Runtime. getRuntime(). GC () and runtime getRuntime(). The difference between runfinalization()

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Preface

Many friends may not have used or even seen these two methods , But as a java Development , You must have seen System.gc();
With this idea , Let's move on

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One 、Runtime.getRuntime().gc()

1. And System.gc() contrast

Let's drive System.gc() Source code , I found that it was actually a call Runtime.getRuntime().gc() , So they are equivalent . It's just that we usually use jdk For us System class

2. Official statement

Runtime.getRuntime().gc() Method , The official comment is like this ：

Which translates as ：

stay Java Run the garbage collector in the virtual machine .
Call the gc The method shows that Java Virtual machines spend energy recycling unused objects , To make the memory they currently occupy available for Java Virtual machine reuse . When control returns from a method call ,Java The virtual machine has done its best to reclaim space from all unused objects . There is no guarantee that this work will recycle any specific number of unused objects , Reclaim any specific amount of space , Or at any particular time （ If any ） Complete before or forever after the method returns . There is no guarantee that this effort will determine the accessibility changes of any specific number of objects , Or any specific number Reference Objects will be cleared and queued .
call System.gc() In fact, it is equivalent to calling ：Runtime.getRuntime().gc()

Two 、Runtime.getRuntime().runFinalization()

1. And System.runFinalization() contrast

Again , Let's drive System.runFinalization() Source code , I found that it was actually a call Runtime.getRuntime().runFinalization() , So they are equivalent .

2. Official statement

Runtime.getRuntime().runFinalization() Method , The official comment is like this ：

Which translates as ：

Run the termination method of any object waiting for termination . Calling this method indicates Java Virtual machines will work hard finalize Found and discarded but finalize Method of an object that has not been run . When control returns from a method call ,Java The virtual machine has done its best to complete all unfinished terminations .
call System.runFinalization() In fact, it is equivalent to calling ： Runtime.getRuntime().runFinalization()

Runtime.getRuntime().runFinalization() Is and object Of finalize() Used in combination with

however , from Java9 Start ,object Of finalize() The method has been labeled @Deprecated Out of date

therefore , It is no longer recommended to use Runtime.getRuntime().runFinalization() Conduct gc 了

Be careful ： In the latest java18 in ,finalize() and runFinalization() Has been marked as deprecated ,, It is no longer a simple expiration , It may be deleted in the future

3. Why not recommend finalize()

1. The timing of the call is uncertain
   although finalize() Method will be called sooner or later , However, the uncontrollability of this call timing may lead to system level exceptions due to the delayed release of resources . Because computer resources are limited , When it is clear to release some resources （ For example, in the example above reader A series of resources under control ）, Other methods should be used to release these resources immediately
2. Affect code portability
   Because of every kind of JVM The built-in garbage collection algorithms are all different , So maybe in your JVM in , You worked hard to write the use finalize The case of method works well , But transplant to different JVM In the middle of the day , It is likely to collapse in a mess ！
3. The high cost
   If a class is overloaded finalize Method and implements some logic inside the method , that JVM Before constructing or destroying objects of this class , Will do a lot of extra work . Obviously , If a class is not overloaded finalize Method , Then when destroying, just deal with the memory in the heap , And if it's overloaded finalize Method words , To carry out finalize Method , In case there are some exceptions or errors in the execution process , The cost of consumption is even higher .
4. Abnormal loss
   In case fianlize Exception thrown in method , that finalize Will terminate the operation , And the exception thrown will also be discarded , Eventually, the object instance will be in a zombie state of semi destruction and semi survival , Lead to unexpected consequences ！

This clip is referenced from ：

https://www.bilibili.com/read/cv4055723

3、 ... and 、 test GC

Maybe we are all professional players of eight part essay , Proficient in the underlying principles of various technical frameworks , Recite all kinds of sql Optimization and gc principle , Of course , This is in the case of an interview , In development , Most of us have never been exposed to these , Just pretend it during the interview .

however , In order to make us feel more intuitively GC The effect , It's also a simple one for us GC Introductory cases , I use the actual code to show you the mysterious GC.

I don't know if you've known about mbean,java The code will treat all kinds of information as one by one mbean In the mbeanServer management , for example OS Information about ,jvm Memory usage , And it can be dynamically modified .

1. View the current operating system (OS) Take up memory

stay mbeanServer in ,OS The information of only corresponds to ObjectName：java.lang:type=OperatingSystem

MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer();
ObjectName objectName = new ObjectName("java.lang:type=OperatingSystem");
// OS Maximum memory of 
Long totalPhysicalMemorySize = (Long)mBeanServer.getAttribute(objectName, "TotalPhysicalMemorySize");
// OS Of free memory 
Long freePhysicalMemorySize = (Long)mBeanServer.getAttribute(objectName, "FreePhysicalMemorySize");
Long totalRAM = totalPhysicalMemorySize / 1024 / 1024;
Long freeRAM = freePhysicalMemorySize / 1024 / 1024;
System.out.println(" Total memory of operating system ：" + totalRAM + " MB");
System.out.println(" The memory of the operating system is used ：" + (totalRAM - freeRAM) + " MB");

2. see jvm Memory heap middle-aged and old generation

stay mbeanServer Memory pool ObjectName With java.lang:type=MemoryPool start

It can be used * To match all the jvm Memory pool , Including what we are familiar with Old age 、 The younger generation （Eden District ,survivor District ）、 Meta space 、 Code cache, etc .

Acquired CompositeDataSupport yes MXBean The type of , It contains attributes max、used、init、commited

MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer();
ObjectName objectName = new ObjectName("java.lang:type=MemoryPool,*");
Set<ObjectInstance> memoryPools = mBeanServer.queryMBeans(objectName, null);
for (ObjectInstance memoryPool : memoryPools) {
    
	ObjectName objectName = memoryPool.getObjectName();
	String name = (String)mBeanServer.getAttribute(objectName, "Name");
	CompositeDataSupport cd = (CompositeDataSupport)mBeanServer.getAttribute(objectName, "Usage");
}

3. Complete example

The description in the code is very detailed , Just look at it ：

Direct operation main Method ,

Code logic ：

1. To look at first main Method used in the basic operation jvm Memory
2. Make a statement List, Put in the data , That is, some jvm Of memory （ Let's start with the younger generation , When memory is insufficient, transfer to the old age ）
3. Then check the occupied jvm Memory
4. hold List Remove the reference to , Again gc
5. Then check the occupied jvm Memory （ By this time, I have already LIst The memory garbage occupied by data is recycled ）

public class TestGc {
    

    static List<Integer> list;

    static MBeanServer mBeanServer;

    static ObjectName os_objectName;

    static ObjectName mp_objectName;

    static{
    
        //  obtain  mbeanServer
        mBeanServer = ManagementFactory.getPlatformMBeanServer();
        try {
    
            // jvm When running, information such as operating system memory will be put into mBeanServer,ObjectName The only correspondence is ：java.lang:type=OperatingSystem
            os_objectName = new ObjectName("java.lang:type=OperatingSystem");
            //  stay java8 Of mBeanServer Memory pool MBean Of ObjectName With  java.lang:type=MemoryPool  start 
            mp_objectName = new ObjectName("java.lang:type=MemoryPool,*");
        } catch (MalformedObjectNameException e) {
    
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws Exception {
    
        osRAM();

        System.out.println("main After method startup ,jvm Memory heap usage of the younger and older generations ：");

        jvmRAM();

        list = new ArrayList<>();
        for (int i = 0; i < 10000000; i++) {
    
            list.add(i);
        }

        System.out.println("new Out List And put the data ,jvm Memory heap usage of the younger and older generations ：");

        jvmRAM();

        list = null;  //  If you don't cancel the reference here , Even call  gc  It won't recycle  list Memory footprint 
        System.gc();
        System.out.println(" Cancel List References to , perform gc after ,jvm Memory heap usage of the younger and older generations ：");

        jvmRAM();
    }

    public static void osRAM() throws Exception {
    
        // OS Maximum memory of 
        Long totalPhysicalMemorySize = (Long)mBeanServer.getAttribute(os_objectName, "TotalPhysicalMemorySize");
        // OS Of free memory 
        Long freePhysicalMemorySize = (Long)mBeanServer.getAttribute(os_objectName, "FreePhysicalMemorySize");
        Long totalRAM = totalPhysicalMemorySize / 1024 / 1024;
        Long freeRAM = freePhysicalMemorySize / 1024 / 1024;
        System.out.println(" Total memory of operating system ：" + totalRAM + " MB");
        System.out.println(" The memory of the operating system is used ：" + (totalRAM - freeRAM) + " MB");
        System.out.println("----------------------------------------------------");
    }

    public static void jvmRAM() throws Exception {
    
        // jvm The young generation and the old generation 
        long eden = 0;
        long survivor = 0;
        long old = 0;
        Set<ObjectInstance> memoryPools = mBeanServer.queryMBeans(mp_objectName, null);
        for (ObjectInstance memoryPool : memoryPools) {
    
            ObjectName objectName = memoryPool.getObjectName();
            String name = (String)mBeanServer.getAttribute(objectName, "Name");
            CompositeDataSupport cd = (CompositeDataSupport)mBeanServer.getAttribute(objectName, "Usage");
            if(cd != null){
    
                if("PS Eden Space".equals(name)){
    
                    eden = (Long) cd.get("used");
                }else if("PS Survivor Space".equals(name)){
    
                    survivor = (Long) cd.get("used");
                }else if("PS Old Gen".equals(name)){
    
                    old = (Long) cd.get("used");
                }
            }
        }
        System.out.println(" The younger generation ：" + (eden + survivor) / 1024 / 1024 + " MB [eden " + eden / 1024 / 1024 + " MB + survivor " + survivor / 1024 / 1024 + " MB]");
        System.out.println(" Old age ：" + old / 1024 / 1024 + " MB");
        System.out.println("----------------------------------------------------");
    }
}

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summary

Welcome to point out my mistake ！