一文看盡Java-Thread
阿新 • • 發佈：2019-09-02
一、前言
主要分成兩部說起:Thread原始碼解讀和常見面試題解答，廢話不多說開始；
二、原始碼解讀
首先看下建構函式，建構函式都是通過呼叫init方法對屬性進行初始化，主要是對執行緒組、執行緒名字、棧大小等資訊進行初始化；init內部通過呼叫currentThread本地方法，獲取當前的執行緒，這個本地方法封裝在JVM中,有興趣的可以看下這個這個連結查詢下JVM實現https://hg.openjdk.java.net/jdk8u，接下來對ThreadGroup的判斷，如果沒有傳入執行緒組的話， 第一是使用SecurityManager中的ThreadGroup， 如果從SecurityManager 中獲取不到ThreadGroup()， 那麼就從當前執行緒中獲取執行緒組，最後做了檢驗和些引數的賦值，整體上相對比較簡單；
private void init(ThreadGroup g, Runnable target, String name,
long stackSize) {
init(g, target, name, stackSize, null);
}
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name.toCharArray();
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null) {
g = security.getThreadGroup();
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup();
}
}
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
if (parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
public Thread() {
init(null, null, "Thread-" + nextThreadNum(), 0);
}
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
Thread(Runnable target, AccessControlContext acc) {
init(null, target, "Thread-" + nextThreadNum(), 0, acc);
}
// 執行緒名
public Thread(String name) {
init(null, null, name, 0);
}
//執行緒組和執行緒名
public Thread(ThreadGroup group, String name) {
init(group, null, name, 0);
}
//執行緒任務，執行緒名
public Thread(Runnable target, String name){
init(null, target, name, 0);
}
// 執行緒組， 執行緒任務， 執行緒名 ，棧大小
public Thread(ThreadGroup group, Runnable target, String name, long stackSize) {
init(group, target, name, stackSize);
}接下來看下主要的屬性：
// 類載入的時候，呼叫本地的註冊本地方靜態方法， 這個方法是本地方法
private static native void registerNatives();
static {
registerNatives();
}
private volatile char name[];
private int priority;
private Thread threadQ;
private long eetop;
/* Whether or not to single_step this thread. */
private boolean single_step;
/* Whether or not the thread is a daemon thread. */
// 設設定這個執行緒是否是守護執行緒
private boolean daemon = false;
/* JVM state */
private boolean stillborn = false;
/* What will be run. */
// 要執行的run方法的物件
private Runnable target;
/* The group of this thread */
// 這個執行緒的執行緒組
private ThreadGroup group;
/* The context ClassLoader for this thread */
// 這個執行緒的上下文類載入器
private ClassLoader contextClassLoader;
/* The inherited AccessControlContext of this thread */
private AccessControlContext inheritedAccessControlContext;
/* For autonumbering anonymous threads. */
private static int threadInitNumber;
private static synchronized int nextThreadNum() {
return threadInitNumber++;
}
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
/*
* InheritableThreadLocal values pertaining to this thread. This map is
* maintained by the InheritableThreadLocal class.
*/
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
/*
* The requested stack size for this thread, or 0 if the creator did
* not specify a stack size. It is up to the VM to do whatever it
* likes with this number; some VMs will ignore it.
*/
// 給這個執行緒設定的棧的大小,預設為0
private long stackSize;
/*
* JVM-private state that persists after native thread termination.
*/
private long nativeParkEventPointer;
/*
* Thread ID
*/
//執行緒id
private long tid;
/* For generating thread ID */
private static long threadSeqNumber;
/* Java thread status for tools,
* initialized to indicate thread 'not yet started'
*/
private volatile int threadStatus = 0;
private static synchronized long nextThreadID() {
return ++threadSeqNumber;
}
/**
* The argument supplied to the current call to
* java.util.concurrent.locks.LockSupport.park.
* Set by (private) java.util.concurrent.locks.LockSupport.setBlocker
* Accessed using java.util.concurrent.locks.LockSupport.getBlocker
*/
volatile Object parkBlocker;
/* The object in which this thread is blocked in an interruptible I/O
* operation, if any. The blocker's interrupt method should be invoked
* after setting this thread's interrupt status.
*/
private volatile Interruptible blocker;
private final Object blockerLock = new Object();
/* Set the blocker field; invoked via sun.misc.SharedSecrets from java.nio code
*/
void blockedOn(Interruptible b) {
synchronized (blockerLock) {
blocker = b;
}
}
/**
* The minimum priority that a thread can have.
*/
// 執行緒執行的最低優先順序 為1
public final static int MIN_PRIORITY = 1;
/**
* The default priority that is assigned to a thread.
*/
// 執行緒預設的執行優先順序為 5
public final static int NORM_PRIORITY = 5;
/**
* The maximum priority that a thread can have.
*/
// 執行緒執行的最高的優先順序為 10
public final static int MAX_PRIORITY = 10;最後介紹下方法的作用和執行緒狀態，原始碼都比較簡單，沒必進行過多的介紹，都是通過呼叫JVM的本地方法實現；
執行緒狀態：
三、常見面試題
1.執行緒與程序的區別?
程序是資源分配最小的單位，執行緒是CPU排程最小的單位；
執行緒屬於程序，共享程序分配的資源；
程序屬於搶佔式排程，資源不相互共享；
2.start和run的區別?
run是Thread的一個普通的方法；
start方法會建立一個新的子執行緒並啟動；
3.sleep與wait的區別?
sleep是Thread方法，wait是Object的方法；
wait方法只能在synchroized方法或者塊中使用；
Thread.sleep只會讓出CPU，不會改變鎖的行為；
Object.wait不僅會讓出CPU，同時還會釋放佔有同步資源的鎖；
4.執行緒狀態的轉化?
圖中將WAITING 和TIMED_WAITING 兩個狀態合併為WAITING ，沒有分開，大家不要搞錯；
5.如何處理執行緒的返回值?
主執行緒等待法，使用while等待主執行緒返回值；
join阻塞當前執行緒以等待子執行緒；
通過FuTureTask獲取子執行緒的返回值；
public class MyCallable implements Callable<String> {
@Override
public String call() throws Exception {
String value="test";
System.out.println("start");
Thread.sleep(5000);
System.out.println("end");
return value;
}
}
public class FutureTaskDemo {
public static void main(String[] main) throws ExecutionException, InterruptedException {
FutureTask<String> futureTask=new FutureTask<String>(new MyCallable());
new Thread(futureTask).start();
if (!futureTask.isDone()){
System.out.println("waiting");
}
System.out.println("return"+futureTask.get());
}
}
View Code
通過執行緒池獲取返回值；
public class ThreadPoolDemo {
public static void main(String[] args){
ExecutorService executorService= Executors.newCachedThreadPool();
Future<String> futureTask=executorService.submit(new MyCallable());
if (!futureTask.isDone()){
System.out.println("wait");
}
try {
System.out.println(futureTask.get());
}catch (InterruptedException ex){
ex.printStackTrace();
}catch (ExecutionException ex){
ex.printStackTrace();
}finally {
executorService.shutdown();
}
}
}
View Code
6.Thread和Runnable？
Thread是類，Runnable是介面，Thread是Runnable實現；
類的繼承單一原則，Runnable是更高層次的抽象；
四、結束
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