECOND);
System.out.println("first: "+MyEnum.FIRST.makeMyName()+" second: "+MyEnum.SECOND.makeMyName());
System.out.println("first: "+MyEnum.FIRST.getMyName()+" second: "+MyEnum.SECOND.getMyName());
}
}
And you get:
$ javac Main.java
$ java Main
first: FIRST second: SECOND
first: <FIRST> second: <SECOND>
first: nullFIRSTnull second: nullSECONDnull
Note that you cannot move the static variable declaration one line upper, the code will not compile.
Application Class:
import android.app.Application;
import android.content.Context;
public class MyApplication extends Application {
private static Context mContext;
public void onCreate() {
super.onCreate();
mContext = getApplicationContext();
}
public static Context getAppContext() {
return mContext;
}
}
Declare the Application in the AndroidManifest:
<application android:name=".MyApplication"
...
/>
Usage:
MyApplication.getAppContext()
You are trying to create a wrapper to get Application Context and there is a possibility that it might return "null" pointer.
As per my understanding, I guess its better approach to call- any of the 2
Context.getApplicationContext() or Activity.getApplication().
It is a good approach. I use it myself as well. I would only suggest to override onCreate to set the singleton instead of using a constructor.
And since you mentioned SQLiteOpenHelper: In onCreate () you can open the database as well.
Personally I think the documentation got it wrong in saying that There is normally no need to subclass Application. I think the opposite is true: You should always subclass Application.
I would use Application Context to get a System Service in the constructor. This eases testing & benefits from composition
public class MyActivity extends Activity {
private final NotificationManager notificationManager;
public MyActivity() {
this(MyApp.getContext().getSystemService(NOTIFICATION_SERVICE));
}
public MyActivity(NotificationManager notificationManager) {
this.notificationManager = notificationManager;
}
// onCreate etc
}
Test class would then use the overloaded constructor.
Android would use the default constructor.
I like it, but I would suggest a singleton instead:
package com.mobidrone;
import android.app.Application;
import android.content.Context;
public class ApplicationContext extends Application
{
private static ApplicationContext instance = null;
private ApplicationContext()
{
instance = this;
}
public static Context getInstance()
{
if (null == instance)
{
instance = new ApplicationContext();
}
return instance;
}
}
I'm using the same approach, I suggest to write the singleton a little better:
public static MyApp getInstance() {
if (instance == null) {
synchronized (MyApp.class) {
if (instance == null) {
instance = new MyApp ();
}
}
}
return instance;
}
but I'm not using everywhere, I use getContext() and getApplicationContext() where I can do it!
I know the original question was posted 13 years ago, and this is the Kotlin version of getting context everywhere.
class MyApplication : Application() {
companion object {
#JvmStatic
private var instance: MyApplication? = null
#JvmStatic
public final fun getContext(): Context? {
return instance
}
}
override fun onCreate() {
instance = this
super.onCreate()
}
}