ThreadContext (core 5.6.16 API)

java.lang.Object
- org.elasticsearch.common.util.concurrent.ThreadContext

All Implemented Interfaces:

java.io.Closeable, java.lang.AutoCloseable, Writeable
```
public final class ThreadContext
extends java.lang.Object
implements java.io.Closeable, Writeable
```
A ThreadContext is a map of string headers and a transient map of keyed objects that are associated with a thread. It allows to store and retrieve header information across method calls, network calls as well as threads spawned from a thread that has a ThreadContext associated with. Threads spawned from a ThreadPool have out of the box support for ThreadContext and all threads spawned will inherit the ThreadContext from the thread that it is forking from.". Network calls will also preserve the senders headers automatically.
Consumers of ThreadContext usually don't need to interact with adding or stashing contexts. Every elasticsearch thread is managed by a thread pool or executor being responsible for stashing and restoring the threads context. For instance if a network request is received, all headers are deserialized from the network and directly added as the headers of the threads ThreadContext (see readHeaders(StreamInput). In order to not modify the context that is currently active on this thread the network code uses a try/with pattern to stash it's current context, read headers into a fresh one and once the request is handled or a handler thread is forked (which in turn inherits the context) it restores the previous context. For instance:
```
     // current context is stashed and replaced with a default context
     try (StoredContext context = threadContext.stashContext()) {
         threadContext.readHeaders(in); // read headers into current context
         if (fork) {
             threadPool.execute(() -> request.handle()); // inherits context
         } else {
             request.handle();
         }
     }
     // previous context is restored on StoredContext#close()
 
```

Nested Class Summary

Nested Classes
Modifier and Type Class and Description

static interface ThreadContext.StoredContext
- Nested classes/interfaces inherited from interface org.elasticsearch.common.io.stream.Writeable
  Writeable.Reader<V>, Writeable.Writer<V>

Nested Classes
Modifier and Type	Class and Description
`static interface`	`ThreadContext.StoredContext`

Field Summary

Fields
Modifier and Type Field and Description

static Setting<Settings> DEFAULT_HEADERS_SETTING

static java.lang.String PREFIX

Fields
Modifier and Type	Field and Description
`static Setting<Settings>`	`DEFAULT_HEADERS_SETTING`
`static java.lang.String`	`PREFIX`

Constructor Summary

Constructors
Constructor and Description

ThreadContext(Settings settings)
Creates a new ThreadContext instance

Constructors
Constructor and Description
`ThreadContext(Settings settings)` Creates a new ThreadContext instance

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method and Description
`void`	`addResponseHeader(java.lang.String key, java.lang.String value)` Add the `value` for the specified `key` Any duplicate `value` is ignored.
`void`	`addResponseHeader(java.lang.String key, java.lang.String value, java.util.function.Function<java.lang.String,java.lang.String> uniqueValue)` Add the `value` for the specified `key` with the specified `uniqueValue` used for de-duplication.
`void`	`close()`
`void`	`copyHeaders(java.lang.Iterable<java.util.Map.Entry<java.lang.String,java.lang.String>> headers)` Copies all header key, value pairs into the current context
`java.lang.String`	`getHeader(java.lang.String key)` Returns the header for the given key or `null` if not present
`java.util.Map<java.lang.String,java.lang.String>`	`getHeaders()` Returns all of the request contexts headers
`java.util.Map<java.lang.String,java.util.List<java.lang.String>>`	`getResponseHeaders()` Get a copy of all response headers.
`<T> T`	`getTransient(java.lang.String key)` Returns a transient header object or `null` if there is no header for the given key
`boolean`	`isSystemContext()` Returns `true` iff this context is a system context
`void`	`markAsSystemContext()` Marks this thread context as an internal system context.
`java.util.function.Supplier<ThreadContext.StoredContext>`	`newRestorableContext(boolean preserveResponseHeaders)` Returns a supplier that gathers a `newStoredContext(boolean)` and restores it once the returned supplier is invoked.
`ThreadContext.StoredContext`	`newStoredContext(boolean preserveResponseHeaders)` Just like `stashContext()` but no default context is set.
`java.lang.Runnable`	`preserveContext(java.lang.Runnable command)` Saves the current thread context and wraps command in a Runnable that restores that context before running command.
`void`	`putHeader(java.util.Map<java.lang.String,java.lang.String> header)` Puts all of the given headers into this context
`void`	`putHeader(java.lang.String key, java.lang.String value)` Puts a header into the context
`void`	`putTransient(java.lang.String key, java.lang.Object value)` Puts a transient header object into this context
`void`	`readHeaders(StreamInput in)` Reads the headers from the stream into the current context
`ThreadContext.StoredContext`	`stashAndMergeHeaders(java.util.Map<java.lang.String,java.lang.String> headers)` Removes the current context and resets a new context that contains a merge of the current headers and the given headers.
`ThreadContext.StoredContext`	`stashContext()` Removes the current context and resets a default context.
`java.lang.Runnable`	`unwrap(java.lang.Runnable command)` Unwraps a command that was previously wrapped by `preserveContext(Runnable)`.
`java.util.function.Supplier<ThreadContext.StoredContext>`	`wrapRestorable(ThreadContext.StoredContext storedContext)` Same as `newRestorableContext(boolean)` but wraps an existing context to restore.
`void`	`writeTo(StreamOutput out)` Write this into the StreamOutput.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Field Detail
  - PREFIX
```
public static final java.lang.String PREFIX
```
    See Also:
    
    Constant Field Values
  - DEFAULT_HEADERS_SETTING
```
public static final Setting<Settings> DEFAULT_HEADERS_SETTING
```
- Constructor Detail
  - ThreadContext
```
public ThreadContext(Settings settings)
```
    Creates a new ThreadContext instance
    
    Parameters:
    
    settings - the settings to read the default request headers from
- Method Detail
  - close
```
public void close()
           throws java.io.IOException
```
    Specified by:
    
    close in interface java.io.Closeable
    
    Specified by:
    
    close in interface java.lang.AutoCloseable
    
    Throws:
    
    java.io.IOException
  - stashContext
```
public ThreadContext.StoredContext stashContext()
```
    Removes the current context and resets a default context. The removed context can be restored when closing the returned ThreadContext.StoredContext
  - stashAndMergeHeaders
```
public ThreadContext.StoredContext stashAndMergeHeaders(java.util.Map<java.lang.String,java.lang.String> headers)
```
    Removes the current context and resets a new context that contains a merge of the current headers and the given headers. The removed context can be restored when closing the returned ThreadContext.StoredContext. The merge strategy is that headers that are already existing are preserved unless they are defaults.
  - newStoredContext
```
public ThreadContext.StoredContext newStoredContext(boolean preserveResponseHeaders)
```
    Just like stashContext() but no default context is set.
    
    Parameters:
    
    preserveResponseHeaders - if set to true the response headers of the restore thread will be preserved.
  - newRestorableContext
```
public java.util.function.Supplier<ThreadContext.StoredContext> newRestorableContext(boolean preserveResponseHeaders)
```
    Returns a supplier that gathers a newStoredContext(boolean) and restores it once the returned supplier is invoked. The context returned from the supplier is a stored version of the suppliers callers context that should be restored once the originally gathered context is not needed anymore. For instance this method should be used like this:
```
     Supplier<ThreadContext.StoredContext> restorable = context.newRestorableContext(true);
     new Thread() {
         public void run() {
             try (ThreadContext.StoredContext ctx = restorable.get()) {
                 // execute with the parents context and restore the threads context afterwards
             }
         }

     }.start();
 
```
    Parameters:
    
    preserveResponseHeaders - if set to true the response headers of the restore thread will be preserved.
    
    Returns:
    
    a restorable context supplier
  - wrapRestorable
```
public java.util.function.Supplier<ThreadContext.StoredContext> wrapRestorable(ThreadContext.StoredContext storedContext)
```
    Same as newRestorableContext(boolean) but wraps an existing context to restore.
    
    Parameters:
    
    storedContext - the context to restore
  - writeTo
```
public void writeTo(StreamOutput out)
             throws java.io.IOException
```
    Description copied from interface: Writeable
    
    Write this into the StreamOutput.
    
    Specified by:
    
    writeTo in interface Writeable
    
    Throws:
    
    java.io.IOException
  - readHeaders
```
public void readHeaders(StreamInput in)
                 throws java.io.IOException
```
    Reads the headers from the stream into the current context
    
    Throws:
    
    java.io.IOException
  - getHeader
```
public java.lang.String getHeader(java.lang.String key)
```
    Returns the header for the given key or null if not present
  - getHeaders
```
public java.util.Map<java.lang.String,java.lang.String> getHeaders()
```
    Returns all of the request contexts headers
  - getResponseHeaders
```
public java.util.Map<java.lang.String,java.util.List<java.lang.String>> getResponseHeaders()
```
    Get a copy of all response headers.
    
    Returns:
    
    Never null.
  - copyHeaders
```
public void copyHeaders(java.lang.Iterable<java.util.Map.Entry<java.lang.String,java.lang.String>> headers)
```
    Copies all header key, value pairs into the current context
  - putHeader
```
public void putHeader(java.lang.String key,
                      java.lang.String value)
```
    Puts a header into the context
  - putHeader
```
public void putHeader(java.util.Map<java.lang.String,java.lang.String> header)
```
    Puts all of the given headers into this context
  - putTransient
```
public void putTransient(java.lang.String key,
                         java.lang.Object value)
```
    Puts a transient header object into this context
  - getTransient
```
public <T> T getTransient(java.lang.String key)
```
    Returns a transient header object or null if there is no header for the given key
  - addResponseHeader
```
public void addResponseHeader(java.lang.String key,
                              java.lang.String value)
```
    Add the value for the specified key Any duplicate value is ignored.
    
    Parameters:
    
    key - the header name
    
    value - the header value
  - addResponseHeader
```
public void addResponseHeader(java.lang.String key,
                              java.lang.String value,
                              java.util.function.Function<java.lang.String,java.lang.String> uniqueValue)
```
    Add the value for the specified key with the specified uniqueValue used for de-duplication. Any duplicate value after applying uniqueValue is ignored.
    
    Parameters:
    
    key - the header name
    
    value - the header value
    
    uniqueValue - the function that produces de-duplication values
  - preserveContext
```
public java.lang.Runnable preserveContext(java.lang.Runnable command)
```
    Saves the current thread context and wraps command in a Runnable that restores that context before running command. If command has already been passed through this method then it is returned unaltered rather than wrapped twice.
  - unwrap
```
public java.lang.Runnable unwrap(java.lang.Runnable command)
```
    Unwraps a command that was previously wrapped by preserveContext(Runnable).
  - markAsSystemContext
```
public void markAsSystemContext()
```
    Marks this thread context as an internal system context. This signals that actions in this context are issued by the system itself rather than by a user action.
  - isSystemContext
```
public boolean isSystemContext()
```
    Returns true iff this context is a system context

Class ThreadContext

Nested Class Summary

Nested classes/interfaces inherited from interface org.elasticsearch.common.io.stream.Writeable

Field Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

PREFIX

DEFAULT_HEADERS_SETTING

Constructor Detail

ThreadContext

Method Detail

close

stashContext

stashAndMergeHeaders

newStoredContext

newRestorableContext

wrapRestorable

writeTo

readHeaders

getHeader

getHeaders

getResponseHeaders

copyHeaders

putHeader

putHeader

putTransient

getTransient

addResponseHeader

addResponseHeader

preserveContext

unwrap

markAsSystemContext

isSystemContext