Java Virtual Machine For Mac

Java™ SE 7 Update 4

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2. Java Virtual Machine Download For Mac

The full internal version number for this update release is 1.7.0_04-b20 (where 'b' means 'build') except for Windows on which it is 1.7.0_04-b22 and for Mac OS X on which it is 1.7.0_04-b21. The external version number is 7u4.

Highlights

This update release contains the following enhancements:

• JDK Support for Mac OS X
• New JVM (Java HotSpot Virtual Machine, version 23)
• New Supported Garbage Collector: Garbage First (G1)
• JavaFX 2.1 Runtime co-installs with JRE 7 during auto-update
• JAXP upgraded to 1.4.6
• Java DB upgraded to 10.8.2.2
• SPARC T4 specific crypto optimizations in the security area
• New flag to unlock Commercial Features

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Olson Data 2012c

Java SE 7u4 contains Olson time zone data version 2012c. For more information, refer to Timezone Data Versions in the JRE Software.

Security Baselines

The security baselines for the Java SE platform at the time of the release of Java SE 7u4 are specified in the following table:

For more information about security baselines, see Deploying Java Applets With Family JRE Versions in Java Plug-in for Internet Explorer.

JDK Support for Mac OS X

This release of Java SE 7u4 includes 64-bit JDK support for Mac OS X Lion and above. You can download the Mac OS X JDK from Java SE Downloads page.

For information on installing JDK 7u4 for Mac OS X, see Installation for Mac OS X document.
For support details, see Supported System Configurations page.

JDK support for Mac OS X does not include client deployment technologies like Java Plugin and Java Web Start. JRE for Mac OS X will be available in an upcoming JDK 7 update release. For an early access release of the JRE for Mac OS X with applet support, visit Mac OS X Port Developer Preview Release.

Some serviceability features may not work in Java SE 7u4 on Mac OS X. See Known Issues for more details.

New JVM (Java HotSpot Virtual Machine, version 23)

HotSpot 23 features JRockit JVM feature convergence. Some of the value-add features of the JRockit JVM are re-implemented in the HotSpot JVM. These features include:

• Text dumps with buffered data: buffered JVM state information available in text crash dumps and core/mdmp
• Diagnostics command framework (jcmd) and diagnostic commands
• Enhanced JMX Agent

Parallel Old GC (-XX:+UseParallelOldGC) is now enabled by default whenever -XX:+UseParallelGC is enabled. To revert to the previous default behavior, use the option -XX:-UseParallelOldGC on the java command line.

According to specification, the JVM should not maintain any particular order of methods being returned by reflection. In previous versions, the JVM used to return the methods in the declaration order and some user code may have relied on this behavior. Starting from Java SE 7, the JVM no longer guarantees constant order of methods and JMX MBeanInfo no longer guarantees equality of MBeanInfo between two identical MBeans.

New Supported Garbage Collector: Garbage First (G1)

Starting in Java SE 7u4 the Garbage First Collector is fully supported. The G1 collector is targeted for applications that fully utilize the large amount of memory available in today's multiprocessor servers, while still keeping garbage collection latencies under control. Applications that require a large heap, have a big active data set, have bursty or non-uniform workloads or suffer from long Garbage Collection induced latencies should benefit from switching to G1. For more detailed information about G1 see the G1 documentation page and command line options.

JAXP upgraded to 1.4.6

The new JAXP 1.4.6 version includes fixes for critical issues in the areas of conformance, performance, and regressions.

Java DB upgraded to 10.8.2.2

Java DB 10.8.2.2 is a bugfix release. For a complete list of bugs fixed in this release, see http://db.apache.org/derby/releases/release-10.8.2.2.cgi.

SPARC T4 specific crypto optimizations in the security area

SPARC T4 contains on-chip(native) cryptographic implementations. These implementations can be accessed by Java applications utilizing either SunPKCS11-Solaris provider(available since Java SE 5.0 release) or OracleUcrypto Provider(new in 7u4 release).

Canon pixma mx490 software install. The new OracleUcrypto Service Provider accesses the underlying native(T4) crypto library without going through the PKCS11 layer and is configured by default to be the most preferred provider for Solaris OS.

New flag to unlock Commercial Features

The Java SE 7 Update 4 release introduces a new flag, -XX:+UnlockCommercialFeatures. This flag enables Oracle Java SE users to control when licensed features are allowed to run. All commercial features started or controlled via the command line or dynamically during execution will be gated by this flag. By default, commercial features are not allowed to execute, and any usage requires an active unlocking either on the command line or dynamically during runtime, to help remove any accidental usage.

For information on command line flags, see the command line documentation for Windows and Solaris/Linux platforms. For information about commercial features in Oracle Java SE and the license requirement please refer to Oracle Java SE Advanced and Suite, and the Binary Code License.

Bug Fixes

Java SE 7u4 does not add any fixes for security vulnerabilities beyond those in Java SE 7u3.

Notable Bug Fixes in Java SE 7u4

This list includes some of the notable bug fixes in Java SE 7u4.

Area: java:install
Synopsis: my .jar file can no long run by double-click under Windows Explorer.

The 7u4 installer now correctly registers javaw.exe as handler for jar files. After installing 7u4 build 22(Windows), jars can be launched correctly by double clicking them.

See 7165926.

Area: java/classes_awt 4
Synopsis: [macosx] Document Usage of -XstartOnFirstThread and -Xdock.

The command line arguments -XstartOnFirstThread, -Xdock:icon, and -Xdock:name are now documented as officially supported options on the Mac OS platform. The following command will print out these options at the command line:
java -help
See 7131038.

For other bug fixes, see Java SE 7u4 Bug Fixes.

Known Issues

Area: java/classes_awt
Synopsis: [macosx] dock menu don't show available frames.

Right-clicking a dock icon of a Java app will not list all open Java top-level windows in the menu as Apple JDK used to do. In order to activate a window, find the window on the screen and click it directly. This is a known issue that will be addressed in a future release.
See 7149062.

Area: java command
Synopsis: Wildcard expansion for single entry classpath does not work on Windows platforms.

The Java command and Setting the classpath documents describe how the wildcard character (*) can be used in a classpath element to expand into a list of the .jar files in the associated directory, separated by the classpath separator (;).

This wildcard expansion does not work in a Windows command shell for a single element classpath due to the Microsoft bug described in Wildcard Handling is Broken. A workaround is to add another element to the classpath. For example, instead of the following:

use:

See 7146424.

Area: java command
Synopsis: Wildcard expansion behavior change on Windows platforms.

In the earlier JDK releases, a wildcard (* or ?) in double quotes on a DOS command line did not get expanded. For example, when using the following command:

the following string is passed to xxx:

In the current JDK release, wildcards in quoted arguments get expanded, and the matched file names (.java) are passed to xxx.

A workaround to produce the previous behavior is to escape the double quotes. For example, instead of the following command:

use:

Also ensure that your application code can accept both '*.java' as well as *.java as values.

Area: Diagnostics
Synopsis: [macosx] Issues with using Serviceability Agent(SA).

The following developer services are not available on Mac OS X:

• Process attach and diagnostics for jtools, such as jinfo, jstack, and jmap:
  

  The system architecture is not recognized and the tools will fail with the error message:
  'Error attaching to process: Bsd only supported on x86/amd64'.

• Parsing core files:
  

  Support for reading core files and providing diagnostic information will not be available. Core files from Java crashes are platform specific. The parser for core files on Mac OS X (MACH-O format) is not yet implemented.

Area: java/debugger
Synopsis: JDI/JDB debugger command line option parsing does not allow comma delimited options on Windows platform

When launching JDB from the command line on Windows platform, if a comma delimited JVM option is passed to either the debugger or the debuggee, it may fail with an IllegalArgumentException. For example, consider the following:

In the above command, even though the comma delimited option has quotation marks, these quotes are not preserved for the launched process.

A workaround for this issue is to use additional qualifiers (') with the quotation marks. For example:

This command works correctly as the JVM option now retains the quotation marks.
See 7154809.

Area: java/classes_nio
Synopsis: Selector.select hang due to poll(2) bug [macosx]

An issue with the Mac OS X poll(2) system call has been found to cause problems for networking applications based on the java.nio socket channel classes. This issue has been detected with the Oracle WebLogic Server on Mac OS X.

A workaround to mitigate the problem is setting the following system property on the Java command line:

Java Virtual Machine Error Mac

See 7159361.

Area: hotspot/runtime_arguments
Synopsis: JVM not printing unknown -XX options

If an unrecognized -XX option is supplied to the JVM, it exits. However, the resulting error message does not clearly state that the reason for the JVM exiting is because of the unrecognized option.
See 7162488.

Here is an example of the error output:

Whether you have used Java to develop programs or not, you might have heard about the Java Virtual Machine (JVM) at some point or another.

JVM is the core of the Java ecosystem, and makes it possible for Java-based software programs to follow the 'write once, run anywhere' approach. You can write Java code on one machine, and run it on any other machine using the JVM.

JVM was initially designed to support only Java. However, over the time, many other languages such as Scala, Kotlin and Groovy were adopted on the Java platform. All of these languages are collectively known as JVM languages.

In this article, we will learn more about the JVM, how it works, and the various components that it is made of.

Before we jump into the JVM, let's revisit the concept of a Virtual Machine (VM).

A virtual machine is a virtual representation of a physical computer. We can call the virtual machine the guest machine, and the physical computer it runs on is the host machine.

A single physical machine can run multiple virtual machines, each with their own operating system and applications. These virtual machines are isolated from each other.

In programming languages like C and C++, the code is first compiled into platform-specific machine code. These languages are called compiled languages.

On the other hand, in languages like JavaScript and Python, the computer executes the instructions directly without having to compile them. These languages are called interpreted languages.

Java uses a combination of both techniques. Java code is first compiled into byte code to generate a class file. This class file is then interpreted by the Java Virtual Machine for the underlying platform. The same class file can be executed on any version of JVM running on any platform and operating system.

Similar to virtual machines, the JVM creates an isolated space on a host machine. This space can be used to execute Java programs irrespective of the platform or operating system of the machine.

The JVM consists of three distinct components:

1. Class Loader
2. Runtime Memory/Data Area
3. Execution Engine

Let's take a look at each of them in more detail.

Class Loader

When you compile a .java source file, it is converted into byte code as a .class file. When you try to use this class in your program, the class loader loads it into the main memory.

The first class to be loaded into memory is usually the class that contains the main() method.

There are three phases in the class loading process: loading, linking, and initialization.

Loading

Loading involves taking the binary representation (bytecode) of a class or interface with a particular name, and generating the original class or interface from that.

There are three built-in class loaders available in Java:

• Bootstrap Class Loader- This is the root class loader. It is the superclass of Extension Class Loader and loads the standard Java packages like java.lang, java.net, java.util, java.io, and so on. These packages are present inside the rt.jar file and other core libraries present in the $JAVA_HOME/jre/lib directory.
• Extension Class Loader - This is the subclass of the Bootstrap Class Loader and the superclass of the Application Class Loader. This loads the extensions of standard Java libraries which are present in the $JAVA_HOME/jre/lib/ext directory.
• Application Class Loader - This is the final class loader and the subclass of Extension Class Loader. It loads the files present on the classpath. By default, the classpath is set to the current directory of the application. The classpath can also be modified by adding the -classpath or -cp command line option.

The JVM uses the ClassLoader.loadClass() method for loading the class into memory. It tries to load the class based on a fully qualified name.

If a parent class loader is unable to find a class, it delegates the work to a child class loader. If the last child class loader isn't able to load the class either, it throws NoClassDefFoundError or ClassNotFoundException.

Linking

After a class is loaded into memory, it undergoes the linking process. Linking a class or interface involves combining the different elements and dependencies of the program together.

Linking includes the following steps:

Verification: This phase checks the structural correctness of the .class file by checking it against a set of constraints or rules. If verification fails for some reason, we get a VerifyException.

For example, if the code has been built using Java 11, but is being run on a system that has Java 8 installed, the verification phase will fail.

Preparation: In this phase, the JVM allocates memory for the static fields of a class or interface, and initializes them with default values.

For example, assume that you have declared the following variable in your class:

During the preparation phase, JVM allocates memory for the variable enabled and sets its value to the default value for a boolean, which is false.

Resolution: In this phase, symbolic references are replaced with direct references present in the runtime constant pool.

For example, if you have references to other classes or constant variables present in other classes, they are resolved in this phase and replaced with their actual references.

Initialization

Initialization involves executing the initialization method of the class or interface (known as <clinit>). This can include calling the class's constructor, executing the static block, and assigning values to all the static variables. This is the final stage of class loading. Cue point with spotify djay pro software.

For example, when we declared the following code earlier:

The variable enabled was set to its default value of false during the preparation phase. In the initialization phase, this variable is assigned its actual value of true.

Note: the JVM is multi-threaded. It can happen that multiple threads are trying to initialize the same class at the same time. This can lead to concurrency issues. You need to handle thread safety to ensure that the program works properly in a multi-threaded environment.

Runtime Data Area

There are five components inside the runtime data area:

Let's look at each one individually.

Method Area

All the class level data such as the run-time constant pool, field, and method data, and the code for methods and constructors, are stored here.

If the memory available in the method area is not sufficient for the program startup, the JVM throws an OutOfMemoryError.

For example, assume that you have the following class definition:

In this code example, the field level data such as name and age and the constructor details are loaded into the method area.

The method area is created on the virtual machine start-up, and there is only one method area per JVM.

Heap Area

All the objects and their corresponding instance variables are stored here. This is the run-time data area from which memory for all class instances and arrays is allocated.

For example assume that you are declaring the following instance:

In this code example, an instance of Employee is created and loaded into the heap area.

The heap is created on the virtual machine start-up, and there is only one heap area per JVM.

Note: Since the Method and Heap areas share the same memory for multiple threads, the data stored here is not thread safe.

Stack Area

Whenever a new thread is created in the JVM, a separate runtime stack is also created at the same time. All local variables, method calls, and partial results are stored in the stack area.

If the processing being done in a thread requires a larger stack size than what's available, the JVM throws a StackOverflowError.

For every method call, one entry is made in the stack memory which is called the Stack Frame. When the method call is complete, the Stack Frame is destroyed.

The Stack Frame is divided into three sub-parts:

• Local Variables – Each frame contains an array of variables known as its local variables. All local variables and their values are stored here. The length of this array is determined at compile-time.
• Operand Stack – Each frame contains a last-in-first-out (LIFO) stack known as its operand stack. This acts as a runtime workspace to perform any intermediate operations. The maximum depth of this stack is determined at compile-time.
• Frame Data – All symbols corresponding to the method are stored here. This also stores the catch block information in case of exceptions.

Washburn guitar serial number decoder. For example assume that you have the following code:

In this code example, variables like answers and score are placed in the Local Variables array. The Operand Stack contains the variables and operators required to perform the mathematical calculations of subtraction and division.

Note: Since the Stack Area is not shared, it is inherently thread safe.

Program Counter (PC) Registers

The JVM supports multiple threads at the same time. Each thread has its own PC Register to hold the address of the currently executing JVM instruction. Once the instruction is executed, the PC register is updated with the next instruction.

Native Method Stacks

The JVM contains stacks that support native methods. These methods are written in a language other than the Java, such as C and C++. For every new thread, a separate native method stack is also allocated.

Java Virtual Machine Download For Mac

Execution Engine

Once the bytecode has been loaded into the main memory, and details are available in the runtime data area, the next step is to run the program. The Execution Engine handles this by executing the code present in each class.

However, before executing the program, the bytecode needs to be converted into machine language instructions. The JVM can use an interpreter or a JIT compiler for the execution engine.

Interpreter

The interpreter reads and executes the bytecode instructions line by line. Due to the line by line execution, the interpreter is comparatively slower.

Another disadvantage of the interpreter is that when a method is called multiple times, every time a new interpretation is required.

JIT Compiler

The JIT Compiler overcomes the disadvantage of the interpreter. The Execution Engine first uses the interpreter to execute the byte code, but when it finds some repeated code, it uses the JIT compiler.

The JIT compiler then compiles the entire bytecode and changes it to native machine code. This native machine code is used directly for repeated method calls, which improves the performance of the system.

The JIT Compiler has the following components:

1. Intermediate Code Generator - generates intermediate code
2. Code Optimizer - optimizes the intermediate code for better performance
3. Target Code Generator - converts intermediate code to native machine code
4. Profiler - finds the hotspots (code that is executed repeatedly)

To better understand the difference between interpreter and JIT compiler, assume that you have the following code:

An interpreter will fetch the value of sum from memory for each iteration in the loop, add the value of i to it, and write it back to memory. This is a costly operation because it is accessing the memory each time it enters the loop.

However, the JIT compiler will recognize that this code has a HotSpot, and will perform optimizations on it. It will store a local copy of sum in the PC register for the thread and will keep adding the value of i to it in the loop. Once the loop is complete, it will write the value of sum back to memory.

Note: a JIT compiler takes more time to compile the code than for the interpreter to interpret the code line by line. If you are going to run a program only once, using the interpreter is better.

Garbage Collector

The Garbage Collector (GC) collects and removes unreferenced objects from the heap area. It is the process of reclaiming the runtime unused memory automatically by destroying them.

Garbage collection makes Java memory efficient because because it removes the unreferenced objects from heap memory and makes free space for new objects. It involves two phases:

1. Mark - in this step, the GC identifies the unused objects in memory
2. Sweep - in this step, the GC removes the objects identified during the previous phase

Garbage Collections is done automatically by the JVM at regular intervals and does not need to be handled separately. It can also be triggered by calling System.gc(), but the execution is not guaranteed.

The JVM contains 3 different types of garbage collectors:

1. Serial GC - This is the simplest implementation of GC, and is designed for small applications running on single-threaded environments. It uses a single thread for garbage collection. When it runs, it leads to a 'stop the world' event where the entire application is paused. The JVM argument to use Serial Garbage Collector is -XX:+UseSerialGC
2. Parallel GC - This is the default implementation of GC in the JVM, and is also known as Throughput Collector. It uses multiple threads for garbage collection, but still pauses the application when running. The JVM argument to use Parallel Garbage Collector is -XX:+UseParallelGC.
3. Garbage First (G1) GC - G1GC was designed for multi-threaded applications that have a large heap size available (more than 4GB). It partitions the heap into a set of equal size regions, and uses multiple threads to scan them. G1GC identifies the regions with the most garbage and performs garbage collection on that region first. The JVM argument to use G1 Garbage Collector is -XX:+UseG1GC

Note: There is another type of garbage collector called Concurrent Mark Sweep (CMS) GC. However, it has been deprecated since Java 9 and completely removed in Java 14 in favour of G1GC.

Java Native Interface (JNI)

At times, it is necessary to use native (non-Java) code (for example, C/C++). This can be in cases where we need to interact with hardware, or to overcome the memory management and performance constraints in Java. Java supports the execution of native code via the Java Native Interface (JNI).

JNI acts as a bridge for permitting the supporting packages for other programming languages such as C, C++, and so on. This is especially helpful in cases where you need to write code that is not entirely supported by Java, like some platform specific features that can only be written in C.

You can use the native keyword to indicate that the method implementation will be provided by a native library. You will also need to invoke System.loadLibrary() to load the shared native library into memory, and make its functions available to Java.

Native Method Libraries

Native Method Libraries are libraries that are written in other programming languages, such as C, C++, and assembly. These libraries are usually present in the form of .dll or .so files. These native libraries can be loaded through JNI.

• ClassNotFoundExcecption - This occurs when the Class Loader is trying to load classes using Class.forName(), ClassLoader.loadClass() or ClassLoader.findSystemClass() but no definition for the class with the specified name is found.
• NoClassDefFoundError - This occurs when a compiler has successfully compiled the class, but the Class Loader is not able to locate the class file at the runtime.
• OutOfMemoryError - This occurs when the JVM cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.
• StackOverflowError - This occurs if the JVM runs out of space while creating new stack frames while processing a thread.

In this article, we discussed the Java Virtual Machine's architecture and its various components. Often we do not dig deep into the internal mechanics of the JVM or care about how it works while our code is working.

It is only when something goes wrong, and we need to tweak the JVM or fix a memory leak, that we try to understand its internal mechanics.

This is also a very popular interview question, both at junior and senior levels for backend roles. A deep understanding of the JVM helps you write better code and avoid pitfalls related to stack and memory errors.

Thank you for staying with me so far. Hope you liked the article. You can connect with me on LinkedIn where I regularly discuss technology and life. Also take a look at some of my other articles and my YouTube channel. Happy reading. 🙂