jvm – Make Me Engineer

Determining location of JVM executable during runtime

You could always just use os.name to check if the user is running Windows or not. That’ll work on OS X, Linux and Windows at String jvm_location; if (System.getProperty(“os.name”).startsWith(“Win”)) { jvm_location = System.getProperties().getProperty(“java.home”) + File.separator + “bin” + File.separator + “java.exe”; } else { jvm_location = System.getProperties().getProperty(“java.home”) + File.separator + “bin” + File.separator + “java”; … Read more

Singleton across JVM or Application instance or Tomcat instance

If you have a singleton class and you run two webapps that use this class in Tomcat both webapps will get 2 different instances of this singleton in JVM running the Tomcat. But if your webapp will use a singleton from JRE or Tomcat shared libs, eg Runtime.getRuntime webapps will get the same instance of … Read more

Why a sawtooth shaped graph?

The sawtooth pattern in the heap usage can be explained by the fact that several local variables are created during the invocation of the System.out.println invocation. Most notably in the Oracle/Sun JRE, several HeapCharBuffer instances are created in the young generation, as noted in the following snapshot obtained using the memory profiler of VisualVM: The … Read more

Android Studio Build gradle OutOfMemoryError

One of the methods below should work for you: METHOD 1 : Open gradle.properties file from your project tree add this line at the memory allocation line org.gradle.jvmargs=-XX\:MaxHeapSize\=256m -Xmx256m or org.gradle.jvmargs=-XX\:MaxHeapSize\=512m -Xmx512m or org.gradle.jvmargs=-XX\:MaxHeapSize\=1024m -Xmx1024m Depending on the Memory of your computer. Then Invalidate Caches/Restart project from the File > Invalidate Caches/Restart To clean up … Read more

Where does the JVM store primitive variables?

Simplistic answer: it depends on where the variable is declared, not on its type. Local variables are stored on the stack. Instance and static variables are stored on the heap. Don’t forget that for reference type variables, the value of a variable is a reference, not the object. (Arrays are reference types too – so … Read more

How to emit and execute Java bytecode at runtime?

Here is a working “hello world” made with ObjectWeb ASM (a library which I recommend): package hello; import java.lang.reflect.Method; import org.objectweb.asm.ClassWriter; import org.objectweb.asm.Label; import org.objectweb.asm.MethodVisitor; import org.objectweb.asm.Opcodes; public class HelloWorldASM implements Opcodes { public static byte[] compile(String name) { ClassWriter cw = new ClassWriter(0); MethodVisitor mv; cw.visit(V1_6, ACC_PUBLIC + ACC_SUPER, “hello/HelloWorld”, null, “java/lang/Object”, null); cw.visitSource(“HelloWorld.java”, … Read more

What are the circumstances under which a finally {} block will NOT execute?

If you call System.exit() the program exits immediately without finally being called. A JVM Crash e.g. Segmentation Fault, will also prevent finally being called. i.e. the JVM stops immediately at this point and produces a crash report. An infinite loop would also prevent a finally being called. The finally block is always called when a … Read more

Tail Call Optimisation in Java

Why can’t Java use the same approach ? I can’t say which approach will be used, but it’s better-explained in Project Loom’s proposal: As adding the ability to manipulate call stacks to the JVM will undoubtedly be required, it is also the goal of this project to add an even lighter-weight construct that will allow … Read more

How can I disable IPv6 stack use for IPv4 IPs on JRE?

I wanted to use this for some program I hadn’t control for running that Java app so ended with this _JAVA_OPTIONS=-Djava.net.preferIPv4Stack=true environment variable. (read about _JAVA_OPTIONS here) If you are using Windows, just run this command on Windows cmd: setx _JAVA_OPTIONS -Djava.net.preferIPv4Stack=true Thanks to Jason Nichols for reminding this JVM argument 🙂

What is the default maximum heap size for Sun’s JVM from Java SE 6?

java 1.6.0_21 or later, or so… $ java -XX:+PrintFlagsFinal -version 2>&1 | grep MaxHeapSize uintx MaxHeapSize := 12660904960 {product} It looks like the min(1G) has been removed. Or on Windows using findstr C:\>java -XX:+PrintFlagsFinal -version 2>&1 | findstr MaxHeapSize

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