I was trying to modify class dynamically, such as call sleep() before a line. I attached agent to a jvm during runtime using Attach method. Then I got target class from jvm, and modified it(Add a line to call sleep()). And I got redine class error. I am using JDK1.6. I am using ASM core API to modify class.
The Error:
Caused by: java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the schema (add/remove fields)
at sun.instrument.InstrumentationImpl.retransformClasses0(Native Method)
at sun.instrument.InstrumentationImpl.retransformClasses(InstrumentationImpl.java:124)
Is there something wrong with ASM code? Actually my ASM code finished its job(to Add a line to call sleep()). Does current jvm not support retransform class? It seems failed to execute retransformClasses(). Does retransformClasses() not support the ASM operation(to add a line into a method to call sleep())? Any ideas? thx
EDIT:
The class which I want modify:
import java.util.concurrent.TimeUnit;
public class Person {
public String name = "abc";
public String address = "xxxxx" ;
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void sayHello() throws InterruptedException {
System.out.println("aaaaaaaaaa");
System.out.println("Hello World!");
TimeUnit.SECONDS.sleep(120);
System.out.println("dd");
}
public void sayHello2() {
System.out.println("aaaaaaaaaa1");
System.out.println("Hello World!2");
}
public static void main (String args[]) {
try {
Person p = new Person();
p.sayHello(); // linenumber #9. A line to call Sleep() should be added before #here.
p.sayHello2();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
My ASM code:
public void visitMethodInsn(int arg0, String arg1, String arg2, String arg3) {
Label la=new Label();
mv.visitLabel(la);
int linenumber=la.getOffset();
if(linenumber==9) {
mv.visitFieldInsn(Opcodes.GETSTATIC, "java/util/concurrent/TimeUnit", "SECONDS", "Ljava/util/concurrent/TimeUnit;");
mv.visitLdcInsn(new Long("5"));
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/util/concurrent/TimeUnit", "sleep", "(J)V");
super.visitMethodInsn(arg0, arg1, arg2, arg3);
}
}
Not looking at your code yet, I think I can suggest something. When a class is first loaded, in addition to storing the class's byte codes, the JVM also has tables where it keeps track of the types of fields and the signatures of methods in each class.
The error you are seeing would suggest that the class was loaded, this signature information was stored and then you tried to add the method after that.
If you instead put your agent jar onto the command line, you can do things before the class is loaded for the first time. If you add your method before the signature info is stored away, you should be good.
If you have to connect the agent after the process is already launched, you may be able to transform the class but you may only be able to transform it without changing the set of fields, their types, or the methods, or their signatures. In other words, you may be able to change the byte codes but you have to not invalidate the previously-stored meta information.
Related
just trying out things in java, found following issue.
DefaultAndStaticMethodMain.java:8: error: not a statement
implementation1::sendNotification;
^
1 error
Following is my code.
ParentInterface:
public interface ParentInterface {
default void callForCompletion() {
System.out.println("<<<< Notification sending completed. >>>>");
}
}
Child interface:
public interface ChildInterface extends ParentInterface {
public abstract void sendNotification();
static String printNotificationSentMessage() {
return "Notification is sent successfully.";
}
}
Implementation 1:
public class Implementation1 implements ChildInterface {
#Override
public void sendNotification() {
System.out.println("Implementation --- 1");
System.out.println("Sending notification via email >>>");
}
}
Implementation 2:
public class Implementation2 implements ChildInterface {
#Override
public void sendNotification() {
System.out.println("Implementation ---- 2.");
System.out.println("Sending notification via SMS >>>");
}
}
Main method:
public class DefaultAndStaticMethodMain {
public static void main(String[] args) {
Implementation1 implementation1 = new Implementation1();
implementation1::sendNotification; // Compilation error as shown above.
Implementation2 implementation2 = new Implementation2();
implementation2.sendNotification();
// Following works fine.
// Arrays.asList(implementation1, implementation2).stream().forEach(SomeInterfaceToBeRenamed::sendNotification);
}
}
I am not sure what am I doing wrong, I have JDK 13 installed in local machine and working with IntelliJ 2019.3 with JDK 11. I checked that IntelliJ supports JDK 13
Thanks.
Update
By mistake I left a semi-colon over there, removed it, please check again.
A method reference is not the same as a method call. Those are two distinct things.
A method call is a standalone expression, or, more precisely, an expression statement. That means that in your case implementation2.sendNotification() works, as you would expect.
A method reference, however,
is used to refer to the invocation of a method without actually performing the invocation
and is not a standalone expression. It can only be used where a lambda expression can also be used. A method reference as a standalone expression does not compile, just like an arithmetic expression without assignment (e.g. 3 + 17;). This is enforced by the Java Language Specification, § 14.8 and § 15.13.
More to read:
Please Explain Java 8 Method Reference to instance Method using class name
Method reference in Java 8
What do you intend for the implementation1::sendNotification; line to do? Judging by the implementation2.sendNotification(); line below it looks like you're trying to call sendNotification on implementation1, which is written like this:
implementation1.sendNotification();
The :: notation is a method reference, and (as the error message says) it an identifier, not a statement, and thus can't be a line on its own. Similarly you couldn't write implementation1; (a variable) or ChildInterface; (a class identifier) as a statement.
The .forEach(SomeInterfaceToBeRenamed::sendNotification); line compiles because you are passing the method reference to forEach(), and it in turn invokes each sendNotification() method.
I'm trying to write a java instrumentation agent using byte buddy. My goal is to replace a java standard library method call with a proxy call of my own. I was suggested to use Byte Buddy's MemberSubstitution to achieve this. I used this and this questions from SO for my reference.
I'm using Intellij IDEA for coding. My Agent code is split into multiple files as follows:
MyFirstAgent.java
public class MyFirstAgent {
public static void premain(String agentArgs, Instrumentation inst) {
new AgentBuilder.Default()
.type(ElementMatchers.any())
.transform(new ByteBuddyTransformer())
.with(AgentBuilder.Listener.StreamWriting.toSystemOut())
.with(AgentBuilder.TypeStrategy.Default.REDEFINE)
.installOn(inst);
}
ByteBuddyTransformer.java
public class ByteBuddyTransformer implements AgentBuilder.Transformer {
#Override
public DynamicType.Builder<?> transform(DynamicType.Builder<?> builder, TypeDescription typeDescription,
ClassLoader classLoader, JavaModule javaModule) {
try {
return builder.visit(MemberSubstitution.relaxed()
.method(named("add"))
.replaceWith(MyClass.class.getMethod("printLine"))
.on(any()));
} catch (NoSuchMethodException e) {
e.printStackTrace();
}
return builder;
}
}
MyClass.java
public class MyClass {
public boolean printLine(){
System.out.println("This is the proxy!");
return true;
}
}
And the application that I want to instrument is in another Intellij IDEA project with the following:
Main.java
public class Main {
public static void main(String[] args) {
ClassToMonitor classToMonitor = new ClassToMonitor();
classToMonitor.bar();
}
}
ClassToMonitor.java
package com.company;
import java.util.ArrayList;
import java.util.Arrays;
public class ClassToMonitor {
public void bar() {
// create an empty array list with an initial capacity
ArrayList<Integer> arrlist = new ArrayList<Integer>(5);
// use add() method to add elements in the list
arrlist.add(15);
// print all the elements available in list
for (Integer number : arrlist) {
System.out.println("Number = " + number);
}
}
}
When I build the fat jar of my agent and run it with my application, I get the following error:
[Byte Buddy] ERROR com.company.ClassToMonitor [jdk.internal.loader.ClassLoaders$AppClassLoader#2626b418, unnamed module #385e9564, loaded=false]
java.lang.IllegalStateException: Cannot invoke public boolean com.company.MyClass.printLine() on [class java.util.ArrayList, E]
I can provide the full error message if required. Also, I'm new to Java and Instrumentation in general so I might be missing something fundamental here, please kindly excuse me and point it out if that's the case.
For substitution to work, the target method needs to accept the same arguments as the replaced method, in your case an int. Also, since you are calling a member, the implicit first argument of your class needs to be the receiver type, i.e. ArrayList or any super type, even Object. Also, your replacement method needs to be static:
public class MyClass {
public static boolean printLine(Object ignored, int ignored2){
System.out.println("This is the proxy!");
return true;
}
}
MemberSubstitution is still not as flexible as it is supposed to be. You can however already inject custom byte code using the chained step if that is what you want.
I'm searching for a concept to forward an object to subobjects.
Example:
I would like to create log files for several main Objects, that include sub objects (imagine a REST server that would log every single connection by ID).
Creating one big log file is simple ( redirect System.out.println, I already encapsulated that)
Example code:
class SubElementA{
public SubElementA(){
Debugger.debug("I am called, too");
}
}
Application.java
package com.dev4ag;
class Application{
private ElementA elA;
private String prefix;
public Application(String name){
this.elA = new ElementA();
this.prefix = name;
}
public void countUp(){
Debugger.debug(this.prefix+": I will now count up");
this.elA.doSomeStuff();
}
}
ElementA.java
package com.dev4ag;
class ElementA{
private int counter;
private SubElementA subElementA;
public void doSomeStuff(){
counter++;
Debugger.debug("Counter is: "+counter);
}
//Constructor
public ElementA(){
subElementA = new SubElementA();
this.counter = 0;
};
}
SubElementA.java
package com.dev4ag;
class SubElementA{
public SubElementA(){
Debugger.debug("I am called, too");
}
}
Debugger.java
package com.dev4ag;
public class Debugger {
public static void debug(String output){
//Just imagine we would write to a file here ;)
System.out.println(output);
}
}
(it was more easy to write system.out.println than to create a file, just imagine, Debugger.debug would write to a file).
Now I am thinking about a solution to create one Debug output target for each App. I could definitely change debug to not being static and create a debug object within Application.
But is there any way to use this object in the sub classes without forwarding the debug object either through Constructor or setter function, which would mean to have to add an object for the debugger to each class?
What would be the most beautiful solution for that?
Note that this solution might decrease performance a lot and it is pretty dirty way, but some loggers include such data.
But you can use Thread.currentThread().getStackTrace() to get stacktrace like in error and get class and method from where your method was called.
If you are using java9+ then you should probably use StackWalker API instead, especially that it have nice filters and other useful features.
So then you could guess app by class/method names on the stack.
I have Java-related question:
I want to know is there a way to create path to class (in program) by using a variable(s).
Im making a program that will download pictures from certain sites and show them to a user. However, different sites have different forms, that's why I have to define a series of functions specific to each. They cannot be put in the same class because functions that preform same job (just for another site) would have to have same names. I'm trying to make adding support for another site later as simple as possible.
Anyway, the question is, could I call a function in program using a variable to determine its location.
For example: code.picturesite.functionINeed();
code is the package containing all of the coding, and picturesite is not a class but rather a variable containing the name of the desired class - that way I can only change value of the variable to call a different function (or the same function in a different class).
I don't really expect that to be possible (this was more for you to understand the nature of the problem), but is there another way to do what I'm trying to achieve here?
Yes, there is a way. It's called reflection.
Given a String containing the class name, you can get an instance like this:
Class<?> c = Class.forName("com.foo.SomeClass");
Object o = c.newInstance(); // assuming there's a default constructor
If there isn't a default constructor, you can get a reference to one via c.getConstructor(param1.getClass(), param2.getClass(), etc)
Given a String containing the method name and an instance, you can invoke that method like this:
Method m = o.getClass().getMethod("someMethod", param1.getClass(), param2.getClass(), etc);
Object result = m.invoke(o, param1, param2, etc);
I'm not immediately seeing anything in your question that couldn't be solved by, instead of having a variable containing a class name, having a variable containing an instance of that class -- to call a function on the class, you would have to know it implements that function, so you could put the function in an interface.
interface SiteThatCanFoo {
void foo();
}
And
class SiteA extends Site implements SiteThatCanFoo {
public void foo() {
System.out.println("Foo");
}
}
Then:
Site currentSite = getCurrentSite(); // or getSiteObjectForName(siteName), or similar
if (SiteThatCanFoo.isAssignableFrom(currentSite.class)) {
((SiteThatCanFoo)currentSite).foo();
}
So you want to do something like this (check ImageDownloader.getImageFrom method)
class SiteADownloader {
public static Image getImage(URI uri) {
System.out.println("invoking SiteADownloader on "+uri);
Image i = null;
// logic for dowlnoading image from siteA
return i;
}
}
class SiteBDownloader {
public static Image getImage(URI uri) {
System.out.println("invoking SiteBDownloader on "+uri);
Image i = null;
// logic for dowlnoading image from siteB
return i;
}
}
// MAIN CLASS
class ImageDownloader {
public static Image getImageFrom(String serverName, URI uri) {
Image i = null;
try {
// load class
Class<?> c = Class.forName(serverName + "Downloader");
// find method to dowload img
Method m = c.getDeclaredMethod("getImage", URI.class);
// invoke method and store result (method should be invoked on
// object, in case of static methods they are invoked on class
// object stored earlier in c reference
i = (Image) m.invoke(c, uri);
} catch (NoSuchMethodException | SecurityException
| IllegalAccessException | IllegalArgumentException
| InvocationTargetException | ClassNotFoundException e) {
e.printStackTrace();
}
return i;
}
// time for test
public static void main(String[] args) {
try {
Image img = ImageDownloader.getImageFrom("SiteB", new URI(
"adress"));
} catch (URISyntaxException e) {
e.printStackTrace();
}
}
}
I've run through the Google Web Toolkit StockWatcher Tutorial using Eclipse and the Google Plugin, and I'm attempting to make some basic changes to it so I can better understand the RPC framework.
I've modified the "getStocks" method on the StockServiceImpl server-side class so that it returns an array of Stock objects instead of String objects. The application compiles perfectly, but the Google Web Toolkit is returning the following error:
"No source code is available for type com.google.gwt.sample.stockwatcher.server.Stock; did you forget to inherit a required module?"
It seems that the client-side classes can't find an implementation of the Stock object, even though the class has been imported. For reference, here is a screenshot of my package hierarchy:
I suspect that I'm missing something in web.xml, but I have no idea what it is. Can anyone point me in the right direction?
EDIT: Forgot to mention that the Stock class is persistable, so it needs to stay on the server-side.
After much trial and error, I managed to find a way to do this. It might not be the best way, but it works. Hopefully this post can save someone else a lot of time and effort.
These instructions assume that you have completed both the basic StockWatcher tutorial and the Google App Engine StockWatcher modifications.
Create a Client-Side Implementation of the Stock Class
There are a couple of things to keep in mind about GWT:
Server-side classes can import client-side classes, but not vice-versa (usually).
The client-side can't import any Google App Engine libraries (i.e. com.google.appengine.api.users.User)
Due to both items above, the client can never implement the Stock class that we created in com.google.gwt.sample.stockwatcher.server. Instead, we'll create a new client-side Stock class called StockClient.
StockClient.java:
package com.google.gwt.sample.stockwatcher.client;
import java.io.Serializable;
import java.util.Date;
public class StockClient implements Serializable {
private Long id;
private String symbol;
private Date createDate;
public StockClient() {
this.createDate = new Date();
}
public StockClient(String symbol) {
this.symbol = symbol;
this.createDate = new Date();
}
public StockClient(Long id, String symbol, Date createDate) {
this();
this.id = id;
this.symbol = symbol;
this.createDate = createDate;
}
public Long getId() {
return this.id;
}
public String getSymbol() {
return this.symbol;
}
public Date getCreateDate() {
return this.createDate;
}
public void setId(Long id) {
this.id = id;
}
public void setSymbol(String symbol) {
this.symbol = symbol;
}
}
Modify Client Classes to Use StockClient[] instead of String[]
Now we make some simple modifications to the client classes so that they know that the RPC call returns StockClient[] instead of String[].
StockService.java:
package com.google.gwt.sample.stockwatcher.client;
import com.google.gwt.sample.stockwatcher.client.NotLoggedInException;
import com.google.gwt.user.client.rpc.RemoteService;
import com.google.gwt.user.client.rpc.RemoteServiceRelativePath;
#RemoteServiceRelativePath("stock")
public interface StockService extends RemoteService {
public Long addStock(String symbol) throws NotLoggedInException;
public void removeStock(String symbol) throws NotLoggedInException;
public StockClient[] getStocks() throws NotLoggedInException;
}
StockServiceAsync.java:
package com.google.gwt.sample.stockwatcher.client;
import com.google.gwt.sample.stockwatcher.client.StockClient;
import com.google.gwt.user.client.rpc.AsyncCallback;
public interface StockServiceAsync {
public void addStock(String symbol, AsyncCallback<Long> async);
public void removeStock(String symbol, AsyncCallback<Void> async);
public void getStocks(AsyncCallback<StockClient[]> async);
}
StockWatcher.java:
Add one import:
import com.google.gwt.sample.stockwatcher.client.StockClient;
All other code stays the same, except addStock, loadStocks, and displayStocks:
private void loadStocks() {
stockService = GWT.create(StockService.class);
stockService.getStocks(new AsyncCallback<String[]>() {
public void onFailure(Throwable error) {
handleError(error);
}
public void onSuccess(String[] symbols) {
displayStocks(symbols);
}
});
}
private void displayStocks(String[] symbols) {
for (String symbol : symbols) {
displayStock(symbol);
}
}
private void addStock() {
final String symbol = newSymbolTextBox.getText().toUpperCase().trim();
newSymbolTextBox.setFocus(true);
// Stock code must be between 1 and 10 chars that are numbers, letters,
// or dots.
if (!symbol.matches("^[0-9a-zA-Z\\.]{1,10}$")) {
Window.alert("'" + symbol + "' is not a valid symbol.");
newSymbolTextBox.selectAll();
return;
}
newSymbolTextBox.setText("");
// Don't add the stock if it's already in the table.
if (stocks.contains(symbol))
return;
addStock(new StockClient(symbol));
}
private void addStock(final StockClient stock) {
stockService.addStock(stock.getSymbol(), new AsyncCallback<Long>() {
public void onFailure(Throwable error) {
handleError(error);
}
public void onSuccess(Long id) {
stock.setId(id);
displayStock(stock.getSymbol());
}
});
}
Modify the StockServiceImpl Class to Return StockClient[]
Finally, we modify the getStocks method of the StockServiceImpl class so that it translates the server-side Stock classes into client-side StockClient classes before returning the array.
StockServiceImpl.java
import com.google.gwt.sample.stockwatcher.client.StockClient;
We need to change the addStock method slightly so that the generated ID is returned:
public Long addStock(String symbol) throws NotLoggedInException {
Stock stock = new Stock(getUser(), symbol);
checkLoggedIn();
PersistenceManager pm = getPersistenceManager();
try {
pm.makePersistent(stock);
} finally {
pm.close();
}
return stock.getId();
}
All other methods stay the same, except getStocks:
public StockClient[] getStocks() throws NotLoggedInException {
checkLoggedIn();
PersistenceManager pm = getPersistenceManager();
List<StockClient> stockclients = new ArrayList<StockClient>();
try {
Query q = pm.newQuery(Stock.class, "user == u");
q.declareParameters("com.google.appengine.api.users.User u");
q.setOrdering("createDate");
List<Stock> stocks = (List<Stock>) q.execute(getUser());
for (Stock stock : stocks)
{
stockclients.add(new StockClient(stock.getId(), stock.getSymbol(), stock.getCreateDate()));
}
} finally {
pm.close();
}
return (StockClient[]) stockclients.toArray(new StockClient[0]);
}
Summary
The code above works perfectly for me when deployed to Google App Engine, but triggers an error in Google Web Toolkit Hosted Mode:
SEVERE: [1244408678890000] javax.servlet.ServletContext log: Exception while dispatching incoming RPC call
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract com.google.gwt.sample.stockwatcher.client.StockClient[] com.google.gwt.sample.stockwatcher.client.StockService.getStocks() throws com.google.gwt.sample.stockwatcher.client.NotLoggedInException' threw an unexpected exception: java.lang.NullPointerException: Name is null
Let me know if you encounter the same problem or not. The fact that it works in Google App Engine seems to indicate a bug in Hosted Mode.
GWT needs the .java file in addition to the .class file. Additionally, Stock needs to be in the "client" location of a GWT module.
The GWT compiler doesn't know about Stock, because it's not in a location it looks in. You can either move it to the client folder, or if it makes more sense leave it where it is and create a ModuleName.gwt.xml that references any other classes you want, and get your Main.gwt.xml file to inherit from that.
eg: DomainGwt.gwt.xml
<module>
<inherits name='com.google.gwt.user.User'/>
<source path="javapackagesabovethispackagegohere"/>
</module>
and:
<module rename-to="gwt_ui">
<inherits name="com.google.gwt.user.User"/>
<inherits name="au.com.groundhog.groundpics.DomainGwt"/>
<entry-point class="au.com.groundhog.groundpics.gwt.client.GPicsUIEntryPoint"/>
</module>
There's a better answer here: GWT Simple RPC use case problem : Code included
Basically, you can add parameters to your APPNAME.gwt.xml file so the compiler to give the compiler a path to the server-side class.
I was getting the same issue and the "mvn gwt:compile" output was not very helpful.
Instead, when I tried deploying to tomcat (via the maven tomcat plugin: mvn tomcat:deploy) I got helpful error messages.
A few things I had to fix up:
Make the object that is sent from the client to the server implement Serializable
Add an empty-arg constructor to that same object
Yes, it is sure that we need to use the Serialization for getting the server objects to the client. These modile?? file settings won't work to use the Stock class in the client side.
In your case you have only one class Stock and you can create a StockClient in client side. It is easy. But what will be the solution if anyone having more classes. Something like the properties of this class are also some other type of classes.
Example: stock.getEOD(date).getHigh();
getEOD will return another class with the given date and that class has the getHigh method.
What to do in such big cases? I don't think creating all classes implementing serialization in client side is good for that. Then we have to write code in both server and client. all classes two times.
Keying off of rustyshelf's answer above ...
In my case I needed to edit the ModuleName.gwt.xml file and add the following:
<source path='client'/>
<source path='shared'/>
I created my project with the New->Web Application Project wizard but unchecked the Generate project sample code option. I then created the shared package. Had I not unchecked that, the package would have been created for me and the xml file modified per the above.
There is a far more simple and easy solution for that. If you want to send an object of your custom designed class from server side to client side you should define this custom class in shared package.
For example for your case the you just have to carry the Stock.java class (by drag and drop) into
com.google.gwt.sample.stockwatcher.shared
package. However from your package hierarchy screenshot it seems that you had deleted this shared package. Just re-create this package and drop the Stock.java inside it and let the game begin.